chapter 5 link layer - uic computer science 5: link layer ... 802.11 wifi) shared rf (satellite)...
TRANSCRIPT
Chapter 5 Link Layer
Computer Networking A Top Down Approach 6th edition Jim Kurose Keith Ross Addison-Wesley March 2012
A note on the use of these ppt slides Wersquore making these slides freely available to all (faculty students readers)
Theyrsquore in PowerPoint form so you see the animations and can add modify
and delete slides (including this one) and slide content to suit your needs
They obviously represent a lot of work on our part In return for use we only
ask the following If you use these slides (eg in a class) that you mention their source
(after all wersquod like people to use our book)
If you post any slides on a www site that you note that they are adapted
from (or perhaps identical to) our slides and note our copyright of this
material
Thanks and enjoy JFKKWR All material copyright 1996-2012 JF Kurose and KW Ross All Rights Reserved
Link Layer 5-1
Link Layer 5-2
Chapter 5 Link layer
our goals understand principles behind link layer
services error detection correction
sharing a broadcast channel multiple access
link layer addressing
local area networks Ethernet VLANs
instantiation implementation of various link layer technologies
Link Layer 5-3
Link layer LANs outline
51 introduction services
52 error detection correction
53 multiple access protocols
54 LANs addressing ARP
Ethernet
switches
VLANS
55 link virtualization MPLS
56 data center networking
57 a day in the life of a web request
Link Layer 5-4
Link layer introduction
terminology hosts and routers nodes
communication channels that connect adjacent nodes along communication path links
wired links
wireless links
LANs
layer-2 packet frame encapsulates datagram
data-link layer has responsibility of transferring datagram from one node to physically adjacent node over a link
global ISP
Link Layer 5-5
Link layer context
datagram transferred by different link protocols over different links
eg Ethernet on first link frame relay on intermediate links 80211 on last link
each link protocol provides different services
eg may or may not provide rdt over link
transportation analogy trip from Princeton to Lausanne
limo Princeton to JFK
plane JFK to Geneva
train Geneva to Lausanne
tourist = datagram
transport segment = communication link
transportation mode = link layer protocol
travel agent = routing algorithm
Link Layer 5-6
Link layer services
framing link access encapsulate datagram into frame adding header trailer channel access if shared medium ldquoMACrdquo addresses used in frame headers to identify
source dest
bull different from IP address
reliable delivery between adjacent nodes we learned how to do this already (chapter 3) seldom used on low bit-error link (fiber some twisted
pair) wireless links high error rates
bull Q why both link-level and end-end reliability
Link Layer 5-7
flow control pacing between adjacent sending and receiving nodes
error detection errors caused by signal attenuation noise
receiver detects presence of errors
bull signals sender for retransmission or drops frame
error correction receiver identifies and corrects bit error(s) without resorting to
retransmission
half-duplex and full-duplex with half duplex nodes at both ends of link can transmit but not
at same time
Link layer services (more)
Link Layer 5-8
Where is the link layer implemented
in each and every host
link layer implemented in ldquoadaptorrdquo (aka network interface card NIC) or on a chip
Ethernet card 80211 card Ethernet chipset
implements link physical layer
attaches into hostrsquos system buses
combination of hardware software firmware
controller
physical
transmission
cpu memory
host
bus
(eg PCI)
network adapter
card
application
transport
network
link
link
physical
Link Layer 5-9
Adaptors communicating
sending side
encapsulates datagram in frame
adds error checking bits rdt flow control etc
receiving side
looks for errors rdt flow control etc
extracts datagram passes to upper layer at receiving side
controller controller
sending host receiving host
datagram datagram
datagram
frame
Link Layer 5-10
Link layer LANs outline
51 introduction services
52 error detection correction
53 multiple access protocols
54 LANs addressing ARP
Ethernet
switches
VLANS
55 link virtualization MPLS
56 data center networking
57 a day in the life of a web request
Link Layer 5-11
Error detection
EDC= Error Detection and Correction bits (redundancy)
D = Data protected by error checking may include header fields
bull Error detection not 100 reliable
bull protocol may miss some errors but rarely
bull larger EDC field yields better detection and correction
otherwise
Link Layer 5-12
Parity checking
single bit parity detect single bit
errors
two-dimensional bit parity detect and correct single bit errors
0 0
Link Layer 5-13
Internet checksum (review)
sender treat segment contents
as sequence of 16-bit integers
checksum addition (1rsquos complement sum) of segment contents
sender puts checksum value into UDP checksum field
receiver compute checksum of
received segment check if computed
checksum equals checksum field value NO - error detected YES - no error detected
But maybe errors nonetheless
goal detect ldquoerrorsrdquo (eg flipped bits) in transmitted packet (note used at transport layer only)
Link Layer 5-14
Cyclic redundancy check
more powerful error-detection coding
view data bits D as a binary number
choose r+1 bit pattern (generator) G
goal choose r CRC bits R such that ltDRgt exactly divisible by G (modulo 2)
receiver knows G divides ltDRgt by G If non-zero remainder error detected
can detect all burst errors less than r+1 bits
widely used in practice (Ethernet 80211 WiFi ATM)
Link Layer 5-15
CRC example
want D2r XOR R = nG
equivalently D2r = nG XOR R
equivalently if we divide D2r by
G want remainder R to satisfy
R = remainder[ ] D2r
G
1001 101110000 1001
1
101
01000
000 1010 1001
010 000
100 000 1000 0000 1000
D G
R
r = 3
Link Layer 5-16
Link layer LANs outline
51 introduction services
52 error detection correction
53 multiple access protocols
54 LANs addressing ARP
Ethernet
switches
VLANS
55 link virtualization MPLS
56 data center networking
57 a day in the life of a web request
Link Layer 5-17
Multiple access links protocols
two types of ldquolinksrdquo
point-to-point PPP for dial-up access
point-to-point link between Ethernet switch host
broadcast (shared wire or medium) old-fashioned Ethernet
upstream HFC
80211 wireless LAN
shared wire (eg cabled Ethernet)
shared RF (eg 80211 WiFi)
shared RF (satellite)
humans at a cocktail party
(shared air acoustical)
Link Layer 5-18
Multiple access protocols
single shared broadcast channel
two or more simultaneous transmissions by nodes interference
collision if node receives two or more signals at the same time
multiple access protocol distributed algorithm that determines how nodes share
channel ie determine when node can transmit
communication about channel sharing must use channel itself no out-of-band channel for coordination
Link Layer 5-19
An ideal multiple access protocol
given broadcast channel of rate R bps
desiderata 1 when one node wants to transmit it can send at rate R
2 when M nodes want to transmit each can send at average rate RM
3 fully decentralized
bull no special node to coordinate transmissions
bull no synchronization of clocks slots
4 simple
Link Layer 5-20
MAC protocols taxonomy
three broad classes
channel partitioning divide channel into smaller ldquopiecesrdquo (time slots frequency code)
allocate piece to node for exclusive use
random access channel not divided allow collisions
ldquorecoverrdquo from collisions
ldquotaking turnsrdquo nodes take turns but nodes with more to send can take longer
turns
Link Layer 5-21
Channel partitioning MAC protocols TDMA
TDMA time division multiple access access to channel in rounds each station gets fixed length slot (length = pkt
trans time) in each round unused slots go idle example 6-station LAN 134 have pkt slots
256 idle
1 3 4 1 3 4
6-slot
frame
6-slot
frame
Link Layer 5-22
FDMA frequency division multiple access channel spectrum divided into frequency bands
each station assigned fixed frequency band
unused transmission time in frequency bands go idle
example 6-station LAN 134 have pkt frequency bands 256 idle
frequency b
ands
FDM cable
Channel partitioning MAC protocols FDMA
Link Layer 5-23
Random access protocols
when node has packet to send transmit at full channel data rate R no a priori coordination among nodes
two or more transmitting nodes ldquocollisionrdquo random access MAC protocol specifies
how to detect collisions how to recover from collisions (eg via delayed
retransmissions)
examples of random access MAC protocols slotted ALOHA ALOHA CSMA CSMACD CSMACA
Link Layer 5-24
Slotted ALOHA
assumptions all frames same size
time divided into equal size slots (time to transmit 1 frame)
nodes start to transmit only slot beginning
nodes are synchronized
if 2 or more nodes transmit in slot all nodes detect collision
operation
when node obtains fresh frame transmits in next slot
if no collision node can send new frame in next slot
if collision node retransmits frame in each subsequent slot with prob p until success
Link Layer 5-25
Pros single active node can
continuously transmit at full rate of channel
highly decentralized only slots in nodes need to be in sync
simple
Cons
collisions wasting slots
idle slots
nodes may be able to detect collision in less than time to transmit packet
clock synchronization
Slotted ALOHA
1 1 1 1
2
3
2 2
3 3
node 1
node 2
node 3
C C C S S S E E E
Link Layer 5-26
suppose N nodes with many frames to send each transmits in slot with probability p
prob that given node has success in a slot = p(1-p)N-1
prob that any node has a success = Np(1-p)N-1
max efficiency find p that maximizes Np(1-p)N-1
for many nodes take limit of Np(1-p)N-1 as N goes to infinity gives
max efficiency = 1e = 37
efficiency long-run fraction of successful slots (many nodes all with many frames to send)
at best channel used for useful transmissions 37 of time
Slotted ALOHA efficiency
Link Layer 5-27
Pure (unslotted) ALOHA
unslotted Aloha simpler no synchronization
when frame first arrives
transmit immediately
collision probability increases
frame sent at t0 collides with other frames sent in [t0-1t0+1]
Link Layer 5-28
Pure ALOHA efficiency
P(success by given node) = P(node transmits)
P(no other node transmits in [t0-1t0]
P(no other node transmits in [t0-1t0]
= p (1-p)N-1 (1-p)N-1
= p (1-p)2(N-1)
hellip choosing optimum p and then letting n
= 1(2e) = 18
even worse than slotted Aloha
Link Layer 5-29
CSMA (carrier sense multiple access)
CSMA listen before transmit if channel sensed idle transmit entire frame
if channel sensed busy defer transmission
human analogy donrsquot interrupt others
Link Layer 5-30
CSMA collisions
collisions can still occur propagation delay means two nodes may not hear each otherrsquos transmission
collision entire packet transmission time wasted distance amp propagation
delay play role in in determining collision probability
spatial layout of nodes
Link Layer 5-31
CSMACD (collision detection)
CSMACD carrier sensing deferral as in CSMA collisions detected within short time
colliding transmissions aborted reducing channel wastage
collision detection easy in wired LANs measure signal strengths compare
transmitted received signals
difficult in wireless LANs received signal strength overwhelmed by local transmission strength
human analogy the polite conversationalist
Link Layer 5-32
CSMACD (collision detection)
spatial layout of nodes
Link Layer 5-33
Ethernet CSMACD algorithm
1 NIC receives datagram from network layer creates frame
2 If NIC senses channel idle starts frame transmission If NIC senses channel busy waits until channel idle then transmits
3 If NIC transmits entire frame without detecting another transmission NIC is done with frame
4 If NIC detects another transmission while transmitting aborts and sends jam signal
5 After aborting NIC enters binary (exponential) backoff
after mth collision NIC chooses K at random from 012 hellip 2m-1 NIC waits K512 bit times returns to Step 2
longer backoff interval with more collisions
Link Layer 5-34
CSMACD efficiency
Tprop = max prop delay between 2 nodes in LAN
ttrans = time to transmit max-size frame
efficiency goes to 1
as tprop goes to 0
as ttrans goes to infinity
better performance than ALOHA and simple cheap decentralized
transpropttefficiency
51
1
Link Layer 5-35
ldquoTaking turnsrdquo MAC protocols
channel partitioning MAC protocols share channel efficiently and fairly at high load
inefficient at low load delay in channel access 1N bandwidth allocated even if only 1 active node
random access MAC protocols efficient at low load single node can fully utilize
channel
high load collision overhead
ldquotaking turnsrdquo protocols look for best of both worlds
Link Layer 5-36
polling master node ldquoinvitesrdquo
slave nodes to transmit in turn
typically used with ldquodumbrdquo slave devices
concerns
polling overhead
latency
single point of failure (master)
master
slaves
poll
data
data
ldquoTaking turnsrdquo MAC protocols
Link Layer 5-37
token passing control token passed
from one node to next sequentially
token message
concerns
token overhead
latency
single point of failure (token)
T
data
(nothing
to send)
T
ldquoTaking turnsrdquo MAC protocols
cable headend
CMTS
ISP
cable modem termination system
multiple 40Mbps downstream (broadcast) channels
single CMTS transmits into channels
multiple 30 Mbps upstream channels
multiple access all users contend for certain upstream channel time slots (others assigned)
Cable access network
cable modem
splitter
hellip
hellip
Internet framesTV channels control transmitted downstream at different frequencies
upstream Internet frames TV control transmitted upstream at different frequencies in time slots
Link Layer 5-39
DOCSIS data over cable service interface spec FDM over upstream downstream frequency channels
TDM upstream some slots assigned some have contention
downstream MAP frame assigns upstream slots
request for upstream slots (and data) transmitted random access (binary backoff) in selected slots
MAP frame for
Interval [t1 t2]
Residences with cable modems
Downstream channel i
Upstream channel j
t1 t2
Assigned minislots containing cable modem
upstream data frames
Minislots containing
minislots request frames
cable headend
CMTS
Cable access network
Link Layer 5-40
Summary of MAC protocols
channel partitioning by time frequency or code Time Division Frequency Division
random access (dynamic)
ALOHA S-ALOHA CSMA CSMACD
carrier sensing easy in some technologies (wire) hard in others (wireless)
CSMACD used in Ethernet
CSMACA used in 80211
taking turns
polling from central site token passing
bluetooth FDDI token ring
Link Layer 5-41
Link layer LANs outline
51 introduction services
52 error detection correction
53 multiple access protocols
54 LANs addressing ARP
Ethernet
switches
VLANS
55 link virtualization MPLS
56 data center networking
57 a day in the life of a web request
Link Layer 5-42
MAC addresses and ARP
32-bit IP address network-layer address for interface
used for layer 3 (network layer) forwarding
MAC (or LAN or physical or Ethernet) address function used lsquolocallyrdquo to get frame from one interface to
another physically-connected interface (same network in IP-addressing sense)
48 bit MAC address (for most LANs) burned in NIC ROM also sometimes software settable
eg 1A-2F-BB-76-09-AD
hexadecimal (base 16) notation
(each ldquonumberrdquo represents 4 bits)
Link Layer 5-43
LAN addresses and ARP
each adapter on LAN has unique LAN address
adapter
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN
(wired or
wireless)
Link Layer 5-44
LAN addresses (more)
MAC address allocation administered by IEEE
manufacturer buys portion of MAC address space (to assure uniqueness)
analogy MAC address like Social Security Number
IP address like postal address
MAC flat address portability can move LAN card from one LAN to another
IP hierarchical address not portable address depends on IP subnet to which node is
attached
Link Layer 5-45
ARP address resolution protocol
ARP table each IP node (host router) on LAN has table
IPMAC address mappings for some LAN nodes
lt IP address MAC address TTLgt
TTL (Time To Live) time after which address mapping will be forgotten (typically 20 min)
Question how to determine
interfacersquos MAC address
knowing its IP address
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN
137196723
137196778
137196714
137196788
Link Layer 5-46
ARP protocol same LAN
A wants to send datagram to B Brsquos MAC address not in
Arsquos ARP table
A broadcasts ARP query packet containing Bs IP address dest MAC address = FF-FF-
FF-FF-FF-FF
all nodes on LAN receive ARP query
B receives ARP packet replies to A with its (Bs) MAC address frame sent to Arsquos MAC
address (unicast)
A caches (saves) IP-to-MAC address pair in its ARP table until information becomes old (times out) soft state information that
times out (goes away) unless refreshed
ARP is ldquoplug-and-playrdquo nodes create their ARP
tables without intervention from net administrator
Link Layer 5-47
walkthrough send datagram from A to B via R
focus on addressing ndash at IP (datagram) and MAC layer (frame)
assume A knows Brsquos IP address
assume A knows IP address of first hop router R (how)
assume A knows Rrsquos MAC address (how)
Addressing routing to another LAN
R
1A-23-F9-CD-06-9B 222222222220
111111111110 E6-E9-00-17-BB-4B CC-49-DE-D0-AB-7D
111111111112
111111111111
74-29-9C-E8-FF-55
A
222222222222
49-BD-D2-C7-56-2A
222222222221 88-B2-2F-54-1A-0F
B
R
1A-23-F9-CD-06-9B 222222222220
111111111110 E6-E9-00-17-BB-4B CC-49-DE-D0-AB-7D
111111111112
111111111111
74-29-9C-E8-FF-55
A
222222222222
49-BD-D2-C7-56-2A
222222222221 88-B2-2F-54-1A-0F
B
Link Layer 5-48
Addressing routing to another LAN
IP
Eth
Phy
IP src 111111111111
IP dest 222222222222
A creates IP datagram with IP source A destination B
A creates link-layer frame with Rs MAC address as dest frame contains A-to-B IP datagram
MAC src 74-29-9C-E8-FF-55
MAC dest E6-E9-00-17-BB-4B
R
1A-23-F9-CD-06-9B 222222222220
111111111110 E6-E9-00-17-BB-4B CC-49-DE-D0-AB-7D
111111111112
111111111111
74-29-9C-E8-FF-55
A
222222222222
49-BD-D2-C7-56-2A
222222222221 88-B2-2F-54-1A-0F
B
Link Layer 5-49
Addressing routing to another LAN
IP
Eth
Phy
frame sent from A to R
IP
Eth
Phy
frame received at R datagram removed passed up to IP
MAC src 74-29-9C-E8-FF-55
MAC dest E6-E9-00-17-BB-4B
IP src 111111111111
IP dest 222222222222
IP src 111111111111
IP dest 222222222222
R
1A-23-F9-CD-06-9B 222222222220
111111111110 E6-E9-00-17-BB-4B CC-49-DE-D0-AB-7D
111111111112
111111111111
74-29-9C-E8-FF-55
A
222222222222
49-BD-D2-C7-56-2A
222222222221 88-B2-2F-54-1A-0F
B
Link Layer 5-50
Addressing routing to another LAN
IP src 111111111111
IP dest 222222222222
R forwards datagram with IP source A destination B
R creates link-layer frame with Bs MAC address as dest frame contains A-to-B IP datagram
MAC src 1A-23-F9-CD-06-9B
MAC dest 49-BD-D2-C7-56-2A
IP
Eth
Phy
IP
Eth
Phy
R
1A-23-F9-CD-06-9B 222222222220
111111111110 E6-E9-00-17-BB-4B CC-49-DE-D0-AB-7D
111111111112
111111111111
74-29-9C-E8-FF-55
A
222222222222
49-BD-D2-C7-56-2A
222222222221 88-B2-2F-54-1A-0F
B
Link Layer 5-51
Addressing routing to another LAN R forwards datagram with IP source A destination B
R creates link-layer frame with Bs MAC address as dest frame contains A-to-B IP datagram
IP src 111111111111
IP dest 222222222222
MAC src 1A-23-F9-CD-06-9B
MAC dest 49-BD-D2-C7-56-2A
IP
Eth
Phy
IP
Eth
Phy
R
1A-23-F9-CD-06-9B 222222222220
111111111110 E6-E9-00-17-BB-4B CC-49-DE-D0-AB-7D
111111111112
111111111111
74-29-9C-E8-FF-55
A
222222222222
49-BD-D2-C7-56-2A
222222222221 88-B2-2F-54-1A-0F
B
Link Layer 5-52
Addressing routing to another LAN R forwards datagram with IP source A destination B
R creates link-layer frame with Bs MAC address as dest frame contains A-to-B IP datagram
IP src 111111111111
IP dest 222222222222
MAC src 1A-23-F9-CD-06-9B
MAC dest 49-BD-D2-C7-56-2A
IP
Eth
Phy
Link Layer 5-53
Link layer LANs outline
51 introduction services
52 error detection correction
53 multiple access protocols
54 LANs addressing ARP
Ethernet
switches
VLANS
55 link virtualization MPLS
56 data center networking
57 a day in the life of a web request
Link Layer 5-54
Ethernet
ldquodominantrdquo wired LAN technology
cheap $20 for NIC
first widely used LAN technology
simpler cheaper than token LANs and ATM
kept up with speed race 10 Mbps ndash 10 Gbps
Metcalfersquos Ethernet sketch
Link Layer 5-55
Ethernet physical topology
bus popular through mid 90s all nodes in same collision domain (can collide with each
other)
star prevails today active switch in center
each ldquospokerdquo runs a (separate) Ethernet protocol (nodes
do not collide with each other)
switch
bus coaxial cable star
Link Layer 5-56
Ethernet frame structure
sending adapter encapsulates IP datagram (or other network layer protocol packet) in Ethernet frame
preamble
7 bytes with pattern 10101010 followed by one byte with pattern 10101011
used to synchronize receiver sender clock rates
dest address
source address
data (payload) CRC preamble
type
Link Layer 5-57
Ethernet frame structure (more)
addresses 6 byte source destination MAC addresses if adapter receives frame with matching destination
address or with broadcast address (eg ARP packet) it passes data in frame to network layer protocol
otherwise adapter discards frame
type indicates higher layer protocol (mostly IP but others possible eg Novell IPX AppleTalk)
CRC cyclic redundancy check at receiver error detected frame is dropped
dest address
source address
data (payload) CRC preamble
type
Link Layer 5-58
Ethernet unreliable connectionless
connectionless no handshaking between sending and receiving NICs
unreliable receiving NIC doesnt send acks or nacks to sending NIC
data in dropped frames recovered only if initial sender uses higher layer rdt (eg TCP) otherwise dropped data lost
Ethernetrsquos MAC protocol unslotted CSMACD wth binary backoff
Link Layer 5-59
8023 Ethernet standards link amp physical layers
many different Ethernet standards
common MAC protocol and frame format
different speeds 2 Mbps 10 Mbps 100 Mbps 1Gbps 10G bps
different physical layer media fiber cable
application
transport
network
link
physical
MAC protocol
and frame format
100BASE-TX
100BASE-T4
100BASE-FX 100BASE-T2
100BASE-SX 100BASE-BX
fiber physical layer copper (twister
pair) physical layer
Link Layer 5-60
Link layer LANs outline
51 introduction services
52 error detection correction
53 multiple access protocols
54 LANs addressing ARP
Ethernet
switches
VLANS
55 link virtualization MPLS
56 data center networking
57 a day in the life of a web request
Link Layer 5-61
Ethernet switch link-layer device takes an active role
store forward Ethernet frames
examine incoming framersquos MAC address selectively forward frame to one-or-more outgoing links when frame is to be forwarded on segment uses CSMACD to access segment
transparent
hosts are unaware of presence of switches
plug-and-play self-learning
switches do not need to be configured
Link Layer 5-62
Switch multiple simultaneous transmissions
hosts have dedicated direct connection to switch
switches buffer packets
Ethernet protocol used on each incoming link but no collisions full duplex
each link is its own collision domain
switching A-to-Arsquo and B-to-Brsquo can transmit simultaneously without collisions switch with six interfaces
(123456)
A
Arsquo
B
Brsquo C
Crsquo
1 2
3 4 5
6
Link Layer 5-63
Switch forwarding table
Q how does switch know Arsquo reachable via interface 4 Brsquo reachable via interface 5
switch with six interfaces
(123456)
A
Arsquo
B
Brsquo C
Crsquo
1 2
3 4 5
6 A each switch has a switch table each entry
(MAC address of host interface to
reach host time stamp)
looks like a routing table
Q how are entries created maintained in switch table
something like a routing protocol
A
Arsquo
B
Brsquo C
Crsquo
1 2
3 4 5
6
Link Layer 5-64
Switch self-learning
switch learns which hosts can be reached through which interfaces
when frame received switch ldquolearnsrdquo location of sender incoming LAN segment
records senderlocation pair in switch table
A Arsquo
Source A
Dest Arsquo
MAC addr interface TTL
Switch table
(initially empty) A 1 60
Link Layer 5-65
Switch frame filteringforwarding
when frame received at switch 1 record incoming link MAC address of sending host
2 index switch table using MAC destination address
3 if entry found for destination then
if destination on segment from which frame arrived then drop frame
else forward frame on interface indicated by entry
else flood forward on all interfaces except arriving
interface
A
Arsquo
B
Brsquo C
Crsquo
1 2
3 4 5
6
Link Layer 5-66
Self-learning forwarding example
A Arsquo
Source A
Dest Arsquo
MAC addr interface TTL
switch table
(initially empty) A 1 60
A Arsquo A Arsquo A Arsquo A Arsquo A Arsquo
frame destination Arsquo locaton unknown flood
Arsquo A
destination A location
known
Arsquo 4 60
selectively send
on just one link
Link Layer 5-67
Interconnecting switches
switches can be connected together
Q sending from A to G - how does S1 know to forward frame destined to F via S4 and S3
A self learning (works exactly the same as in single-switch case)
A
B
S1
C D
E
F
S2
S4
S3
H
I
G
Link Layer 5-68
Self-learning multi-switch example
Suppose C sends frame to I I responds to C
Q show switch tables and packet forwarding in S1 S2 S3 S4
A
B
S1
C D
E
F
S2
S4
S3
H
I
G
Link Layer 5-69
Institutional network
to external
network
router
IP subnet
mail server
web server
Link Layer 5-70
Switches vs routers
both are store-and-forward
routers network-layer devices (examine network-layer headers)
switches link-layer devices (examine link-layer headers)
both have forwarding tables
routers compute tables using routing algorithms IP addresses
switches learn forwarding table using flooding learning MAC addresses
application
transport
network
link
physical
network
link
physical
link
physical
switch
datagram
application
transport
network
link
physical
frame
frame
frame
datagram
Link Layer 5-71
VLANs motivation
consider CS user moves office to
EE but wants connect to CS switch
single broadcast domain
all layer-2 broadcast traffic (ARP DHCP unknown location of destination MAC address) must cross entire LAN
securityprivacy efficiency issues
Computer
Science Electrical
Engineering
Computer
Engineering
Link Layer 5-72
VLANs port-based VLAN switch ports
grouped (by switch management software) so that single physical switch helliphellip
switch(es) supporting
VLAN capabilities can
be configured to
define multiple virtual
LANS over single
physical LAN
infrastructure
Virtual Local
Area Network 1
8
9
16 10 2
7
hellip
Electrical Engineering
(VLAN ports 1-8)
Computer Science
(VLAN ports 9-15)
15
hellip
Electrical Engineering
(VLAN ports 1-8)
hellip
1
8 2
7 9
16 10
15
hellip
Computer Science
(VLAN ports 9-16)
hellip operates as multiple virtual switches
Link Layer 5-73
Port-based VLAN
1
8
9
16 10 2
7
hellip
Electrical Engineering
(VLAN ports 1-8)
Computer Science
(VLAN ports 9-15)
15
hellip
traffic isolation frames tofrom ports 1-8 can only reach ports 1-8 can also define VLAN based on
MAC addresses of endpoints rather than switch port
dynamic membership ports can be dynamically assigned among VLANs
router
forwarding between VLANS done via routing (just as with separate switches) in practice vendors sell combined
switches plus routers
Link Layer 5-74
VLANS spanning multiple switches
trunk port carries frames between VLANS defined over multiple physical switches frames forwarded within VLAN between switches canrsquot be vanilla
8021 frames (must carry VLAN ID info)
8021q protocol addsremoved additional header fields for frames forwarded between trunk ports
1
8
9
10 2
7
hellip
Electrical Engineering
(VLAN ports 1-8)
Computer Science
(VLAN ports 9-15)
15
hellip
2
7 3
Ports 235 belong to EE VLAN
Ports 4678 belong to CS VLAN
5
4 6 8 16
1
Link Layer 5-75
type
2-byte Tag Protocol Identifier
(value 81-00)
Tag Control Information (12 bit VLAN ID field
3 bit priority field like IP TOS)
Recomputed CRC
8021Q VLAN frame format
8021 frame
8021Q frame
dest address
source address
data (payload) CRC preamble
dest address
source address
preamble data (payload) CRC
type
Link Layer 5-76
Link layer LANs outline
51 introduction services
52 error detection correction
53 multiple access protocols
54 LANs addressing ARP
Ethernet
switches
VLANS
55 link virtualization MPLS
56 data center networking
57 a day in the life of a web request
Link Layer 5-77
Multiprotocol label switching (MPLS)
initial goal high-speed IP forwarding using fixed length label (instead of IP address) fast lookup using fixed length identifier (rather than
shortest prefix matching)
borrowing ideas from Virtual Circuit (VC) approach
but IP datagram still keeps IP address
PPP or Ethernet
header IP header remainder of link-layer frame MPLS header
label Exp S TTL
20 3 1 5
Link Layer 5-78
MPLS capable routers
aka label-switched router
forward packets to outgoing interface based only on label value (donrsquot inspect IP address) MPLS forwarding table distinct from IP forwarding tables
flexibility MPLS forwarding decisions can differ from those of IP use destination and source addresses to route flows to
same destination differently (traffic engineering)
re-route flows quickly if link fails pre-computed backup paths (useful for VoIP)
Link Layer 5-79
R2
D
R3 R5
A
R6
MPLS versus IP paths
IP router IP routing path to destination determined
by destination address alone
R4
Link Layer 5-80
R2
D
R3 R4
R5
A
R6
MPLS versus IP paths
IP-only router
IP routing path to destination determined by destination address alone
MPLS and IP router
MPLS routing path to destination can be based on source and dest address fast reroute precompute backup routes in
case of link failure
entry router (R4) can use different MPLS
routes to A based eg on source address
Link Layer 5-81
MPLS signaling
modify OSPF IS-IS link-state flooding protocols to carry info used by MPLS routing eg link bandwidth amount of ldquoreservedrdquo link bandwidth
D
R4
R5
A
R6
entry MPLS router uses RSVP-TE signaling protocol to set up MPLS forwarding at downstream routers
modified link state flooding
RSVP-TE
Link Layer 5-82
R1 R2
D
R3 R4 R5
0
1
0 0
A
R6
in out out
label label dest interface
6 - A 0
in out out
label label dest interface
10 6 A 1
12 9 D 0
in out out
label label dest interface
10 A 0
12 D 0
1
in out out
label label dest interface
8 6 A 0
0
8 A 1
MPLS forwarding tables
Link Layer 5-83
Link layer LANs outline
51 introduction services
52 error detection correction
53 multiple access protocols
54 LANs addressing ARP
Ethernet
switches
VLANS
55 link virtualization MPLS
56 data center networking
57 a day in the life of a web request
Link Layer 5-84
Data center networks
10rsquos to 100rsquos of thousands of hosts often closely coupled in close proximity e-business (eg Amazon)
content-servers (eg YouTube Akamai Apple Microsoft)
search engines data mining (eg Google)
challenges
multiple applications each serving massive numbers of clients
managingbalancing load avoiding processing networking data bottlenecks
Inside a 40-ft Microsoft container
Chicago data center
Link Layer 5-85
Server racks
TOR switches
Tier-1 switches
Tier-2 switches
Load balancer
Load balancer
B
1 2 3 4 5 6 7 8
A C
Border router
Access router
Internet
Data center networks
load balancer application-layer routing receives external client requests
directs workload within data center
returns results to external client (hiding data
center internals from client)
Server racks
TOR switches
Tier-1 switches
Tier-2 switches
1 2 3 4 5 6 7 8
Data center networks
rich interconnection among switches racks
increased throughput between racks (multiple routing
paths possible)
increased reliability via redundancy
Link Layer 5-87
Link layer LANs outline
51 introduction services
52 error detection correction
53 multiple access protocols
54 LANs addressing ARP
Ethernet
switches
VLANS
55 link virtualization MPLS
56 data center networking
57 a day in the life of a web request
Link Layer 5-88
Synthesis a day in the life of a web request
journey down protocol stack complete application transport network link
putting-it-all-together synthesis goal identify review understand protocols (at all
layers) involved in seemingly simple scenario requesting www page
scenario student attaches laptop to campus network requestsreceives wwwgooglecom
Link Layer 5-89
A day in the life scenario
Comcast network
68800013
Googlersquos network
64233160019 64233169105
web server
DNS server
school network
68802024
web page
browser
router
(runs DHCP)
Link Layer 5-90
A day in the lifehellip connecting to the Internet
connecting laptop needs to get its own IP address addr of first-hop router addr of DNS server use DHCP
DHCP
UDP
IP
Eth
Phy
DHCP
DHCP
DHCP
DHCP
DHCP
DHCP
UDP
IP
Eth
Phy
DHCP
DHCP
DHCP
DHCP DHCP
DHCP request encapsulated in UDP encapsulated in IP encapsulated in 8023 Ethernet
Ethernet frame broadcast
(dest FFFFFFFFFFFF) on LAN received at router running DHCP server
Ethernet demuxed to IP demuxed UDP demuxed to DHCP
router
(runs DHCP)
Link Layer 5-91
DHCP server formulates DHCP ACK containing clientrsquos IP address IP address of first-hop router for client name amp IP address of DNS server
DHCP
UDP
IP
Eth
Phy
DHCP
DHCP
DHCP
DHCP
DHCP
UDP
IP
Eth
Phy
DHCP
DHCP
DHCP
DHCP
DHCP
encapsulation at DHCP server frame forwarded (switch learning) through LAN demultiplexing at client
Client now has IP address knows name amp addr of DNS
server IP address of its first-hop router
DHCP client receives DHCP ACK reply
A day in the lifehellip connecting to the Internet
router
(runs DHCP)
Link Layer 5-92
A day in the lifehellip ARP (before DNS before HTTP)
before sending HTTP request need IP address of wwwgooglecom DNS
DNS
UDP
IP
Eth
Phy
DNS
DNS
DNS
DNS query created encapsulated in UDP encapsulated in IP encapsulated in Eth To send frame to router need MAC address of router interface ARP
ARP query broadcast received by
router which replies with ARP reply giving MAC address of router interface
client now knows MAC address of first hop router so can now send frame containing DNS query
ARP query
Eth
Phy
ARP
ARP
ARP reply
router
(runs DHCP)
Link Layer 5-93
DNS
UDP
IP
Eth
Phy
DNS
DNS
DNS
DNS
DNS
IP datagram containing DNS query forwarded via LAN switch from client to 1st hop router
IP datagram forwarded from campus network into comcast network routed (tables created by RIP OSPF IS-IS andor BGP routing protocols) to DNS server
demuxrsquoed to DNS server
DNS server replies to client with IP address of wwwgooglecom
Comcast network
68800013
DNS server
DNS
UDP
IP
Eth
Phy
DNS
DNS
DNS
DNS
A day in the lifehellip using DNS
router
(runs DHCP)
Link Layer 5-94
A day in the lifehellipTCP connection carrying HTTP
HTTP
TCP
IP
Eth
Phy
HTTP
to send HTTP request client first opens TCP socket to web server
TCP SYN segment (step 1 in 3-way handshake) inter-domain routed to web server
TCP connection established 64233169105
web server
SYN
SYN
SYN
SYN
TCP
IP
Eth
Phy
SYN
SYN
SYN
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
web server responds with TCP SYNACK (step 2 in 3-way handshake)
router
(runs DHCP)
Link Layer 5-95
A day in the lifehellip HTTP requestreply
HTTP
TCP
IP
Eth
Phy
HTTP
HTTP request sent into TCP socket
IP datagram containing HTTP request routed to wwwgooglecom
IP datagram containing HTTP reply routed back to client
64233169105
web server
HTTP
TCP
IP
Eth
Phy web server responds with
HTTP reply (containing web page)
HTTP
HTTP
HTTP HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
web page finally () displayed
Link Layer 5-96
Chapter 5 Summary
principles behind data link layer services error detection correction
sharing a broadcast channel multiple access
link layer addressing
instantiation and implementation of various link layer technologies Ethernet
switched LANS VLANs
virtualized networks as a link layer MPLS
synthesis a day in the life of a web request
Link Layer 5-97
Chapter 5 letrsquos take a breath
journey down protocol stack complete (except PHY)
solid understanding of networking principles practice
hellip could stop here hellip but lots of interesting topics wireless
multimedia
security
network management
Link Layer 5-2
Chapter 5 Link layer
our goals understand principles behind link layer
services error detection correction
sharing a broadcast channel multiple access
link layer addressing
local area networks Ethernet VLANs
instantiation implementation of various link layer technologies
Link Layer 5-3
Link layer LANs outline
51 introduction services
52 error detection correction
53 multiple access protocols
54 LANs addressing ARP
Ethernet
switches
VLANS
55 link virtualization MPLS
56 data center networking
57 a day in the life of a web request
Link Layer 5-4
Link layer introduction
terminology hosts and routers nodes
communication channels that connect adjacent nodes along communication path links
wired links
wireless links
LANs
layer-2 packet frame encapsulates datagram
data-link layer has responsibility of transferring datagram from one node to physically adjacent node over a link
global ISP
Link Layer 5-5
Link layer context
datagram transferred by different link protocols over different links
eg Ethernet on first link frame relay on intermediate links 80211 on last link
each link protocol provides different services
eg may or may not provide rdt over link
transportation analogy trip from Princeton to Lausanne
limo Princeton to JFK
plane JFK to Geneva
train Geneva to Lausanne
tourist = datagram
transport segment = communication link
transportation mode = link layer protocol
travel agent = routing algorithm
Link Layer 5-6
Link layer services
framing link access encapsulate datagram into frame adding header trailer channel access if shared medium ldquoMACrdquo addresses used in frame headers to identify
source dest
bull different from IP address
reliable delivery between adjacent nodes we learned how to do this already (chapter 3) seldom used on low bit-error link (fiber some twisted
pair) wireless links high error rates
bull Q why both link-level and end-end reliability
Link Layer 5-7
flow control pacing between adjacent sending and receiving nodes
error detection errors caused by signal attenuation noise
receiver detects presence of errors
bull signals sender for retransmission or drops frame
error correction receiver identifies and corrects bit error(s) without resorting to
retransmission
half-duplex and full-duplex with half duplex nodes at both ends of link can transmit but not
at same time
Link layer services (more)
Link Layer 5-8
Where is the link layer implemented
in each and every host
link layer implemented in ldquoadaptorrdquo (aka network interface card NIC) or on a chip
Ethernet card 80211 card Ethernet chipset
implements link physical layer
attaches into hostrsquos system buses
combination of hardware software firmware
controller
physical
transmission
cpu memory
host
bus
(eg PCI)
network adapter
card
application
transport
network
link
link
physical
Link Layer 5-9
Adaptors communicating
sending side
encapsulates datagram in frame
adds error checking bits rdt flow control etc
receiving side
looks for errors rdt flow control etc
extracts datagram passes to upper layer at receiving side
controller controller
sending host receiving host
datagram datagram
datagram
frame
Link Layer 5-10
Link layer LANs outline
51 introduction services
52 error detection correction
53 multiple access protocols
54 LANs addressing ARP
Ethernet
switches
VLANS
55 link virtualization MPLS
56 data center networking
57 a day in the life of a web request
Link Layer 5-11
Error detection
EDC= Error Detection and Correction bits (redundancy)
D = Data protected by error checking may include header fields
bull Error detection not 100 reliable
bull protocol may miss some errors but rarely
bull larger EDC field yields better detection and correction
otherwise
Link Layer 5-12
Parity checking
single bit parity detect single bit
errors
two-dimensional bit parity detect and correct single bit errors
0 0
Link Layer 5-13
Internet checksum (review)
sender treat segment contents
as sequence of 16-bit integers
checksum addition (1rsquos complement sum) of segment contents
sender puts checksum value into UDP checksum field
receiver compute checksum of
received segment check if computed
checksum equals checksum field value NO - error detected YES - no error detected
But maybe errors nonetheless
goal detect ldquoerrorsrdquo (eg flipped bits) in transmitted packet (note used at transport layer only)
Link Layer 5-14
Cyclic redundancy check
more powerful error-detection coding
view data bits D as a binary number
choose r+1 bit pattern (generator) G
goal choose r CRC bits R such that ltDRgt exactly divisible by G (modulo 2)
receiver knows G divides ltDRgt by G If non-zero remainder error detected
can detect all burst errors less than r+1 bits
widely used in practice (Ethernet 80211 WiFi ATM)
Link Layer 5-15
CRC example
want D2r XOR R = nG
equivalently D2r = nG XOR R
equivalently if we divide D2r by
G want remainder R to satisfy
R = remainder[ ] D2r
G
1001 101110000 1001
1
101
01000
000 1010 1001
010 000
100 000 1000 0000 1000
D G
R
r = 3
Link Layer 5-16
Link layer LANs outline
51 introduction services
52 error detection correction
53 multiple access protocols
54 LANs addressing ARP
Ethernet
switches
VLANS
55 link virtualization MPLS
56 data center networking
57 a day in the life of a web request
Link Layer 5-17
Multiple access links protocols
two types of ldquolinksrdquo
point-to-point PPP for dial-up access
point-to-point link between Ethernet switch host
broadcast (shared wire or medium) old-fashioned Ethernet
upstream HFC
80211 wireless LAN
shared wire (eg cabled Ethernet)
shared RF (eg 80211 WiFi)
shared RF (satellite)
humans at a cocktail party
(shared air acoustical)
Link Layer 5-18
Multiple access protocols
single shared broadcast channel
two or more simultaneous transmissions by nodes interference
collision if node receives two or more signals at the same time
multiple access protocol distributed algorithm that determines how nodes share
channel ie determine when node can transmit
communication about channel sharing must use channel itself no out-of-band channel for coordination
Link Layer 5-19
An ideal multiple access protocol
given broadcast channel of rate R bps
desiderata 1 when one node wants to transmit it can send at rate R
2 when M nodes want to transmit each can send at average rate RM
3 fully decentralized
bull no special node to coordinate transmissions
bull no synchronization of clocks slots
4 simple
Link Layer 5-20
MAC protocols taxonomy
three broad classes
channel partitioning divide channel into smaller ldquopiecesrdquo (time slots frequency code)
allocate piece to node for exclusive use
random access channel not divided allow collisions
ldquorecoverrdquo from collisions
ldquotaking turnsrdquo nodes take turns but nodes with more to send can take longer
turns
Link Layer 5-21
Channel partitioning MAC protocols TDMA
TDMA time division multiple access access to channel in rounds each station gets fixed length slot (length = pkt
trans time) in each round unused slots go idle example 6-station LAN 134 have pkt slots
256 idle
1 3 4 1 3 4
6-slot
frame
6-slot
frame
Link Layer 5-22
FDMA frequency division multiple access channel spectrum divided into frequency bands
each station assigned fixed frequency band
unused transmission time in frequency bands go idle
example 6-station LAN 134 have pkt frequency bands 256 idle
frequency b
ands
FDM cable
Channel partitioning MAC protocols FDMA
Link Layer 5-23
Random access protocols
when node has packet to send transmit at full channel data rate R no a priori coordination among nodes
two or more transmitting nodes ldquocollisionrdquo random access MAC protocol specifies
how to detect collisions how to recover from collisions (eg via delayed
retransmissions)
examples of random access MAC protocols slotted ALOHA ALOHA CSMA CSMACD CSMACA
Link Layer 5-24
Slotted ALOHA
assumptions all frames same size
time divided into equal size slots (time to transmit 1 frame)
nodes start to transmit only slot beginning
nodes are synchronized
if 2 or more nodes transmit in slot all nodes detect collision
operation
when node obtains fresh frame transmits in next slot
if no collision node can send new frame in next slot
if collision node retransmits frame in each subsequent slot with prob p until success
Link Layer 5-25
Pros single active node can
continuously transmit at full rate of channel
highly decentralized only slots in nodes need to be in sync
simple
Cons
collisions wasting slots
idle slots
nodes may be able to detect collision in less than time to transmit packet
clock synchronization
Slotted ALOHA
1 1 1 1
2
3
2 2
3 3
node 1
node 2
node 3
C C C S S S E E E
Link Layer 5-26
suppose N nodes with many frames to send each transmits in slot with probability p
prob that given node has success in a slot = p(1-p)N-1
prob that any node has a success = Np(1-p)N-1
max efficiency find p that maximizes Np(1-p)N-1
for many nodes take limit of Np(1-p)N-1 as N goes to infinity gives
max efficiency = 1e = 37
efficiency long-run fraction of successful slots (many nodes all with many frames to send)
at best channel used for useful transmissions 37 of time
Slotted ALOHA efficiency
Link Layer 5-27
Pure (unslotted) ALOHA
unslotted Aloha simpler no synchronization
when frame first arrives
transmit immediately
collision probability increases
frame sent at t0 collides with other frames sent in [t0-1t0+1]
Link Layer 5-28
Pure ALOHA efficiency
P(success by given node) = P(node transmits)
P(no other node transmits in [t0-1t0]
P(no other node transmits in [t0-1t0]
= p (1-p)N-1 (1-p)N-1
= p (1-p)2(N-1)
hellip choosing optimum p and then letting n
= 1(2e) = 18
even worse than slotted Aloha
Link Layer 5-29
CSMA (carrier sense multiple access)
CSMA listen before transmit if channel sensed idle transmit entire frame
if channel sensed busy defer transmission
human analogy donrsquot interrupt others
Link Layer 5-30
CSMA collisions
collisions can still occur propagation delay means two nodes may not hear each otherrsquos transmission
collision entire packet transmission time wasted distance amp propagation
delay play role in in determining collision probability
spatial layout of nodes
Link Layer 5-31
CSMACD (collision detection)
CSMACD carrier sensing deferral as in CSMA collisions detected within short time
colliding transmissions aborted reducing channel wastage
collision detection easy in wired LANs measure signal strengths compare
transmitted received signals
difficult in wireless LANs received signal strength overwhelmed by local transmission strength
human analogy the polite conversationalist
Link Layer 5-32
CSMACD (collision detection)
spatial layout of nodes
Link Layer 5-33
Ethernet CSMACD algorithm
1 NIC receives datagram from network layer creates frame
2 If NIC senses channel idle starts frame transmission If NIC senses channel busy waits until channel idle then transmits
3 If NIC transmits entire frame without detecting another transmission NIC is done with frame
4 If NIC detects another transmission while transmitting aborts and sends jam signal
5 After aborting NIC enters binary (exponential) backoff
after mth collision NIC chooses K at random from 012 hellip 2m-1 NIC waits K512 bit times returns to Step 2
longer backoff interval with more collisions
Link Layer 5-34
CSMACD efficiency
Tprop = max prop delay between 2 nodes in LAN
ttrans = time to transmit max-size frame
efficiency goes to 1
as tprop goes to 0
as ttrans goes to infinity
better performance than ALOHA and simple cheap decentralized
transpropttefficiency
51
1
Link Layer 5-35
ldquoTaking turnsrdquo MAC protocols
channel partitioning MAC protocols share channel efficiently and fairly at high load
inefficient at low load delay in channel access 1N bandwidth allocated even if only 1 active node
random access MAC protocols efficient at low load single node can fully utilize
channel
high load collision overhead
ldquotaking turnsrdquo protocols look for best of both worlds
Link Layer 5-36
polling master node ldquoinvitesrdquo
slave nodes to transmit in turn
typically used with ldquodumbrdquo slave devices
concerns
polling overhead
latency
single point of failure (master)
master
slaves
poll
data
data
ldquoTaking turnsrdquo MAC protocols
Link Layer 5-37
token passing control token passed
from one node to next sequentially
token message
concerns
token overhead
latency
single point of failure (token)
T
data
(nothing
to send)
T
ldquoTaking turnsrdquo MAC protocols
cable headend
CMTS
ISP
cable modem termination system
multiple 40Mbps downstream (broadcast) channels
single CMTS transmits into channels
multiple 30 Mbps upstream channels
multiple access all users contend for certain upstream channel time slots (others assigned)
Cable access network
cable modem
splitter
hellip
hellip
Internet framesTV channels control transmitted downstream at different frequencies
upstream Internet frames TV control transmitted upstream at different frequencies in time slots
Link Layer 5-39
DOCSIS data over cable service interface spec FDM over upstream downstream frequency channels
TDM upstream some slots assigned some have contention
downstream MAP frame assigns upstream slots
request for upstream slots (and data) transmitted random access (binary backoff) in selected slots
MAP frame for
Interval [t1 t2]
Residences with cable modems
Downstream channel i
Upstream channel j
t1 t2
Assigned minislots containing cable modem
upstream data frames
Minislots containing
minislots request frames
cable headend
CMTS
Cable access network
Link Layer 5-40
Summary of MAC protocols
channel partitioning by time frequency or code Time Division Frequency Division
random access (dynamic)
ALOHA S-ALOHA CSMA CSMACD
carrier sensing easy in some technologies (wire) hard in others (wireless)
CSMACD used in Ethernet
CSMACA used in 80211
taking turns
polling from central site token passing
bluetooth FDDI token ring
Link Layer 5-41
Link layer LANs outline
51 introduction services
52 error detection correction
53 multiple access protocols
54 LANs addressing ARP
Ethernet
switches
VLANS
55 link virtualization MPLS
56 data center networking
57 a day in the life of a web request
Link Layer 5-42
MAC addresses and ARP
32-bit IP address network-layer address for interface
used for layer 3 (network layer) forwarding
MAC (or LAN or physical or Ethernet) address function used lsquolocallyrdquo to get frame from one interface to
another physically-connected interface (same network in IP-addressing sense)
48 bit MAC address (for most LANs) burned in NIC ROM also sometimes software settable
eg 1A-2F-BB-76-09-AD
hexadecimal (base 16) notation
(each ldquonumberrdquo represents 4 bits)
Link Layer 5-43
LAN addresses and ARP
each adapter on LAN has unique LAN address
adapter
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN
(wired or
wireless)
Link Layer 5-44
LAN addresses (more)
MAC address allocation administered by IEEE
manufacturer buys portion of MAC address space (to assure uniqueness)
analogy MAC address like Social Security Number
IP address like postal address
MAC flat address portability can move LAN card from one LAN to another
IP hierarchical address not portable address depends on IP subnet to which node is
attached
Link Layer 5-45
ARP address resolution protocol
ARP table each IP node (host router) on LAN has table
IPMAC address mappings for some LAN nodes
lt IP address MAC address TTLgt
TTL (Time To Live) time after which address mapping will be forgotten (typically 20 min)
Question how to determine
interfacersquos MAC address
knowing its IP address
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN
137196723
137196778
137196714
137196788
Link Layer 5-46
ARP protocol same LAN
A wants to send datagram to B Brsquos MAC address not in
Arsquos ARP table
A broadcasts ARP query packet containing Bs IP address dest MAC address = FF-FF-
FF-FF-FF-FF
all nodes on LAN receive ARP query
B receives ARP packet replies to A with its (Bs) MAC address frame sent to Arsquos MAC
address (unicast)
A caches (saves) IP-to-MAC address pair in its ARP table until information becomes old (times out) soft state information that
times out (goes away) unless refreshed
ARP is ldquoplug-and-playrdquo nodes create their ARP
tables without intervention from net administrator
Link Layer 5-47
walkthrough send datagram from A to B via R
focus on addressing ndash at IP (datagram) and MAC layer (frame)
assume A knows Brsquos IP address
assume A knows IP address of first hop router R (how)
assume A knows Rrsquos MAC address (how)
Addressing routing to another LAN
R
1A-23-F9-CD-06-9B 222222222220
111111111110 E6-E9-00-17-BB-4B CC-49-DE-D0-AB-7D
111111111112
111111111111
74-29-9C-E8-FF-55
A
222222222222
49-BD-D2-C7-56-2A
222222222221 88-B2-2F-54-1A-0F
B
R
1A-23-F9-CD-06-9B 222222222220
111111111110 E6-E9-00-17-BB-4B CC-49-DE-D0-AB-7D
111111111112
111111111111
74-29-9C-E8-FF-55
A
222222222222
49-BD-D2-C7-56-2A
222222222221 88-B2-2F-54-1A-0F
B
Link Layer 5-48
Addressing routing to another LAN
IP
Eth
Phy
IP src 111111111111
IP dest 222222222222
A creates IP datagram with IP source A destination B
A creates link-layer frame with Rs MAC address as dest frame contains A-to-B IP datagram
MAC src 74-29-9C-E8-FF-55
MAC dest E6-E9-00-17-BB-4B
R
1A-23-F9-CD-06-9B 222222222220
111111111110 E6-E9-00-17-BB-4B CC-49-DE-D0-AB-7D
111111111112
111111111111
74-29-9C-E8-FF-55
A
222222222222
49-BD-D2-C7-56-2A
222222222221 88-B2-2F-54-1A-0F
B
Link Layer 5-49
Addressing routing to another LAN
IP
Eth
Phy
frame sent from A to R
IP
Eth
Phy
frame received at R datagram removed passed up to IP
MAC src 74-29-9C-E8-FF-55
MAC dest E6-E9-00-17-BB-4B
IP src 111111111111
IP dest 222222222222
IP src 111111111111
IP dest 222222222222
R
1A-23-F9-CD-06-9B 222222222220
111111111110 E6-E9-00-17-BB-4B CC-49-DE-D0-AB-7D
111111111112
111111111111
74-29-9C-E8-FF-55
A
222222222222
49-BD-D2-C7-56-2A
222222222221 88-B2-2F-54-1A-0F
B
Link Layer 5-50
Addressing routing to another LAN
IP src 111111111111
IP dest 222222222222
R forwards datagram with IP source A destination B
R creates link-layer frame with Bs MAC address as dest frame contains A-to-B IP datagram
MAC src 1A-23-F9-CD-06-9B
MAC dest 49-BD-D2-C7-56-2A
IP
Eth
Phy
IP
Eth
Phy
R
1A-23-F9-CD-06-9B 222222222220
111111111110 E6-E9-00-17-BB-4B CC-49-DE-D0-AB-7D
111111111112
111111111111
74-29-9C-E8-FF-55
A
222222222222
49-BD-D2-C7-56-2A
222222222221 88-B2-2F-54-1A-0F
B
Link Layer 5-51
Addressing routing to another LAN R forwards datagram with IP source A destination B
R creates link-layer frame with Bs MAC address as dest frame contains A-to-B IP datagram
IP src 111111111111
IP dest 222222222222
MAC src 1A-23-F9-CD-06-9B
MAC dest 49-BD-D2-C7-56-2A
IP
Eth
Phy
IP
Eth
Phy
R
1A-23-F9-CD-06-9B 222222222220
111111111110 E6-E9-00-17-BB-4B CC-49-DE-D0-AB-7D
111111111112
111111111111
74-29-9C-E8-FF-55
A
222222222222
49-BD-D2-C7-56-2A
222222222221 88-B2-2F-54-1A-0F
B
Link Layer 5-52
Addressing routing to another LAN R forwards datagram with IP source A destination B
R creates link-layer frame with Bs MAC address as dest frame contains A-to-B IP datagram
IP src 111111111111
IP dest 222222222222
MAC src 1A-23-F9-CD-06-9B
MAC dest 49-BD-D2-C7-56-2A
IP
Eth
Phy
Link Layer 5-53
Link layer LANs outline
51 introduction services
52 error detection correction
53 multiple access protocols
54 LANs addressing ARP
Ethernet
switches
VLANS
55 link virtualization MPLS
56 data center networking
57 a day in the life of a web request
Link Layer 5-54
Ethernet
ldquodominantrdquo wired LAN technology
cheap $20 for NIC
first widely used LAN technology
simpler cheaper than token LANs and ATM
kept up with speed race 10 Mbps ndash 10 Gbps
Metcalfersquos Ethernet sketch
Link Layer 5-55
Ethernet physical topology
bus popular through mid 90s all nodes in same collision domain (can collide with each
other)
star prevails today active switch in center
each ldquospokerdquo runs a (separate) Ethernet protocol (nodes
do not collide with each other)
switch
bus coaxial cable star
Link Layer 5-56
Ethernet frame structure
sending adapter encapsulates IP datagram (or other network layer protocol packet) in Ethernet frame
preamble
7 bytes with pattern 10101010 followed by one byte with pattern 10101011
used to synchronize receiver sender clock rates
dest address
source address
data (payload) CRC preamble
type
Link Layer 5-57
Ethernet frame structure (more)
addresses 6 byte source destination MAC addresses if adapter receives frame with matching destination
address or with broadcast address (eg ARP packet) it passes data in frame to network layer protocol
otherwise adapter discards frame
type indicates higher layer protocol (mostly IP but others possible eg Novell IPX AppleTalk)
CRC cyclic redundancy check at receiver error detected frame is dropped
dest address
source address
data (payload) CRC preamble
type
Link Layer 5-58
Ethernet unreliable connectionless
connectionless no handshaking between sending and receiving NICs
unreliable receiving NIC doesnt send acks or nacks to sending NIC
data in dropped frames recovered only if initial sender uses higher layer rdt (eg TCP) otherwise dropped data lost
Ethernetrsquos MAC protocol unslotted CSMACD wth binary backoff
Link Layer 5-59
8023 Ethernet standards link amp physical layers
many different Ethernet standards
common MAC protocol and frame format
different speeds 2 Mbps 10 Mbps 100 Mbps 1Gbps 10G bps
different physical layer media fiber cable
application
transport
network
link
physical
MAC protocol
and frame format
100BASE-TX
100BASE-T4
100BASE-FX 100BASE-T2
100BASE-SX 100BASE-BX
fiber physical layer copper (twister
pair) physical layer
Link Layer 5-60
Link layer LANs outline
51 introduction services
52 error detection correction
53 multiple access protocols
54 LANs addressing ARP
Ethernet
switches
VLANS
55 link virtualization MPLS
56 data center networking
57 a day in the life of a web request
Link Layer 5-61
Ethernet switch link-layer device takes an active role
store forward Ethernet frames
examine incoming framersquos MAC address selectively forward frame to one-or-more outgoing links when frame is to be forwarded on segment uses CSMACD to access segment
transparent
hosts are unaware of presence of switches
plug-and-play self-learning
switches do not need to be configured
Link Layer 5-62
Switch multiple simultaneous transmissions
hosts have dedicated direct connection to switch
switches buffer packets
Ethernet protocol used on each incoming link but no collisions full duplex
each link is its own collision domain
switching A-to-Arsquo and B-to-Brsquo can transmit simultaneously without collisions switch with six interfaces
(123456)
A
Arsquo
B
Brsquo C
Crsquo
1 2
3 4 5
6
Link Layer 5-63
Switch forwarding table
Q how does switch know Arsquo reachable via interface 4 Brsquo reachable via interface 5
switch with six interfaces
(123456)
A
Arsquo
B
Brsquo C
Crsquo
1 2
3 4 5
6 A each switch has a switch table each entry
(MAC address of host interface to
reach host time stamp)
looks like a routing table
Q how are entries created maintained in switch table
something like a routing protocol
A
Arsquo
B
Brsquo C
Crsquo
1 2
3 4 5
6
Link Layer 5-64
Switch self-learning
switch learns which hosts can be reached through which interfaces
when frame received switch ldquolearnsrdquo location of sender incoming LAN segment
records senderlocation pair in switch table
A Arsquo
Source A
Dest Arsquo
MAC addr interface TTL
Switch table
(initially empty) A 1 60
Link Layer 5-65
Switch frame filteringforwarding
when frame received at switch 1 record incoming link MAC address of sending host
2 index switch table using MAC destination address
3 if entry found for destination then
if destination on segment from which frame arrived then drop frame
else forward frame on interface indicated by entry
else flood forward on all interfaces except arriving
interface
A
Arsquo
B
Brsquo C
Crsquo
1 2
3 4 5
6
Link Layer 5-66
Self-learning forwarding example
A Arsquo
Source A
Dest Arsquo
MAC addr interface TTL
switch table
(initially empty) A 1 60
A Arsquo A Arsquo A Arsquo A Arsquo A Arsquo
frame destination Arsquo locaton unknown flood
Arsquo A
destination A location
known
Arsquo 4 60
selectively send
on just one link
Link Layer 5-67
Interconnecting switches
switches can be connected together
Q sending from A to G - how does S1 know to forward frame destined to F via S4 and S3
A self learning (works exactly the same as in single-switch case)
A
B
S1
C D
E
F
S2
S4
S3
H
I
G
Link Layer 5-68
Self-learning multi-switch example
Suppose C sends frame to I I responds to C
Q show switch tables and packet forwarding in S1 S2 S3 S4
A
B
S1
C D
E
F
S2
S4
S3
H
I
G
Link Layer 5-69
Institutional network
to external
network
router
IP subnet
mail server
web server
Link Layer 5-70
Switches vs routers
both are store-and-forward
routers network-layer devices (examine network-layer headers)
switches link-layer devices (examine link-layer headers)
both have forwarding tables
routers compute tables using routing algorithms IP addresses
switches learn forwarding table using flooding learning MAC addresses
application
transport
network
link
physical
network
link
physical
link
physical
switch
datagram
application
transport
network
link
physical
frame
frame
frame
datagram
Link Layer 5-71
VLANs motivation
consider CS user moves office to
EE but wants connect to CS switch
single broadcast domain
all layer-2 broadcast traffic (ARP DHCP unknown location of destination MAC address) must cross entire LAN
securityprivacy efficiency issues
Computer
Science Electrical
Engineering
Computer
Engineering
Link Layer 5-72
VLANs port-based VLAN switch ports
grouped (by switch management software) so that single physical switch helliphellip
switch(es) supporting
VLAN capabilities can
be configured to
define multiple virtual
LANS over single
physical LAN
infrastructure
Virtual Local
Area Network 1
8
9
16 10 2
7
hellip
Electrical Engineering
(VLAN ports 1-8)
Computer Science
(VLAN ports 9-15)
15
hellip
Electrical Engineering
(VLAN ports 1-8)
hellip
1
8 2
7 9
16 10
15
hellip
Computer Science
(VLAN ports 9-16)
hellip operates as multiple virtual switches
Link Layer 5-73
Port-based VLAN
1
8
9
16 10 2
7
hellip
Electrical Engineering
(VLAN ports 1-8)
Computer Science
(VLAN ports 9-15)
15
hellip
traffic isolation frames tofrom ports 1-8 can only reach ports 1-8 can also define VLAN based on
MAC addresses of endpoints rather than switch port
dynamic membership ports can be dynamically assigned among VLANs
router
forwarding between VLANS done via routing (just as with separate switches) in practice vendors sell combined
switches plus routers
Link Layer 5-74
VLANS spanning multiple switches
trunk port carries frames between VLANS defined over multiple physical switches frames forwarded within VLAN between switches canrsquot be vanilla
8021 frames (must carry VLAN ID info)
8021q protocol addsremoved additional header fields for frames forwarded between trunk ports
1
8
9
10 2
7
hellip
Electrical Engineering
(VLAN ports 1-8)
Computer Science
(VLAN ports 9-15)
15
hellip
2
7 3
Ports 235 belong to EE VLAN
Ports 4678 belong to CS VLAN
5
4 6 8 16
1
Link Layer 5-75
type
2-byte Tag Protocol Identifier
(value 81-00)
Tag Control Information (12 bit VLAN ID field
3 bit priority field like IP TOS)
Recomputed CRC
8021Q VLAN frame format
8021 frame
8021Q frame
dest address
source address
data (payload) CRC preamble
dest address
source address
preamble data (payload) CRC
type
Link Layer 5-76
Link layer LANs outline
51 introduction services
52 error detection correction
53 multiple access protocols
54 LANs addressing ARP
Ethernet
switches
VLANS
55 link virtualization MPLS
56 data center networking
57 a day in the life of a web request
Link Layer 5-77
Multiprotocol label switching (MPLS)
initial goal high-speed IP forwarding using fixed length label (instead of IP address) fast lookup using fixed length identifier (rather than
shortest prefix matching)
borrowing ideas from Virtual Circuit (VC) approach
but IP datagram still keeps IP address
PPP or Ethernet
header IP header remainder of link-layer frame MPLS header
label Exp S TTL
20 3 1 5
Link Layer 5-78
MPLS capable routers
aka label-switched router
forward packets to outgoing interface based only on label value (donrsquot inspect IP address) MPLS forwarding table distinct from IP forwarding tables
flexibility MPLS forwarding decisions can differ from those of IP use destination and source addresses to route flows to
same destination differently (traffic engineering)
re-route flows quickly if link fails pre-computed backup paths (useful for VoIP)
Link Layer 5-79
R2
D
R3 R5
A
R6
MPLS versus IP paths
IP router IP routing path to destination determined
by destination address alone
R4
Link Layer 5-80
R2
D
R3 R4
R5
A
R6
MPLS versus IP paths
IP-only router
IP routing path to destination determined by destination address alone
MPLS and IP router
MPLS routing path to destination can be based on source and dest address fast reroute precompute backup routes in
case of link failure
entry router (R4) can use different MPLS
routes to A based eg on source address
Link Layer 5-81
MPLS signaling
modify OSPF IS-IS link-state flooding protocols to carry info used by MPLS routing eg link bandwidth amount of ldquoreservedrdquo link bandwidth
D
R4
R5
A
R6
entry MPLS router uses RSVP-TE signaling protocol to set up MPLS forwarding at downstream routers
modified link state flooding
RSVP-TE
Link Layer 5-82
R1 R2
D
R3 R4 R5
0
1
0 0
A
R6
in out out
label label dest interface
6 - A 0
in out out
label label dest interface
10 6 A 1
12 9 D 0
in out out
label label dest interface
10 A 0
12 D 0
1
in out out
label label dest interface
8 6 A 0
0
8 A 1
MPLS forwarding tables
Link Layer 5-83
Link layer LANs outline
51 introduction services
52 error detection correction
53 multiple access protocols
54 LANs addressing ARP
Ethernet
switches
VLANS
55 link virtualization MPLS
56 data center networking
57 a day in the life of a web request
Link Layer 5-84
Data center networks
10rsquos to 100rsquos of thousands of hosts often closely coupled in close proximity e-business (eg Amazon)
content-servers (eg YouTube Akamai Apple Microsoft)
search engines data mining (eg Google)
challenges
multiple applications each serving massive numbers of clients
managingbalancing load avoiding processing networking data bottlenecks
Inside a 40-ft Microsoft container
Chicago data center
Link Layer 5-85
Server racks
TOR switches
Tier-1 switches
Tier-2 switches
Load balancer
Load balancer
B
1 2 3 4 5 6 7 8
A C
Border router
Access router
Internet
Data center networks
load balancer application-layer routing receives external client requests
directs workload within data center
returns results to external client (hiding data
center internals from client)
Server racks
TOR switches
Tier-1 switches
Tier-2 switches
1 2 3 4 5 6 7 8
Data center networks
rich interconnection among switches racks
increased throughput between racks (multiple routing
paths possible)
increased reliability via redundancy
Link Layer 5-87
Link layer LANs outline
51 introduction services
52 error detection correction
53 multiple access protocols
54 LANs addressing ARP
Ethernet
switches
VLANS
55 link virtualization MPLS
56 data center networking
57 a day in the life of a web request
Link Layer 5-88
Synthesis a day in the life of a web request
journey down protocol stack complete application transport network link
putting-it-all-together synthesis goal identify review understand protocols (at all
layers) involved in seemingly simple scenario requesting www page
scenario student attaches laptop to campus network requestsreceives wwwgooglecom
Link Layer 5-89
A day in the life scenario
Comcast network
68800013
Googlersquos network
64233160019 64233169105
web server
DNS server
school network
68802024
web page
browser
router
(runs DHCP)
Link Layer 5-90
A day in the lifehellip connecting to the Internet
connecting laptop needs to get its own IP address addr of first-hop router addr of DNS server use DHCP
DHCP
UDP
IP
Eth
Phy
DHCP
DHCP
DHCP
DHCP
DHCP
DHCP
UDP
IP
Eth
Phy
DHCP
DHCP
DHCP
DHCP DHCP
DHCP request encapsulated in UDP encapsulated in IP encapsulated in 8023 Ethernet
Ethernet frame broadcast
(dest FFFFFFFFFFFF) on LAN received at router running DHCP server
Ethernet demuxed to IP demuxed UDP demuxed to DHCP
router
(runs DHCP)
Link Layer 5-91
DHCP server formulates DHCP ACK containing clientrsquos IP address IP address of first-hop router for client name amp IP address of DNS server
DHCP
UDP
IP
Eth
Phy
DHCP
DHCP
DHCP
DHCP
DHCP
UDP
IP
Eth
Phy
DHCP
DHCP
DHCP
DHCP
DHCP
encapsulation at DHCP server frame forwarded (switch learning) through LAN demultiplexing at client
Client now has IP address knows name amp addr of DNS
server IP address of its first-hop router
DHCP client receives DHCP ACK reply
A day in the lifehellip connecting to the Internet
router
(runs DHCP)
Link Layer 5-92
A day in the lifehellip ARP (before DNS before HTTP)
before sending HTTP request need IP address of wwwgooglecom DNS
DNS
UDP
IP
Eth
Phy
DNS
DNS
DNS
DNS query created encapsulated in UDP encapsulated in IP encapsulated in Eth To send frame to router need MAC address of router interface ARP
ARP query broadcast received by
router which replies with ARP reply giving MAC address of router interface
client now knows MAC address of first hop router so can now send frame containing DNS query
ARP query
Eth
Phy
ARP
ARP
ARP reply
router
(runs DHCP)
Link Layer 5-93
DNS
UDP
IP
Eth
Phy
DNS
DNS
DNS
DNS
DNS
IP datagram containing DNS query forwarded via LAN switch from client to 1st hop router
IP datagram forwarded from campus network into comcast network routed (tables created by RIP OSPF IS-IS andor BGP routing protocols) to DNS server
demuxrsquoed to DNS server
DNS server replies to client with IP address of wwwgooglecom
Comcast network
68800013
DNS server
DNS
UDP
IP
Eth
Phy
DNS
DNS
DNS
DNS
A day in the lifehellip using DNS
router
(runs DHCP)
Link Layer 5-94
A day in the lifehellipTCP connection carrying HTTP
HTTP
TCP
IP
Eth
Phy
HTTP
to send HTTP request client first opens TCP socket to web server
TCP SYN segment (step 1 in 3-way handshake) inter-domain routed to web server
TCP connection established 64233169105
web server
SYN
SYN
SYN
SYN
TCP
IP
Eth
Phy
SYN
SYN
SYN
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
web server responds with TCP SYNACK (step 2 in 3-way handshake)
router
(runs DHCP)
Link Layer 5-95
A day in the lifehellip HTTP requestreply
HTTP
TCP
IP
Eth
Phy
HTTP
HTTP request sent into TCP socket
IP datagram containing HTTP request routed to wwwgooglecom
IP datagram containing HTTP reply routed back to client
64233169105
web server
HTTP
TCP
IP
Eth
Phy web server responds with
HTTP reply (containing web page)
HTTP
HTTP
HTTP HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
web page finally () displayed
Link Layer 5-96
Chapter 5 Summary
principles behind data link layer services error detection correction
sharing a broadcast channel multiple access
link layer addressing
instantiation and implementation of various link layer technologies Ethernet
switched LANS VLANs
virtualized networks as a link layer MPLS
synthesis a day in the life of a web request
Link Layer 5-97
Chapter 5 letrsquos take a breath
journey down protocol stack complete (except PHY)
solid understanding of networking principles practice
hellip could stop here hellip but lots of interesting topics wireless
multimedia
security
network management
Link Layer 5-3
Link layer LANs outline
51 introduction services
52 error detection correction
53 multiple access protocols
54 LANs addressing ARP
Ethernet
switches
VLANS
55 link virtualization MPLS
56 data center networking
57 a day in the life of a web request
Link Layer 5-4
Link layer introduction
terminology hosts and routers nodes
communication channels that connect adjacent nodes along communication path links
wired links
wireless links
LANs
layer-2 packet frame encapsulates datagram
data-link layer has responsibility of transferring datagram from one node to physically adjacent node over a link
global ISP
Link Layer 5-5
Link layer context
datagram transferred by different link protocols over different links
eg Ethernet on first link frame relay on intermediate links 80211 on last link
each link protocol provides different services
eg may or may not provide rdt over link
transportation analogy trip from Princeton to Lausanne
limo Princeton to JFK
plane JFK to Geneva
train Geneva to Lausanne
tourist = datagram
transport segment = communication link
transportation mode = link layer protocol
travel agent = routing algorithm
Link Layer 5-6
Link layer services
framing link access encapsulate datagram into frame adding header trailer channel access if shared medium ldquoMACrdquo addresses used in frame headers to identify
source dest
bull different from IP address
reliable delivery between adjacent nodes we learned how to do this already (chapter 3) seldom used on low bit-error link (fiber some twisted
pair) wireless links high error rates
bull Q why both link-level and end-end reliability
Link Layer 5-7
flow control pacing between adjacent sending and receiving nodes
error detection errors caused by signal attenuation noise
receiver detects presence of errors
bull signals sender for retransmission or drops frame
error correction receiver identifies and corrects bit error(s) without resorting to
retransmission
half-duplex and full-duplex with half duplex nodes at both ends of link can transmit but not
at same time
Link layer services (more)
Link Layer 5-8
Where is the link layer implemented
in each and every host
link layer implemented in ldquoadaptorrdquo (aka network interface card NIC) or on a chip
Ethernet card 80211 card Ethernet chipset
implements link physical layer
attaches into hostrsquos system buses
combination of hardware software firmware
controller
physical
transmission
cpu memory
host
bus
(eg PCI)
network adapter
card
application
transport
network
link
link
physical
Link Layer 5-9
Adaptors communicating
sending side
encapsulates datagram in frame
adds error checking bits rdt flow control etc
receiving side
looks for errors rdt flow control etc
extracts datagram passes to upper layer at receiving side
controller controller
sending host receiving host
datagram datagram
datagram
frame
Link Layer 5-10
Link layer LANs outline
51 introduction services
52 error detection correction
53 multiple access protocols
54 LANs addressing ARP
Ethernet
switches
VLANS
55 link virtualization MPLS
56 data center networking
57 a day in the life of a web request
Link Layer 5-11
Error detection
EDC= Error Detection and Correction bits (redundancy)
D = Data protected by error checking may include header fields
bull Error detection not 100 reliable
bull protocol may miss some errors but rarely
bull larger EDC field yields better detection and correction
otherwise
Link Layer 5-12
Parity checking
single bit parity detect single bit
errors
two-dimensional bit parity detect and correct single bit errors
0 0
Link Layer 5-13
Internet checksum (review)
sender treat segment contents
as sequence of 16-bit integers
checksum addition (1rsquos complement sum) of segment contents
sender puts checksum value into UDP checksum field
receiver compute checksum of
received segment check if computed
checksum equals checksum field value NO - error detected YES - no error detected
But maybe errors nonetheless
goal detect ldquoerrorsrdquo (eg flipped bits) in transmitted packet (note used at transport layer only)
Link Layer 5-14
Cyclic redundancy check
more powerful error-detection coding
view data bits D as a binary number
choose r+1 bit pattern (generator) G
goal choose r CRC bits R such that ltDRgt exactly divisible by G (modulo 2)
receiver knows G divides ltDRgt by G If non-zero remainder error detected
can detect all burst errors less than r+1 bits
widely used in practice (Ethernet 80211 WiFi ATM)
Link Layer 5-15
CRC example
want D2r XOR R = nG
equivalently D2r = nG XOR R
equivalently if we divide D2r by
G want remainder R to satisfy
R = remainder[ ] D2r
G
1001 101110000 1001
1
101
01000
000 1010 1001
010 000
100 000 1000 0000 1000
D G
R
r = 3
Link Layer 5-16
Link layer LANs outline
51 introduction services
52 error detection correction
53 multiple access protocols
54 LANs addressing ARP
Ethernet
switches
VLANS
55 link virtualization MPLS
56 data center networking
57 a day in the life of a web request
Link Layer 5-17
Multiple access links protocols
two types of ldquolinksrdquo
point-to-point PPP for dial-up access
point-to-point link between Ethernet switch host
broadcast (shared wire or medium) old-fashioned Ethernet
upstream HFC
80211 wireless LAN
shared wire (eg cabled Ethernet)
shared RF (eg 80211 WiFi)
shared RF (satellite)
humans at a cocktail party
(shared air acoustical)
Link Layer 5-18
Multiple access protocols
single shared broadcast channel
two or more simultaneous transmissions by nodes interference
collision if node receives two or more signals at the same time
multiple access protocol distributed algorithm that determines how nodes share
channel ie determine when node can transmit
communication about channel sharing must use channel itself no out-of-band channel for coordination
Link Layer 5-19
An ideal multiple access protocol
given broadcast channel of rate R bps
desiderata 1 when one node wants to transmit it can send at rate R
2 when M nodes want to transmit each can send at average rate RM
3 fully decentralized
bull no special node to coordinate transmissions
bull no synchronization of clocks slots
4 simple
Link Layer 5-20
MAC protocols taxonomy
three broad classes
channel partitioning divide channel into smaller ldquopiecesrdquo (time slots frequency code)
allocate piece to node for exclusive use
random access channel not divided allow collisions
ldquorecoverrdquo from collisions
ldquotaking turnsrdquo nodes take turns but nodes with more to send can take longer
turns
Link Layer 5-21
Channel partitioning MAC protocols TDMA
TDMA time division multiple access access to channel in rounds each station gets fixed length slot (length = pkt
trans time) in each round unused slots go idle example 6-station LAN 134 have pkt slots
256 idle
1 3 4 1 3 4
6-slot
frame
6-slot
frame
Link Layer 5-22
FDMA frequency division multiple access channel spectrum divided into frequency bands
each station assigned fixed frequency band
unused transmission time in frequency bands go idle
example 6-station LAN 134 have pkt frequency bands 256 idle
frequency b
ands
FDM cable
Channel partitioning MAC protocols FDMA
Link Layer 5-23
Random access protocols
when node has packet to send transmit at full channel data rate R no a priori coordination among nodes
two or more transmitting nodes ldquocollisionrdquo random access MAC protocol specifies
how to detect collisions how to recover from collisions (eg via delayed
retransmissions)
examples of random access MAC protocols slotted ALOHA ALOHA CSMA CSMACD CSMACA
Link Layer 5-24
Slotted ALOHA
assumptions all frames same size
time divided into equal size slots (time to transmit 1 frame)
nodes start to transmit only slot beginning
nodes are synchronized
if 2 or more nodes transmit in slot all nodes detect collision
operation
when node obtains fresh frame transmits in next slot
if no collision node can send new frame in next slot
if collision node retransmits frame in each subsequent slot with prob p until success
Link Layer 5-25
Pros single active node can
continuously transmit at full rate of channel
highly decentralized only slots in nodes need to be in sync
simple
Cons
collisions wasting slots
idle slots
nodes may be able to detect collision in less than time to transmit packet
clock synchronization
Slotted ALOHA
1 1 1 1
2
3
2 2
3 3
node 1
node 2
node 3
C C C S S S E E E
Link Layer 5-26
suppose N nodes with many frames to send each transmits in slot with probability p
prob that given node has success in a slot = p(1-p)N-1
prob that any node has a success = Np(1-p)N-1
max efficiency find p that maximizes Np(1-p)N-1
for many nodes take limit of Np(1-p)N-1 as N goes to infinity gives
max efficiency = 1e = 37
efficiency long-run fraction of successful slots (many nodes all with many frames to send)
at best channel used for useful transmissions 37 of time
Slotted ALOHA efficiency
Link Layer 5-27
Pure (unslotted) ALOHA
unslotted Aloha simpler no synchronization
when frame first arrives
transmit immediately
collision probability increases
frame sent at t0 collides with other frames sent in [t0-1t0+1]
Link Layer 5-28
Pure ALOHA efficiency
P(success by given node) = P(node transmits)
P(no other node transmits in [t0-1t0]
P(no other node transmits in [t0-1t0]
= p (1-p)N-1 (1-p)N-1
= p (1-p)2(N-1)
hellip choosing optimum p and then letting n
= 1(2e) = 18
even worse than slotted Aloha
Link Layer 5-29
CSMA (carrier sense multiple access)
CSMA listen before transmit if channel sensed idle transmit entire frame
if channel sensed busy defer transmission
human analogy donrsquot interrupt others
Link Layer 5-30
CSMA collisions
collisions can still occur propagation delay means two nodes may not hear each otherrsquos transmission
collision entire packet transmission time wasted distance amp propagation
delay play role in in determining collision probability
spatial layout of nodes
Link Layer 5-31
CSMACD (collision detection)
CSMACD carrier sensing deferral as in CSMA collisions detected within short time
colliding transmissions aborted reducing channel wastage
collision detection easy in wired LANs measure signal strengths compare
transmitted received signals
difficult in wireless LANs received signal strength overwhelmed by local transmission strength
human analogy the polite conversationalist
Link Layer 5-32
CSMACD (collision detection)
spatial layout of nodes
Link Layer 5-33
Ethernet CSMACD algorithm
1 NIC receives datagram from network layer creates frame
2 If NIC senses channel idle starts frame transmission If NIC senses channel busy waits until channel idle then transmits
3 If NIC transmits entire frame without detecting another transmission NIC is done with frame
4 If NIC detects another transmission while transmitting aborts and sends jam signal
5 After aborting NIC enters binary (exponential) backoff
after mth collision NIC chooses K at random from 012 hellip 2m-1 NIC waits K512 bit times returns to Step 2
longer backoff interval with more collisions
Link Layer 5-34
CSMACD efficiency
Tprop = max prop delay between 2 nodes in LAN
ttrans = time to transmit max-size frame
efficiency goes to 1
as tprop goes to 0
as ttrans goes to infinity
better performance than ALOHA and simple cheap decentralized
transpropttefficiency
51
1
Link Layer 5-35
ldquoTaking turnsrdquo MAC protocols
channel partitioning MAC protocols share channel efficiently and fairly at high load
inefficient at low load delay in channel access 1N bandwidth allocated even if only 1 active node
random access MAC protocols efficient at low load single node can fully utilize
channel
high load collision overhead
ldquotaking turnsrdquo protocols look for best of both worlds
Link Layer 5-36
polling master node ldquoinvitesrdquo
slave nodes to transmit in turn
typically used with ldquodumbrdquo slave devices
concerns
polling overhead
latency
single point of failure (master)
master
slaves
poll
data
data
ldquoTaking turnsrdquo MAC protocols
Link Layer 5-37
token passing control token passed
from one node to next sequentially
token message
concerns
token overhead
latency
single point of failure (token)
T
data
(nothing
to send)
T
ldquoTaking turnsrdquo MAC protocols
cable headend
CMTS
ISP
cable modem termination system
multiple 40Mbps downstream (broadcast) channels
single CMTS transmits into channels
multiple 30 Mbps upstream channels
multiple access all users contend for certain upstream channel time slots (others assigned)
Cable access network
cable modem
splitter
hellip
hellip
Internet framesTV channels control transmitted downstream at different frequencies
upstream Internet frames TV control transmitted upstream at different frequencies in time slots
Link Layer 5-39
DOCSIS data over cable service interface spec FDM over upstream downstream frequency channels
TDM upstream some slots assigned some have contention
downstream MAP frame assigns upstream slots
request for upstream slots (and data) transmitted random access (binary backoff) in selected slots
MAP frame for
Interval [t1 t2]
Residences with cable modems
Downstream channel i
Upstream channel j
t1 t2
Assigned minislots containing cable modem
upstream data frames
Minislots containing
minislots request frames
cable headend
CMTS
Cable access network
Link Layer 5-40
Summary of MAC protocols
channel partitioning by time frequency or code Time Division Frequency Division
random access (dynamic)
ALOHA S-ALOHA CSMA CSMACD
carrier sensing easy in some technologies (wire) hard in others (wireless)
CSMACD used in Ethernet
CSMACA used in 80211
taking turns
polling from central site token passing
bluetooth FDDI token ring
Link Layer 5-41
Link layer LANs outline
51 introduction services
52 error detection correction
53 multiple access protocols
54 LANs addressing ARP
Ethernet
switches
VLANS
55 link virtualization MPLS
56 data center networking
57 a day in the life of a web request
Link Layer 5-42
MAC addresses and ARP
32-bit IP address network-layer address for interface
used for layer 3 (network layer) forwarding
MAC (or LAN or physical or Ethernet) address function used lsquolocallyrdquo to get frame from one interface to
another physically-connected interface (same network in IP-addressing sense)
48 bit MAC address (for most LANs) burned in NIC ROM also sometimes software settable
eg 1A-2F-BB-76-09-AD
hexadecimal (base 16) notation
(each ldquonumberrdquo represents 4 bits)
Link Layer 5-43
LAN addresses and ARP
each adapter on LAN has unique LAN address
adapter
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN
(wired or
wireless)
Link Layer 5-44
LAN addresses (more)
MAC address allocation administered by IEEE
manufacturer buys portion of MAC address space (to assure uniqueness)
analogy MAC address like Social Security Number
IP address like postal address
MAC flat address portability can move LAN card from one LAN to another
IP hierarchical address not portable address depends on IP subnet to which node is
attached
Link Layer 5-45
ARP address resolution protocol
ARP table each IP node (host router) on LAN has table
IPMAC address mappings for some LAN nodes
lt IP address MAC address TTLgt
TTL (Time To Live) time after which address mapping will be forgotten (typically 20 min)
Question how to determine
interfacersquos MAC address
knowing its IP address
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN
137196723
137196778
137196714
137196788
Link Layer 5-46
ARP protocol same LAN
A wants to send datagram to B Brsquos MAC address not in
Arsquos ARP table
A broadcasts ARP query packet containing Bs IP address dest MAC address = FF-FF-
FF-FF-FF-FF
all nodes on LAN receive ARP query
B receives ARP packet replies to A with its (Bs) MAC address frame sent to Arsquos MAC
address (unicast)
A caches (saves) IP-to-MAC address pair in its ARP table until information becomes old (times out) soft state information that
times out (goes away) unless refreshed
ARP is ldquoplug-and-playrdquo nodes create their ARP
tables without intervention from net administrator
Link Layer 5-47
walkthrough send datagram from A to B via R
focus on addressing ndash at IP (datagram) and MAC layer (frame)
assume A knows Brsquos IP address
assume A knows IP address of first hop router R (how)
assume A knows Rrsquos MAC address (how)
Addressing routing to another LAN
R
1A-23-F9-CD-06-9B 222222222220
111111111110 E6-E9-00-17-BB-4B CC-49-DE-D0-AB-7D
111111111112
111111111111
74-29-9C-E8-FF-55
A
222222222222
49-BD-D2-C7-56-2A
222222222221 88-B2-2F-54-1A-0F
B
R
1A-23-F9-CD-06-9B 222222222220
111111111110 E6-E9-00-17-BB-4B CC-49-DE-D0-AB-7D
111111111112
111111111111
74-29-9C-E8-FF-55
A
222222222222
49-BD-D2-C7-56-2A
222222222221 88-B2-2F-54-1A-0F
B
Link Layer 5-48
Addressing routing to another LAN
IP
Eth
Phy
IP src 111111111111
IP dest 222222222222
A creates IP datagram with IP source A destination B
A creates link-layer frame with Rs MAC address as dest frame contains A-to-B IP datagram
MAC src 74-29-9C-E8-FF-55
MAC dest E6-E9-00-17-BB-4B
R
1A-23-F9-CD-06-9B 222222222220
111111111110 E6-E9-00-17-BB-4B CC-49-DE-D0-AB-7D
111111111112
111111111111
74-29-9C-E8-FF-55
A
222222222222
49-BD-D2-C7-56-2A
222222222221 88-B2-2F-54-1A-0F
B
Link Layer 5-49
Addressing routing to another LAN
IP
Eth
Phy
frame sent from A to R
IP
Eth
Phy
frame received at R datagram removed passed up to IP
MAC src 74-29-9C-E8-FF-55
MAC dest E6-E9-00-17-BB-4B
IP src 111111111111
IP dest 222222222222
IP src 111111111111
IP dest 222222222222
R
1A-23-F9-CD-06-9B 222222222220
111111111110 E6-E9-00-17-BB-4B CC-49-DE-D0-AB-7D
111111111112
111111111111
74-29-9C-E8-FF-55
A
222222222222
49-BD-D2-C7-56-2A
222222222221 88-B2-2F-54-1A-0F
B
Link Layer 5-50
Addressing routing to another LAN
IP src 111111111111
IP dest 222222222222
R forwards datagram with IP source A destination B
R creates link-layer frame with Bs MAC address as dest frame contains A-to-B IP datagram
MAC src 1A-23-F9-CD-06-9B
MAC dest 49-BD-D2-C7-56-2A
IP
Eth
Phy
IP
Eth
Phy
R
1A-23-F9-CD-06-9B 222222222220
111111111110 E6-E9-00-17-BB-4B CC-49-DE-D0-AB-7D
111111111112
111111111111
74-29-9C-E8-FF-55
A
222222222222
49-BD-D2-C7-56-2A
222222222221 88-B2-2F-54-1A-0F
B
Link Layer 5-51
Addressing routing to another LAN R forwards datagram with IP source A destination B
R creates link-layer frame with Bs MAC address as dest frame contains A-to-B IP datagram
IP src 111111111111
IP dest 222222222222
MAC src 1A-23-F9-CD-06-9B
MAC dest 49-BD-D2-C7-56-2A
IP
Eth
Phy
IP
Eth
Phy
R
1A-23-F9-CD-06-9B 222222222220
111111111110 E6-E9-00-17-BB-4B CC-49-DE-D0-AB-7D
111111111112
111111111111
74-29-9C-E8-FF-55
A
222222222222
49-BD-D2-C7-56-2A
222222222221 88-B2-2F-54-1A-0F
B
Link Layer 5-52
Addressing routing to another LAN R forwards datagram with IP source A destination B
R creates link-layer frame with Bs MAC address as dest frame contains A-to-B IP datagram
IP src 111111111111
IP dest 222222222222
MAC src 1A-23-F9-CD-06-9B
MAC dest 49-BD-D2-C7-56-2A
IP
Eth
Phy
Link Layer 5-53
Link layer LANs outline
51 introduction services
52 error detection correction
53 multiple access protocols
54 LANs addressing ARP
Ethernet
switches
VLANS
55 link virtualization MPLS
56 data center networking
57 a day in the life of a web request
Link Layer 5-54
Ethernet
ldquodominantrdquo wired LAN technology
cheap $20 for NIC
first widely used LAN technology
simpler cheaper than token LANs and ATM
kept up with speed race 10 Mbps ndash 10 Gbps
Metcalfersquos Ethernet sketch
Link Layer 5-55
Ethernet physical topology
bus popular through mid 90s all nodes in same collision domain (can collide with each
other)
star prevails today active switch in center
each ldquospokerdquo runs a (separate) Ethernet protocol (nodes
do not collide with each other)
switch
bus coaxial cable star
Link Layer 5-56
Ethernet frame structure
sending adapter encapsulates IP datagram (or other network layer protocol packet) in Ethernet frame
preamble
7 bytes with pattern 10101010 followed by one byte with pattern 10101011
used to synchronize receiver sender clock rates
dest address
source address
data (payload) CRC preamble
type
Link Layer 5-57
Ethernet frame structure (more)
addresses 6 byte source destination MAC addresses if adapter receives frame with matching destination
address or with broadcast address (eg ARP packet) it passes data in frame to network layer protocol
otherwise adapter discards frame
type indicates higher layer protocol (mostly IP but others possible eg Novell IPX AppleTalk)
CRC cyclic redundancy check at receiver error detected frame is dropped
dest address
source address
data (payload) CRC preamble
type
Link Layer 5-58
Ethernet unreliable connectionless
connectionless no handshaking between sending and receiving NICs
unreliable receiving NIC doesnt send acks or nacks to sending NIC
data in dropped frames recovered only if initial sender uses higher layer rdt (eg TCP) otherwise dropped data lost
Ethernetrsquos MAC protocol unslotted CSMACD wth binary backoff
Link Layer 5-59
8023 Ethernet standards link amp physical layers
many different Ethernet standards
common MAC protocol and frame format
different speeds 2 Mbps 10 Mbps 100 Mbps 1Gbps 10G bps
different physical layer media fiber cable
application
transport
network
link
physical
MAC protocol
and frame format
100BASE-TX
100BASE-T4
100BASE-FX 100BASE-T2
100BASE-SX 100BASE-BX
fiber physical layer copper (twister
pair) physical layer
Link Layer 5-60
Link layer LANs outline
51 introduction services
52 error detection correction
53 multiple access protocols
54 LANs addressing ARP
Ethernet
switches
VLANS
55 link virtualization MPLS
56 data center networking
57 a day in the life of a web request
Link Layer 5-61
Ethernet switch link-layer device takes an active role
store forward Ethernet frames
examine incoming framersquos MAC address selectively forward frame to one-or-more outgoing links when frame is to be forwarded on segment uses CSMACD to access segment
transparent
hosts are unaware of presence of switches
plug-and-play self-learning
switches do not need to be configured
Link Layer 5-62
Switch multiple simultaneous transmissions
hosts have dedicated direct connection to switch
switches buffer packets
Ethernet protocol used on each incoming link but no collisions full duplex
each link is its own collision domain
switching A-to-Arsquo and B-to-Brsquo can transmit simultaneously without collisions switch with six interfaces
(123456)
A
Arsquo
B
Brsquo C
Crsquo
1 2
3 4 5
6
Link Layer 5-63
Switch forwarding table
Q how does switch know Arsquo reachable via interface 4 Brsquo reachable via interface 5
switch with six interfaces
(123456)
A
Arsquo
B
Brsquo C
Crsquo
1 2
3 4 5
6 A each switch has a switch table each entry
(MAC address of host interface to
reach host time stamp)
looks like a routing table
Q how are entries created maintained in switch table
something like a routing protocol
A
Arsquo
B
Brsquo C
Crsquo
1 2
3 4 5
6
Link Layer 5-64
Switch self-learning
switch learns which hosts can be reached through which interfaces
when frame received switch ldquolearnsrdquo location of sender incoming LAN segment
records senderlocation pair in switch table
A Arsquo
Source A
Dest Arsquo
MAC addr interface TTL
Switch table
(initially empty) A 1 60
Link Layer 5-65
Switch frame filteringforwarding
when frame received at switch 1 record incoming link MAC address of sending host
2 index switch table using MAC destination address
3 if entry found for destination then
if destination on segment from which frame arrived then drop frame
else forward frame on interface indicated by entry
else flood forward on all interfaces except arriving
interface
A
Arsquo
B
Brsquo C
Crsquo
1 2
3 4 5
6
Link Layer 5-66
Self-learning forwarding example
A Arsquo
Source A
Dest Arsquo
MAC addr interface TTL
switch table
(initially empty) A 1 60
A Arsquo A Arsquo A Arsquo A Arsquo A Arsquo
frame destination Arsquo locaton unknown flood
Arsquo A
destination A location
known
Arsquo 4 60
selectively send
on just one link
Link Layer 5-67
Interconnecting switches
switches can be connected together
Q sending from A to G - how does S1 know to forward frame destined to F via S4 and S3
A self learning (works exactly the same as in single-switch case)
A
B
S1
C D
E
F
S2
S4
S3
H
I
G
Link Layer 5-68
Self-learning multi-switch example
Suppose C sends frame to I I responds to C
Q show switch tables and packet forwarding in S1 S2 S3 S4
A
B
S1
C D
E
F
S2
S4
S3
H
I
G
Link Layer 5-69
Institutional network
to external
network
router
IP subnet
mail server
web server
Link Layer 5-70
Switches vs routers
both are store-and-forward
routers network-layer devices (examine network-layer headers)
switches link-layer devices (examine link-layer headers)
both have forwarding tables
routers compute tables using routing algorithms IP addresses
switches learn forwarding table using flooding learning MAC addresses
application
transport
network
link
physical
network
link
physical
link
physical
switch
datagram
application
transport
network
link
physical
frame
frame
frame
datagram
Link Layer 5-71
VLANs motivation
consider CS user moves office to
EE but wants connect to CS switch
single broadcast domain
all layer-2 broadcast traffic (ARP DHCP unknown location of destination MAC address) must cross entire LAN
securityprivacy efficiency issues
Computer
Science Electrical
Engineering
Computer
Engineering
Link Layer 5-72
VLANs port-based VLAN switch ports
grouped (by switch management software) so that single physical switch helliphellip
switch(es) supporting
VLAN capabilities can
be configured to
define multiple virtual
LANS over single
physical LAN
infrastructure
Virtual Local
Area Network 1
8
9
16 10 2
7
hellip
Electrical Engineering
(VLAN ports 1-8)
Computer Science
(VLAN ports 9-15)
15
hellip
Electrical Engineering
(VLAN ports 1-8)
hellip
1
8 2
7 9
16 10
15
hellip
Computer Science
(VLAN ports 9-16)
hellip operates as multiple virtual switches
Link Layer 5-73
Port-based VLAN
1
8
9
16 10 2
7
hellip
Electrical Engineering
(VLAN ports 1-8)
Computer Science
(VLAN ports 9-15)
15
hellip
traffic isolation frames tofrom ports 1-8 can only reach ports 1-8 can also define VLAN based on
MAC addresses of endpoints rather than switch port
dynamic membership ports can be dynamically assigned among VLANs
router
forwarding between VLANS done via routing (just as with separate switches) in practice vendors sell combined
switches plus routers
Link Layer 5-74
VLANS spanning multiple switches
trunk port carries frames between VLANS defined over multiple physical switches frames forwarded within VLAN between switches canrsquot be vanilla
8021 frames (must carry VLAN ID info)
8021q protocol addsremoved additional header fields for frames forwarded between trunk ports
1
8
9
10 2
7
hellip
Electrical Engineering
(VLAN ports 1-8)
Computer Science
(VLAN ports 9-15)
15
hellip
2
7 3
Ports 235 belong to EE VLAN
Ports 4678 belong to CS VLAN
5
4 6 8 16
1
Link Layer 5-75
type
2-byte Tag Protocol Identifier
(value 81-00)
Tag Control Information (12 bit VLAN ID field
3 bit priority field like IP TOS)
Recomputed CRC
8021Q VLAN frame format
8021 frame
8021Q frame
dest address
source address
data (payload) CRC preamble
dest address
source address
preamble data (payload) CRC
type
Link Layer 5-76
Link layer LANs outline
51 introduction services
52 error detection correction
53 multiple access protocols
54 LANs addressing ARP
Ethernet
switches
VLANS
55 link virtualization MPLS
56 data center networking
57 a day in the life of a web request
Link Layer 5-77
Multiprotocol label switching (MPLS)
initial goal high-speed IP forwarding using fixed length label (instead of IP address) fast lookup using fixed length identifier (rather than
shortest prefix matching)
borrowing ideas from Virtual Circuit (VC) approach
but IP datagram still keeps IP address
PPP or Ethernet
header IP header remainder of link-layer frame MPLS header
label Exp S TTL
20 3 1 5
Link Layer 5-78
MPLS capable routers
aka label-switched router
forward packets to outgoing interface based only on label value (donrsquot inspect IP address) MPLS forwarding table distinct from IP forwarding tables
flexibility MPLS forwarding decisions can differ from those of IP use destination and source addresses to route flows to
same destination differently (traffic engineering)
re-route flows quickly if link fails pre-computed backup paths (useful for VoIP)
Link Layer 5-79
R2
D
R3 R5
A
R6
MPLS versus IP paths
IP router IP routing path to destination determined
by destination address alone
R4
Link Layer 5-80
R2
D
R3 R4
R5
A
R6
MPLS versus IP paths
IP-only router
IP routing path to destination determined by destination address alone
MPLS and IP router
MPLS routing path to destination can be based on source and dest address fast reroute precompute backup routes in
case of link failure
entry router (R4) can use different MPLS
routes to A based eg on source address
Link Layer 5-81
MPLS signaling
modify OSPF IS-IS link-state flooding protocols to carry info used by MPLS routing eg link bandwidth amount of ldquoreservedrdquo link bandwidth
D
R4
R5
A
R6
entry MPLS router uses RSVP-TE signaling protocol to set up MPLS forwarding at downstream routers
modified link state flooding
RSVP-TE
Link Layer 5-82
R1 R2
D
R3 R4 R5
0
1
0 0
A
R6
in out out
label label dest interface
6 - A 0
in out out
label label dest interface
10 6 A 1
12 9 D 0
in out out
label label dest interface
10 A 0
12 D 0
1
in out out
label label dest interface
8 6 A 0
0
8 A 1
MPLS forwarding tables
Link Layer 5-83
Link layer LANs outline
51 introduction services
52 error detection correction
53 multiple access protocols
54 LANs addressing ARP
Ethernet
switches
VLANS
55 link virtualization MPLS
56 data center networking
57 a day in the life of a web request
Link Layer 5-84
Data center networks
10rsquos to 100rsquos of thousands of hosts often closely coupled in close proximity e-business (eg Amazon)
content-servers (eg YouTube Akamai Apple Microsoft)
search engines data mining (eg Google)
challenges
multiple applications each serving massive numbers of clients
managingbalancing load avoiding processing networking data bottlenecks
Inside a 40-ft Microsoft container
Chicago data center
Link Layer 5-85
Server racks
TOR switches
Tier-1 switches
Tier-2 switches
Load balancer
Load balancer
B
1 2 3 4 5 6 7 8
A C
Border router
Access router
Internet
Data center networks
load balancer application-layer routing receives external client requests
directs workload within data center
returns results to external client (hiding data
center internals from client)
Server racks
TOR switches
Tier-1 switches
Tier-2 switches
1 2 3 4 5 6 7 8
Data center networks
rich interconnection among switches racks
increased throughput between racks (multiple routing
paths possible)
increased reliability via redundancy
Link Layer 5-87
Link layer LANs outline
51 introduction services
52 error detection correction
53 multiple access protocols
54 LANs addressing ARP
Ethernet
switches
VLANS
55 link virtualization MPLS
56 data center networking
57 a day in the life of a web request
Link Layer 5-88
Synthesis a day in the life of a web request
journey down protocol stack complete application transport network link
putting-it-all-together synthesis goal identify review understand protocols (at all
layers) involved in seemingly simple scenario requesting www page
scenario student attaches laptop to campus network requestsreceives wwwgooglecom
Link Layer 5-89
A day in the life scenario
Comcast network
68800013
Googlersquos network
64233160019 64233169105
web server
DNS server
school network
68802024
web page
browser
router
(runs DHCP)
Link Layer 5-90
A day in the lifehellip connecting to the Internet
connecting laptop needs to get its own IP address addr of first-hop router addr of DNS server use DHCP
DHCP
UDP
IP
Eth
Phy
DHCP
DHCP
DHCP
DHCP
DHCP
DHCP
UDP
IP
Eth
Phy
DHCP
DHCP
DHCP
DHCP DHCP
DHCP request encapsulated in UDP encapsulated in IP encapsulated in 8023 Ethernet
Ethernet frame broadcast
(dest FFFFFFFFFFFF) on LAN received at router running DHCP server
Ethernet demuxed to IP demuxed UDP demuxed to DHCP
router
(runs DHCP)
Link Layer 5-91
DHCP server formulates DHCP ACK containing clientrsquos IP address IP address of first-hop router for client name amp IP address of DNS server
DHCP
UDP
IP
Eth
Phy
DHCP
DHCP
DHCP
DHCP
DHCP
UDP
IP
Eth
Phy
DHCP
DHCP
DHCP
DHCP
DHCP
encapsulation at DHCP server frame forwarded (switch learning) through LAN demultiplexing at client
Client now has IP address knows name amp addr of DNS
server IP address of its first-hop router
DHCP client receives DHCP ACK reply
A day in the lifehellip connecting to the Internet
router
(runs DHCP)
Link Layer 5-92
A day in the lifehellip ARP (before DNS before HTTP)
before sending HTTP request need IP address of wwwgooglecom DNS
DNS
UDP
IP
Eth
Phy
DNS
DNS
DNS
DNS query created encapsulated in UDP encapsulated in IP encapsulated in Eth To send frame to router need MAC address of router interface ARP
ARP query broadcast received by
router which replies with ARP reply giving MAC address of router interface
client now knows MAC address of first hop router so can now send frame containing DNS query
ARP query
Eth
Phy
ARP
ARP
ARP reply
router
(runs DHCP)
Link Layer 5-93
DNS
UDP
IP
Eth
Phy
DNS
DNS
DNS
DNS
DNS
IP datagram containing DNS query forwarded via LAN switch from client to 1st hop router
IP datagram forwarded from campus network into comcast network routed (tables created by RIP OSPF IS-IS andor BGP routing protocols) to DNS server
demuxrsquoed to DNS server
DNS server replies to client with IP address of wwwgooglecom
Comcast network
68800013
DNS server
DNS
UDP
IP
Eth
Phy
DNS
DNS
DNS
DNS
A day in the lifehellip using DNS
router
(runs DHCP)
Link Layer 5-94
A day in the lifehellipTCP connection carrying HTTP
HTTP
TCP
IP
Eth
Phy
HTTP
to send HTTP request client first opens TCP socket to web server
TCP SYN segment (step 1 in 3-way handshake) inter-domain routed to web server
TCP connection established 64233169105
web server
SYN
SYN
SYN
SYN
TCP
IP
Eth
Phy
SYN
SYN
SYN
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
web server responds with TCP SYNACK (step 2 in 3-way handshake)
router
(runs DHCP)
Link Layer 5-95
A day in the lifehellip HTTP requestreply
HTTP
TCP
IP
Eth
Phy
HTTP
HTTP request sent into TCP socket
IP datagram containing HTTP request routed to wwwgooglecom
IP datagram containing HTTP reply routed back to client
64233169105
web server
HTTP
TCP
IP
Eth
Phy web server responds with
HTTP reply (containing web page)
HTTP
HTTP
HTTP HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
web page finally () displayed
Link Layer 5-96
Chapter 5 Summary
principles behind data link layer services error detection correction
sharing a broadcast channel multiple access
link layer addressing
instantiation and implementation of various link layer technologies Ethernet
switched LANS VLANs
virtualized networks as a link layer MPLS
synthesis a day in the life of a web request
Link Layer 5-97
Chapter 5 letrsquos take a breath
journey down protocol stack complete (except PHY)
solid understanding of networking principles practice
hellip could stop here hellip but lots of interesting topics wireless
multimedia
security
network management
Link Layer 5-4
Link layer introduction
terminology hosts and routers nodes
communication channels that connect adjacent nodes along communication path links
wired links
wireless links
LANs
layer-2 packet frame encapsulates datagram
data-link layer has responsibility of transferring datagram from one node to physically adjacent node over a link
global ISP
Link Layer 5-5
Link layer context
datagram transferred by different link protocols over different links
eg Ethernet on first link frame relay on intermediate links 80211 on last link
each link protocol provides different services
eg may or may not provide rdt over link
transportation analogy trip from Princeton to Lausanne
limo Princeton to JFK
plane JFK to Geneva
train Geneva to Lausanne
tourist = datagram
transport segment = communication link
transportation mode = link layer protocol
travel agent = routing algorithm
Link Layer 5-6
Link layer services
framing link access encapsulate datagram into frame adding header trailer channel access if shared medium ldquoMACrdquo addresses used in frame headers to identify
source dest
bull different from IP address
reliable delivery between adjacent nodes we learned how to do this already (chapter 3) seldom used on low bit-error link (fiber some twisted
pair) wireless links high error rates
bull Q why both link-level and end-end reliability
Link Layer 5-7
flow control pacing between adjacent sending and receiving nodes
error detection errors caused by signal attenuation noise
receiver detects presence of errors
bull signals sender for retransmission or drops frame
error correction receiver identifies and corrects bit error(s) without resorting to
retransmission
half-duplex and full-duplex with half duplex nodes at both ends of link can transmit but not
at same time
Link layer services (more)
Link Layer 5-8
Where is the link layer implemented
in each and every host
link layer implemented in ldquoadaptorrdquo (aka network interface card NIC) or on a chip
Ethernet card 80211 card Ethernet chipset
implements link physical layer
attaches into hostrsquos system buses
combination of hardware software firmware
controller
physical
transmission
cpu memory
host
bus
(eg PCI)
network adapter
card
application
transport
network
link
link
physical
Link Layer 5-9
Adaptors communicating
sending side
encapsulates datagram in frame
adds error checking bits rdt flow control etc
receiving side
looks for errors rdt flow control etc
extracts datagram passes to upper layer at receiving side
controller controller
sending host receiving host
datagram datagram
datagram
frame
Link Layer 5-10
Link layer LANs outline
51 introduction services
52 error detection correction
53 multiple access protocols
54 LANs addressing ARP
Ethernet
switches
VLANS
55 link virtualization MPLS
56 data center networking
57 a day in the life of a web request
Link Layer 5-11
Error detection
EDC= Error Detection and Correction bits (redundancy)
D = Data protected by error checking may include header fields
bull Error detection not 100 reliable
bull protocol may miss some errors but rarely
bull larger EDC field yields better detection and correction
otherwise
Link Layer 5-12
Parity checking
single bit parity detect single bit
errors
two-dimensional bit parity detect and correct single bit errors
0 0
Link Layer 5-13
Internet checksum (review)
sender treat segment contents
as sequence of 16-bit integers
checksum addition (1rsquos complement sum) of segment contents
sender puts checksum value into UDP checksum field
receiver compute checksum of
received segment check if computed
checksum equals checksum field value NO - error detected YES - no error detected
But maybe errors nonetheless
goal detect ldquoerrorsrdquo (eg flipped bits) in transmitted packet (note used at transport layer only)
Link Layer 5-14
Cyclic redundancy check
more powerful error-detection coding
view data bits D as a binary number
choose r+1 bit pattern (generator) G
goal choose r CRC bits R such that ltDRgt exactly divisible by G (modulo 2)
receiver knows G divides ltDRgt by G If non-zero remainder error detected
can detect all burst errors less than r+1 bits
widely used in practice (Ethernet 80211 WiFi ATM)
Link Layer 5-15
CRC example
want D2r XOR R = nG
equivalently D2r = nG XOR R
equivalently if we divide D2r by
G want remainder R to satisfy
R = remainder[ ] D2r
G
1001 101110000 1001
1
101
01000
000 1010 1001
010 000
100 000 1000 0000 1000
D G
R
r = 3
Link Layer 5-16
Link layer LANs outline
51 introduction services
52 error detection correction
53 multiple access protocols
54 LANs addressing ARP
Ethernet
switches
VLANS
55 link virtualization MPLS
56 data center networking
57 a day in the life of a web request
Link Layer 5-17
Multiple access links protocols
two types of ldquolinksrdquo
point-to-point PPP for dial-up access
point-to-point link between Ethernet switch host
broadcast (shared wire or medium) old-fashioned Ethernet
upstream HFC
80211 wireless LAN
shared wire (eg cabled Ethernet)
shared RF (eg 80211 WiFi)
shared RF (satellite)
humans at a cocktail party
(shared air acoustical)
Link Layer 5-18
Multiple access protocols
single shared broadcast channel
two or more simultaneous transmissions by nodes interference
collision if node receives two or more signals at the same time
multiple access protocol distributed algorithm that determines how nodes share
channel ie determine when node can transmit
communication about channel sharing must use channel itself no out-of-band channel for coordination
Link Layer 5-19
An ideal multiple access protocol
given broadcast channel of rate R bps
desiderata 1 when one node wants to transmit it can send at rate R
2 when M nodes want to transmit each can send at average rate RM
3 fully decentralized
bull no special node to coordinate transmissions
bull no synchronization of clocks slots
4 simple
Link Layer 5-20
MAC protocols taxonomy
three broad classes
channel partitioning divide channel into smaller ldquopiecesrdquo (time slots frequency code)
allocate piece to node for exclusive use
random access channel not divided allow collisions
ldquorecoverrdquo from collisions
ldquotaking turnsrdquo nodes take turns but nodes with more to send can take longer
turns
Link Layer 5-21
Channel partitioning MAC protocols TDMA
TDMA time division multiple access access to channel in rounds each station gets fixed length slot (length = pkt
trans time) in each round unused slots go idle example 6-station LAN 134 have pkt slots
256 idle
1 3 4 1 3 4
6-slot
frame
6-slot
frame
Link Layer 5-22
FDMA frequency division multiple access channel spectrum divided into frequency bands
each station assigned fixed frequency band
unused transmission time in frequency bands go idle
example 6-station LAN 134 have pkt frequency bands 256 idle
frequency b
ands
FDM cable
Channel partitioning MAC protocols FDMA
Link Layer 5-23
Random access protocols
when node has packet to send transmit at full channel data rate R no a priori coordination among nodes
two or more transmitting nodes ldquocollisionrdquo random access MAC protocol specifies
how to detect collisions how to recover from collisions (eg via delayed
retransmissions)
examples of random access MAC protocols slotted ALOHA ALOHA CSMA CSMACD CSMACA
Link Layer 5-24
Slotted ALOHA
assumptions all frames same size
time divided into equal size slots (time to transmit 1 frame)
nodes start to transmit only slot beginning
nodes are synchronized
if 2 or more nodes transmit in slot all nodes detect collision
operation
when node obtains fresh frame transmits in next slot
if no collision node can send new frame in next slot
if collision node retransmits frame in each subsequent slot with prob p until success
Link Layer 5-25
Pros single active node can
continuously transmit at full rate of channel
highly decentralized only slots in nodes need to be in sync
simple
Cons
collisions wasting slots
idle slots
nodes may be able to detect collision in less than time to transmit packet
clock synchronization
Slotted ALOHA
1 1 1 1
2
3
2 2
3 3
node 1
node 2
node 3
C C C S S S E E E
Link Layer 5-26
suppose N nodes with many frames to send each transmits in slot with probability p
prob that given node has success in a slot = p(1-p)N-1
prob that any node has a success = Np(1-p)N-1
max efficiency find p that maximizes Np(1-p)N-1
for many nodes take limit of Np(1-p)N-1 as N goes to infinity gives
max efficiency = 1e = 37
efficiency long-run fraction of successful slots (many nodes all with many frames to send)
at best channel used for useful transmissions 37 of time
Slotted ALOHA efficiency
Link Layer 5-27
Pure (unslotted) ALOHA
unslotted Aloha simpler no synchronization
when frame first arrives
transmit immediately
collision probability increases
frame sent at t0 collides with other frames sent in [t0-1t0+1]
Link Layer 5-28
Pure ALOHA efficiency
P(success by given node) = P(node transmits)
P(no other node transmits in [t0-1t0]
P(no other node transmits in [t0-1t0]
= p (1-p)N-1 (1-p)N-1
= p (1-p)2(N-1)
hellip choosing optimum p and then letting n
= 1(2e) = 18
even worse than slotted Aloha
Link Layer 5-29
CSMA (carrier sense multiple access)
CSMA listen before transmit if channel sensed idle transmit entire frame
if channel sensed busy defer transmission
human analogy donrsquot interrupt others
Link Layer 5-30
CSMA collisions
collisions can still occur propagation delay means two nodes may not hear each otherrsquos transmission
collision entire packet transmission time wasted distance amp propagation
delay play role in in determining collision probability
spatial layout of nodes
Link Layer 5-31
CSMACD (collision detection)
CSMACD carrier sensing deferral as in CSMA collisions detected within short time
colliding transmissions aborted reducing channel wastage
collision detection easy in wired LANs measure signal strengths compare
transmitted received signals
difficult in wireless LANs received signal strength overwhelmed by local transmission strength
human analogy the polite conversationalist
Link Layer 5-32
CSMACD (collision detection)
spatial layout of nodes
Link Layer 5-33
Ethernet CSMACD algorithm
1 NIC receives datagram from network layer creates frame
2 If NIC senses channel idle starts frame transmission If NIC senses channel busy waits until channel idle then transmits
3 If NIC transmits entire frame without detecting another transmission NIC is done with frame
4 If NIC detects another transmission while transmitting aborts and sends jam signal
5 After aborting NIC enters binary (exponential) backoff
after mth collision NIC chooses K at random from 012 hellip 2m-1 NIC waits K512 bit times returns to Step 2
longer backoff interval with more collisions
Link Layer 5-34
CSMACD efficiency
Tprop = max prop delay between 2 nodes in LAN
ttrans = time to transmit max-size frame
efficiency goes to 1
as tprop goes to 0
as ttrans goes to infinity
better performance than ALOHA and simple cheap decentralized
transpropttefficiency
51
1
Link Layer 5-35
ldquoTaking turnsrdquo MAC protocols
channel partitioning MAC protocols share channel efficiently and fairly at high load
inefficient at low load delay in channel access 1N bandwidth allocated even if only 1 active node
random access MAC protocols efficient at low load single node can fully utilize
channel
high load collision overhead
ldquotaking turnsrdquo protocols look for best of both worlds
Link Layer 5-36
polling master node ldquoinvitesrdquo
slave nodes to transmit in turn
typically used with ldquodumbrdquo slave devices
concerns
polling overhead
latency
single point of failure (master)
master
slaves
poll
data
data
ldquoTaking turnsrdquo MAC protocols
Link Layer 5-37
token passing control token passed
from one node to next sequentially
token message
concerns
token overhead
latency
single point of failure (token)
T
data
(nothing
to send)
T
ldquoTaking turnsrdquo MAC protocols
cable headend
CMTS
ISP
cable modem termination system
multiple 40Mbps downstream (broadcast) channels
single CMTS transmits into channels
multiple 30 Mbps upstream channels
multiple access all users contend for certain upstream channel time slots (others assigned)
Cable access network
cable modem
splitter
hellip
hellip
Internet framesTV channels control transmitted downstream at different frequencies
upstream Internet frames TV control transmitted upstream at different frequencies in time slots
Link Layer 5-39
DOCSIS data over cable service interface spec FDM over upstream downstream frequency channels
TDM upstream some slots assigned some have contention
downstream MAP frame assigns upstream slots
request for upstream slots (and data) transmitted random access (binary backoff) in selected slots
MAP frame for
Interval [t1 t2]
Residences with cable modems
Downstream channel i
Upstream channel j
t1 t2
Assigned minislots containing cable modem
upstream data frames
Minislots containing
minislots request frames
cable headend
CMTS
Cable access network
Link Layer 5-40
Summary of MAC protocols
channel partitioning by time frequency or code Time Division Frequency Division
random access (dynamic)
ALOHA S-ALOHA CSMA CSMACD
carrier sensing easy in some technologies (wire) hard in others (wireless)
CSMACD used in Ethernet
CSMACA used in 80211
taking turns
polling from central site token passing
bluetooth FDDI token ring
Link Layer 5-41
Link layer LANs outline
51 introduction services
52 error detection correction
53 multiple access protocols
54 LANs addressing ARP
Ethernet
switches
VLANS
55 link virtualization MPLS
56 data center networking
57 a day in the life of a web request
Link Layer 5-42
MAC addresses and ARP
32-bit IP address network-layer address for interface
used for layer 3 (network layer) forwarding
MAC (or LAN or physical or Ethernet) address function used lsquolocallyrdquo to get frame from one interface to
another physically-connected interface (same network in IP-addressing sense)
48 bit MAC address (for most LANs) burned in NIC ROM also sometimes software settable
eg 1A-2F-BB-76-09-AD
hexadecimal (base 16) notation
(each ldquonumberrdquo represents 4 bits)
Link Layer 5-43
LAN addresses and ARP
each adapter on LAN has unique LAN address
adapter
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN
(wired or
wireless)
Link Layer 5-44
LAN addresses (more)
MAC address allocation administered by IEEE
manufacturer buys portion of MAC address space (to assure uniqueness)
analogy MAC address like Social Security Number
IP address like postal address
MAC flat address portability can move LAN card from one LAN to another
IP hierarchical address not portable address depends on IP subnet to which node is
attached
Link Layer 5-45
ARP address resolution protocol
ARP table each IP node (host router) on LAN has table
IPMAC address mappings for some LAN nodes
lt IP address MAC address TTLgt
TTL (Time To Live) time after which address mapping will be forgotten (typically 20 min)
Question how to determine
interfacersquos MAC address
knowing its IP address
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN
137196723
137196778
137196714
137196788
Link Layer 5-46
ARP protocol same LAN
A wants to send datagram to B Brsquos MAC address not in
Arsquos ARP table
A broadcasts ARP query packet containing Bs IP address dest MAC address = FF-FF-
FF-FF-FF-FF
all nodes on LAN receive ARP query
B receives ARP packet replies to A with its (Bs) MAC address frame sent to Arsquos MAC
address (unicast)
A caches (saves) IP-to-MAC address pair in its ARP table until information becomes old (times out) soft state information that
times out (goes away) unless refreshed
ARP is ldquoplug-and-playrdquo nodes create their ARP
tables without intervention from net administrator
Link Layer 5-47
walkthrough send datagram from A to B via R
focus on addressing ndash at IP (datagram) and MAC layer (frame)
assume A knows Brsquos IP address
assume A knows IP address of first hop router R (how)
assume A knows Rrsquos MAC address (how)
Addressing routing to another LAN
R
1A-23-F9-CD-06-9B 222222222220
111111111110 E6-E9-00-17-BB-4B CC-49-DE-D0-AB-7D
111111111112
111111111111
74-29-9C-E8-FF-55
A
222222222222
49-BD-D2-C7-56-2A
222222222221 88-B2-2F-54-1A-0F
B
R
1A-23-F9-CD-06-9B 222222222220
111111111110 E6-E9-00-17-BB-4B CC-49-DE-D0-AB-7D
111111111112
111111111111
74-29-9C-E8-FF-55
A
222222222222
49-BD-D2-C7-56-2A
222222222221 88-B2-2F-54-1A-0F
B
Link Layer 5-48
Addressing routing to another LAN
IP
Eth
Phy
IP src 111111111111
IP dest 222222222222
A creates IP datagram with IP source A destination B
A creates link-layer frame with Rs MAC address as dest frame contains A-to-B IP datagram
MAC src 74-29-9C-E8-FF-55
MAC dest E6-E9-00-17-BB-4B
R
1A-23-F9-CD-06-9B 222222222220
111111111110 E6-E9-00-17-BB-4B CC-49-DE-D0-AB-7D
111111111112
111111111111
74-29-9C-E8-FF-55
A
222222222222
49-BD-D2-C7-56-2A
222222222221 88-B2-2F-54-1A-0F
B
Link Layer 5-49
Addressing routing to another LAN
IP
Eth
Phy
frame sent from A to R
IP
Eth
Phy
frame received at R datagram removed passed up to IP
MAC src 74-29-9C-E8-FF-55
MAC dest E6-E9-00-17-BB-4B
IP src 111111111111
IP dest 222222222222
IP src 111111111111
IP dest 222222222222
R
1A-23-F9-CD-06-9B 222222222220
111111111110 E6-E9-00-17-BB-4B CC-49-DE-D0-AB-7D
111111111112
111111111111
74-29-9C-E8-FF-55
A
222222222222
49-BD-D2-C7-56-2A
222222222221 88-B2-2F-54-1A-0F
B
Link Layer 5-50
Addressing routing to another LAN
IP src 111111111111
IP dest 222222222222
R forwards datagram with IP source A destination B
R creates link-layer frame with Bs MAC address as dest frame contains A-to-B IP datagram
MAC src 1A-23-F9-CD-06-9B
MAC dest 49-BD-D2-C7-56-2A
IP
Eth
Phy
IP
Eth
Phy
R
1A-23-F9-CD-06-9B 222222222220
111111111110 E6-E9-00-17-BB-4B CC-49-DE-D0-AB-7D
111111111112
111111111111
74-29-9C-E8-FF-55
A
222222222222
49-BD-D2-C7-56-2A
222222222221 88-B2-2F-54-1A-0F
B
Link Layer 5-51
Addressing routing to another LAN R forwards datagram with IP source A destination B
R creates link-layer frame with Bs MAC address as dest frame contains A-to-B IP datagram
IP src 111111111111
IP dest 222222222222
MAC src 1A-23-F9-CD-06-9B
MAC dest 49-BD-D2-C7-56-2A
IP
Eth
Phy
IP
Eth
Phy
R
1A-23-F9-CD-06-9B 222222222220
111111111110 E6-E9-00-17-BB-4B CC-49-DE-D0-AB-7D
111111111112
111111111111
74-29-9C-E8-FF-55
A
222222222222
49-BD-D2-C7-56-2A
222222222221 88-B2-2F-54-1A-0F
B
Link Layer 5-52
Addressing routing to another LAN R forwards datagram with IP source A destination B
R creates link-layer frame with Bs MAC address as dest frame contains A-to-B IP datagram
IP src 111111111111
IP dest 222222222222
MAC src 1A-23-F9-CD-06-9B
MAC dest 49-BD-D2-C7-56-2A
IP
Eth
Phy
Link Layer 5-53
Link layer LANs outline
51 introduction services
52 error detection correction
53 multiple access protocols
54 LANs addressing ARP
Ethernet
switches
VLANS
55 link virtualization MPLS
56 data center networking
57 a day in the life of a web request
Link Layer 5-54
Ethernet
ldquodominantrdquo wired LAN technology
cheap $20 for NIC
first widely used LAN technology
simpler cheaper than token LANs and ATM
kept up with speed race 10 Mbps ndash 10 Gbps
Metcalfersquos Ethernet sketch
Link Layer 5-55
Ethernet physical topology
bus popular through mid 90s all nodes in same collision domain (can collide with each
other)
star prevails today active switch in center
each ldquospokerdquo runs a (separate) Ethernet protocol (nodes
do not collide with each other)
switch
bus coaxial cable star
Link Layer 5-56
Ethernet frame structure
sending adapter encapsulates IP datagram (or other network layer protocol packet) in Ethernet frame
preamble
7 bytes with pattern 10101010 followed by one byte with pattern 10101011
used to synchronize receiver sender clock rates
dest address
source address
data (payload) CRC preamble
type
Link Layer 5-57
Ethernet frame structure (more)
addresses 6 byte source destination MAC addresses if adapter receives frame with matching destination
address or with broadcast address (eg ARP packet) it passes data in frame to network layer protocol
otherwise adapter discards frame
type indicates higher layer protocol (mostly IP but others possible eg Novell IPX AppleTalk)
CRC cyclic redundancy check at receiver error detected frame is dropped
dest address
source address
data (payload) CRC preamble
type
Link Layer 5-58
Ethernet unreliable connectionless
connectionless no handshaking between sending and receiving NICs
unreliable receiving NIC doesnt send acks or nacks to sending NIC
data in dropped frames recovered only if initial sender uses higher layer rdt (eg TCP) otherwise dropped data lost
Ethernetrsquos MAC protocol unslotted CSMACD wth binary backoff
Link Layer 5-59
8023 Ethernet standards link amp physical layers
many different Ethernet standards
common MAC protocol and frame format
different speeds 2 Mbps 10 Mbps 100 Mbps 1Gbps 10G bps
different physical layer media fiber cable
application
transport
network
link
physical
MAC protocol
and frame format
100BASE-TX
100BASE-T4
100BASE-FX 100BASE-T2
100BASE-SX 100BASE-BX
fiber physical layer copper (twister
pair) physical layer
Link Layer 5-60
Link layer LANs outline
51 introduction services
52 error detection correction
53 multiple access protocols
54 LANs addressing ARP
Ethernet
switches
VLANS
55 link virtualization MPLS
56 data center networking
57 a day in the life of a web request
Link Layer 5-61
Ethernet switch link-layer device takes an active role
store forward Ethernet frames
examine incoming framersquos MAC address selectively forward frame to one-or-more outgoing links when frame is to be forwarded on segment uses CSMACD to access segment
transparent
hosts are unaware of presence of switches
plug-and-play self-learning
switches do not need to be configured
Link Layer 5-62
Switch multiple simultaneous transmissions
hosts have dedicated direct connection to switch
switches buffer packets
Ethernet protocol used on each incoming link but no collisions full duplex
each link is its own collision domain
switching A-to-Arsquo and B-to-Brsquo can transmit simultaneously without collisions switch with six interfaces
(123456)
A
Arsquo
B
Brsquo C
Crsquo
1 2
3 4 5
6
Link Layer 5-63
Switch forwarding table
Q how does switch know Arsquo reachable via interface 4 Brsquo reachable via interface 5
switch with six interfaces
(123456)
A
Arsquo
B
Brsquo C
Crsquo
1 2
3 4 5
6 A each switch has a switch table each entry
(MAC address of host interface to
reach host time stamp)
looks like a routing table
Q how are entries created maintained in switch table
something like a routing protocol
A
Arsquo
B
Brsquo C
Crsquo
1 2
3 4 5
6
Link Layer 5-64
Switch self-learning
switch learns which hosts can be reached through which interfaces
when frame received switch ldquolearnsrdquo location of sender incoming LAN segment
records senderlocation pair in switch table
A Arsquo
Source A
Dest Arsquo
MAC addr interface TTL
Switch table
(initially empty) A 1 60
Link Layer 5-65
Switch frame filteringforwarding
when frame received at switch 1 record incoming link MAC address of sending host
2 index switch table using MAC destination address
3 if entry found for destination then
if destination on segment from which frame arrived then drop frame
else forward frame on interface indicated by entry
else flood forward on all interfaces except arriving
interface
A
Arsquo
B
Brsquo C
Crsquo
1 2
3 4 5
6
Link Layer 5-66
Self-learning forwarding example
A Arsquo
Source A
Dest Arsquo
MAC addr interface TTL
switch table
(initially empty) A 1 60
A Arsquo A Arsquo A Arsquo A Arsquo A Arsquo
frame destination Arsquo locaton unknown flood
Arsquo A
destination A location
known
Arsquo 4 60
selectively send
on just one link
Link Layer 5-67
Interconnecting switches
switches can be connected together
Q sending from A to G - how does S1 know to forward frame destined to F via S4 and S3
A self learning (works exactly the same as in single-switch case)
A
B
S1
C D
E
F
S2
S4
S3
H
I
G
Link Layer 5-68
Self-learning multi-switch example
Suppose C sends frame to I I responds to C
Q show switch tables and packet forwarding in S1 S2 S3 S4
A
B
S1
C D
E
F
S2
S4
S3
H
I
G
Link Layer 5-69
Institutional network
to external
network
router
IP subnet
mail server
web server
Link Layer 5-70
Switches vs routers
both are store-and-forward
routers network-layer devices (examine network-layer headers)
switches link-layer devices (examine link-layer headers)
both have forwarding tables
routers compute tables using routing algorithms IP addresses
switches learn forwarding table using flooding learning MAC addresses
application
transport
network
link
physical
network
link
physical
link
physical
switch
datagram
application
transport
network
link
physical
frame
frame
frame
datagram
Link Layer 5-71
VLANs motivation
consider CS user moves office to
EE but wants connect to CS switch
single broadcast domain
all layer-2 broadcast traffic (ARP DHCP unknown location of destination MAC address) must cross entire LAN
securityprivacy efficiency issues
Computer
Science Electrical
Engineering
Computer
Engineering
Link Layer 5-72
VLANs port-based VLAN switch ports
grouped (by switch management software) so that single physical switch helliphellip
switch(es) supporting
VLAN capabilities can
be configured to
define multiple virtual
LANS over single
physical LAN
infrastructure
Virtual Local
Area Network 1
8
9
16 10 2
7
hellip
Electrical Engineering
(VLAN ports 1-8)
Computer Science
(VLAN ports 9-15)
15
hellip
Electrical Engineering
(VLAN ports 1-8)
hellip
1
8 2
7 9
16 10
15
hellip
Computer Science
(VLAN ports 9-16)
hellip operates as multiple virtual switches
Link Layer 5-73
Port-based VLAN
1
8
9
16 10 2
7
hellip
Electrical Engineering
(VLAN ports 1-8)
Computer Science
(VLAN ports 9-15)
15
hellip
traffic isolation frames tofrom ports 1-8 can only reach ports 1-8 can also define VLAN based on
MAC addresses of endpoints rather than switch port
dynamic membership ports can be dynamically assigned among VLANs
router
forwarding between VLANS done via routing (just as with separate switches) in practice vendors sell combined
switches plus routers
Link Layer 5-74
VLANS spanning multiple switches
trunk port carries frames between VLANS defined over multiple physical switches frames forwarded within VLAN between switches canrsquot be vanilla
8021 frames (must carry VLAN ID info)
8021q protocol addsremoved additional header fields for frames forwarded between trunk ports
1
8
9
10 2
7
hellip
Electrical Engineering
(VLAN ports 1-8)
Computer Science
(VLAN ports 9-15)
15
hellip
2
7 3
Ports 235 belong to EE VLAN
Ports 4678 belong to CS VLAN
5
4 6 8 16
1
Link Layer 5-75
type
2-byte Tag Protocol Identifier
(value 81-00)
Tag Control Information (12 bit VLAN ID field
3 bit priority field like IP TOS)
Recomputed CRC
8021Q VLAN frame format
8021 frame
8021Q frame
dest address
source address
data (payload) CRC preamble
dest address
source address
preamble data (payload) CRC
type
Link Layer 5-76
Link layer LANs outline
51 introduction services
52 error detection correction
53 multiple access protocols
54 LANs addressing ARP
Ethernet
switches
VLANS
55 link virtualization MPLS
56 data center networking
57 a day in the life of a web request
Link Layer 5-77
Multiprotocol label switching (MPLS)
initial goal high-speed IP forwarding using fixed length label (instead of IP address) fast lookup using fixed length identifier (rather than
shortest prefix matching)
borrowing ideas from Virtual Circuit (VC) approach
but IP datagram still keeps IP address
PPP or Ethernet
header IP header remainder of link-layer frame MPLS header
label Exp S TTL
20 3 1 5
Link Layer 5-78
MPLS capable routers
aka label-switched router
forward packets to outgoing interface based only on label value (donrsquot inspect IP address) MPLS forwarding table distinct from IP forwarding tables
flexibility MPLS forwarding decisions can differ from those of IP use destination and source addresses to route flows to
same destination differently (traffic engineering)
re-route flows quickly if link fails pre-computed backup paths (useful for VoIP)
Link Layer 5-79
R2
D
R3 R5
A
R6
MPLS versus IP paths
IP router IP routing path to destination determined
by destination address alone
R4
Link Layer 5-80
R2
D
R3 R4
R5
A
R6
MPLS versus IP paths
IP-only router
IP routing path to destination determined by destination address alone
MPLS and IP router
MPLS routing path to destination can be based on source and dest address fast reroute precompute backup routes in
case of link failure
entry router (R4) can use different MPLS
routes to A based eg on source address
Link Layer 5-81
MPLS signaling
modify OSPF IS-IS link-state flooding protocols to carry info used by MPLS routing eg link bandwidth amount of ldquoreservedrdquo link bandwidth
D
R4
R5
A
R6
entry MPLS router uses RSVP-TE signaling protocol to set up MPLS forwarding at downstream routers
modified link state flooding
RSVP-TE
Link Layer 5-82
R1 R2
D
R3 R4 R5
0
1
0 0
A
R6
in out out
label label dest interface
6 - A 0
in out out
label label dest interface
10 6 A 1
12 9 D 0
in out out
label label dest interface
10 A 0
12 D 0
1
in out out
label label dest interface
8 6 A 0
0
8 A 1
MPLS forwarding tables
Link Layer 5-83
Link layer LANs outline
51 introduction services
52 error detection correction
53 multiple access protocols
54 LANs addressing ARP
Ethernet
switches
VLANS
55 link virtualization MPLS
56 data center networking
57 a day in the life of a web request
Link Layer 5-84
Data center networks
10rsquos to 100rsquos of thousands of hosts often closely coupled in close proximity e-business (eg Amazon)
content-servers (eg YouTube Akamai Apple Microsoft)
search engines data mining (eg Google)
challenges
multiple applications each serving massive numbers of clients
managingbalancing load avoiding processing networking data bottlenecks
Inside a 40-ft Microsoft container
Chicago data center
Link Layer 5-85
Server racks
TOR switches
Tier-1 switches
Tier-2 switches
Load balancer
Load balancer
B
1 2 3 4 5 6 7 8
A C
Border router
Access router
Internet
Data center networks
load balancer application-layer routing receives external client requests
directs workload within data center
returns results to external client (hiding data
center internals from client)
Server racks
TOR switches
Tier-1 switches
Tier-2 switches
1 2 3 4 5 6 7 8
Data center networks
rich interconnection among switches racks
increased throughput between racks (multiple routing
paths possible)
increased reliability via redundancy
Link Layer 5-87
Link layer LANs outline
51 introduction services
52 error detection correction
53 multiple access protocols
54 LANs addressing ARP
Ethernet
switches
VLANS
55 link virtualization MPLS
56 data center networking
57 a day in the life of a web request
Link Layer 5-88
Synthesis a day in the life of a web request
journey down protocol stack complete application transport network link
putting-it-all-together synthesis goal identify review understand protocols (at all
layers) involved in seemingly simple scenario requesting www page
scenario student attaches laptop to campus network requestsreceives wwwgooglecom
Link Layer 5-89
A day in the life scenario
Comcast network
68800013
Googlersquos network
64233160019 64233169105
web server
DNS server
school network
68802024
web page
browser
router
(runs DHCP)
Link Layer 5-90
A day in the lifehellip connecting to the Internet
connecting laptop needs to get its own IP address addr of first-hop router addr of DNS server use DHCP
DHCP
UDP
IP
Eth
Phy
DHCP
DHCP
DHCP
DHCP
DHCP
DHCP
UDP
IP
Eth
Phy
DHCP
DHCP
DHCP
DHCP DHCP
DHCP request encapsulated in UDP encapsulated in IP encapsulated in 8023 Ethernet
Ethernet frame broadcast
(dest FFFFFFFFFFFF) on LAN received at router running DHCP server
Ethernet demuxed to IP demuxed UDP demuxed to DHCP
router
(runs DHCP)
Link Layer 5-91
DHCP server formulates DHCP ACK containing clientrsquos IP address IP address of first-hop router for client name amp IP address of DNS server
DHCP
UDP
IP
Eth
Phy
DHCP
DHCP
DHCP
DHCP
DHCP
UDP
IP
Eth
Phy
DHCP
DHCP
DHCP
DHCP
DHCP
encapsulation at DHCP server frame forwarded (switch learning) through LAN demultiplexing at client
Client now has IP address knows name amp addr of DNS
server IP address of its first-hop router
DHCP client receives DHCP ACK reply
A day in the lifehellip connecting to the Internet
router
(runs DHCP)
Link Layer 5-92
A day in the lifehellip ARP (before DNS before HTTP)
before sending HTTP request need IP address of wwwgooglecom DNS
DNS
UDP
IP
Eth
Phy
DNS
DNS
DNS
DNS query created encapsulated in UDP encapsulated in IP encapsulated in Eth To send frame to router need MAC address of router interface ARP
ARP query broadcast received by
router which replies with ARP reply giving MAC address of router interface
client now knows MAC address of first hop router so can now send frame containing DNS query
ARP query
Eth
Phy
ARP
ARP
ARP reply
router
(runs DHCP)
Link Layer 5-93
DNS
UDP
IP
Eth
Phy
DNS
DNS
DNS
DNS
DNS
IP datagram containing DNS query forwarded via LAN switch from client to 1st hop router
IP datagram forwarded from campus network into comcast network routed (tables created by RIP OSPF IS-IS andor BGP routing protocols) to DNS server
demuxrsquoed to DNS server
DNS server replies to client with IP address of wwwgooglecom
Comcast network
68800013
DNS server
DNS
UDP
IP
Eth
Phy
DNS
DNS
DNS
DNS
A day in the lifehellip using DNS
router
(runs DHCP)
Link Layer 5-94
A day in the lifehellipTCP connection carrying HTTP
HTTP
TCP
IP
Eth
Phy
HTTP
to send HTTP request client first opens TCP socket to web server
TCP SYN segment (step 1 in 3-way handshake) inter-domain routed to web server
TCP connection established 64233169105
web server
SYN
SYN
SYN
SYN
TCP
IP
Eth
Phy
SYN
SYN
SYN
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
web server responds with TCP SYNACK (step 2 in 3-way handshake)
router
(runs DHCP)
Link Layer 5-95
A day in the lifehellip HTTP requestreply
HTTP
TCP
IP
Eth
Phy
HTTP
HTTP request sent into TCP socket
IP datagram containing HTTP request routed to wwwgooglecom
IP datagram containing HTTP reply routed back to client
64233169105
web server
HTTP
TCP
IP
Eth
Phy web server responds with
HTTP reply (containing web page)
HTTP
HTTP
HTTP HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
web page finally () displayed
Link Layer 5-96
Chapter 5 Summary
principles behind data link layer services error detection correction
sharing a broadcast channel multiple access
link layer addressing
instantiation and implementation of various link layer technologies Ethernet
switched LANS VLANs
virtualized networks as a link layer MPLS
synthesis a day in the life of a web request
Link Layer 5-97
Chapter 5 letrsquos take a breath
journey down protocol stack complete (except PHY)
solid understanding of networking principles practice
hellip could stop here hellip but lots of interesting topics wireless
multimedia
security
network management
Link Layer 5-5
Link layer context
datagram transferred by different link protocols over different links
eg Ethernet on first link frame relay on intermediate links 80211 on last link
each link protocol provides different services
eg may or may not provide rdt over link
transportation analogy trip from Princeton to Lausanne
limo Princeton to JFK
plane JFK to Geneva
train Geneva to Lausanne
tourist = datagram
transport segment = communication link
transportation mode = link layer protocol
travel agent = routing algorithm
Link Layer 5-6
Link layer services
framing link access encapsulate datagram into frame adding header trailer channel access if shared medium ldquoMACrdquo addresses used in frame headers to identify
source dest
bull different from IP address
reliable delivery between adjacent nodes we learned how to do this already (chapter 3) seldom used on low bit-error link (fiber some twisted
pair) wireless links high error rates
bull Q why both link-level and end-end reliability
Link Layer 5-7
flow control pacing between adjacent sending and receiving nodes
error detection errors caused by signal attenuation noise
receiver detects presence of errors
bull signals sender for retransmission or drops frame
error correction receiver identifies and corrects bit error(s) without resorting to
retransmission
half-duplex and full-duplex with half duplex nodes at both ends of link can transmit but not
at same time
Link layer services (more)
Link Layer 5-8
Where is the link layer implemented
in each and every host
link layer implemented in ldquoadaptorrdquo (aka network interface card NIC) or on a chip
Ethernet card 80211 card Ethernet chipset
implements link physical layer
attaches into hostrsquos system buses
combination of hardware software firmware
controller
physical
transmission
cpu memory
host
bus
(eg PCI)
network adapter
card
application
transport
network
link
link
physical
Link Layer 5-9
Adaptors communicating
sending side
encapsulates datagram in frame
adds error checking bits rdt flow control etc
receiving side
looks for errors rdt flow control etc
extracts datagram passes to upper layer at receiving side
controller controller
sending host receiving host
datagram datagram
datagram
frame
Link Layer 5-10
Link layer LANs outline
51 introduction services
52 error detection correction
53 multiple access protocols
54 LANs addressing ARP
Ethernet
switches
VLANS
55 link virtualization MPLS
56 data center networking
57 a day in the life of a web request
Link Layer 5-11
Error detection
EDC= Error Detection and Correction bits (redundancy)
D = Data protected by error checking may include header fields
bull Error detection not 100 reliable
bull protocol may miss some errors but rarely
bull larger EDC field yields better detection and correction
otherwise
Link Layer 5-12
Parity checking
single bit parity detect single bit
errors
two-dimensional bit parity detect and correct single bit errors
0 0
Link Layer 5-13
Internet checksum (review)
sender treat segment contents
as sequence of 16-bit integers
checksum addition (1rsquos complement sum) of segment contents
sender puts checksum value into UDP checksum field
receiver compute checksum of
received segment check if computed
checksum equals checksum field value NO - error detected YES - no error detected
But maybe errors nonetheless
goal detect ldquoerrorsrdquo (eg flipped bits) in transmitted packet (note used at transport layer only)
Link Layer 5-14
Cyclic redundancy check
more powerful error-detection coding
view data bits D as a binary number
choose r+1 bit pattern (generator) G
goal choose r CRC bits R such that ltDRgt exactly divisible by G (modulo 2)
receiver knows G divides ltDRgt by G If non-zero remainder error detected
can detect all burst errors less than r+1 bits
widely used in practice (Ethernet 80211 WiFi ATM)
Link Layer 5-15
CRC example
want D2r XOR R = nG
equivalently D2r = nG XOR R
equivalently if we divide D2r by
G want remainder R to satisfy
R = remainder[ ] D2r
G
1001 101110000 1001
1
101
01000
000 1010 1001
010 000
100 000 1000 0000 1000
D G
R
r = 3
Link Layer 5-16
Link layer LANs outline
51 introduction services
52 error detection correction
53 multiple access protocols
54 LANs addressing ARP
Ethernet
switches
VLANS
55 link virtualization MPLS
56 data center networking
57 a day in the life of a web request
Link Layer 5-17
Multiple access links protocols
two types of ldquolinksrdquo
point-to-point PPP for dial-up access
point-to-point link between Ethernet switch host
broadcast (shared wire or medium) old-fashioned Ethernet
upstream HFC
80211 wireless LAN
shared wire (eg cabled Ethernet)
shared RF (eg 80211 WiFi)
shared RF (satellite)
humans at a cocktail party
(shared air acoustical)
Link Layer 5-18
Multiple access protocols
single shared broadcast channel
two or more simultaneous transmissions by nodes interference
collision if node receives two or more signals at the same time
multiple access protocol distributed algorithm that determines how nodes share
channel ie determine when node can transmit
communication about channel sharing must use channel itself no out-of-band channel for coordination
Link Layer 5-19
An ideal multiple access protocol
given broadcast channel of rate R bps
desiderata 1 when one node wants to transmit it can send at rate R
2 when M nodes want to transmit each can send at average rate RM
3 fully decentralized
bull no special node to coordinate transmissions
bull no synchronization of clocks slots
4 simple
Link Layer 5-20
MAC protocols taxonomy
three broad classes
channel partitioning divide channel into smaller ldquopiecesrdquo (time slots frequency code)
allocate piece to node for exclusive use
random access channel not divided allow collisions
ldquorecoverrdquo from collisions
ldquotaking turnsrdquo nodes take turns but nodes with more to send can take longer
turns
Link Layer 5-21
Channel partitioning MAC protocols TDMA
TDMA time division multiple access access to channel in rounds each station gets fixed length slot (length = pkt
trans time) in each round unused slots go idle example 6-station LAN 134 have pkt slots
256 idle
1 3 4 1 3 4
6-slot
frame
6-slot
frame
Link Layer 5-22
FDMA frequency division multiple access channel spectrum divided into frequency bands
each station assigned fixed frequency band
unused transmission time in frequency bands go idle
example 6-station LAN 134 have pkt frequency bands 256 idle
frequency b
ands
FDM cable
Channel partitioning MAC protocols FDMA
Link Layer 5-23
Random access protocols
when node has packet to send transmit at full channel data rate R no a priori coordination among nodes
two or more transmitting nodes ldquocollisionrdquo random access MAC protocol specifies
how to detect collisions how to recover from collisions (eg via delayed
retransmissions)
examples of random access MAC protocols slotted ALOHA ALOHA CSMA CSMACD CSMACA
Link Layer 5-24
Slotted ALOHA
assumptions all frames same size
time divided into equal size slots (time to transmit 1 frame)
nodes start to transmit only slot beginning
nodes are synchronized
if 2 or more nodes transmit in slot all nodes detect collision
operation
when node obtains fresh frame transmits in next slot
if no collision node can send new frame in next slot
if collision node retransmits frame in each subsequent slot with prob p until success
Link Layer 5-25
Pros single active node can
continuously transmit at full rate of channel
highly decentralized only slots in nodes need to be in sync
simple
Cons
collisions wasting slots
idle slots
nodes may be able to detect collision in less than time to transmit packet
clock synchronization
Slotted ALOHA
1 1 1 1
2
3
2 2
3 3
node 1
node 2
node 3
C C C S S S E E E
Link Layer 5-26
suppose N nodes with many frames to send each transmits in slot with probability p
prob that given node has success in a slot = p(1-p)N-1
prob that any node has a success = Np(1-p)N-1
max efficiency find p that maximizes Np(1-p)N-1
for many nodes take limit of Np(1-p)N-1 as N goes to infinity gives
max efficiency = 1e = 37
efficiency long-run fraction of successful slots (many nodes all with many frames to send)
at best channel used for useful transmissions 37 of time
Slotted ALOHA efficiency
Link Layer 5-27
Pure (unslotted) ALOHA
unslotted Aloha simpler no synchronization
when frame first arrives
transmit immediately
collision probability increases
frame sent at t0 collides with other frames sent in [t0-1t0+1]
Link Layer 5-28
Pure ALOHA efficiency
P(success by given node) = P(node transmits)
P(no other node transmits in [t0-1t0]
P(no other node transmits in [t0-1t0]
= p (1-p)N-1 (1-p)N-1
= p (1-p)2(N-1)
hellip choosing optimum p and then letting n
= 1(2e) = 18
even worse than slotted Aloha
Link Layer 5-29
CSMA (carrier sense multiple access)
CSMA listen before transmit if channel sensed idle transmit entire frame
if channel sensed busy defer transmission
human analogy donrsquot interrupt others
Link Layer 5-30
CSMA collisions
collisions can still occur propagation delay means two nodes may not hear each otherrsquos transmission
collision entire packet transmission time wasted distance amp propagation
delay play role in in determining collision probability
spatial layout of nodes
Link Layer 5-31
CSMACD (collision detection)
CSMACD carrier sensing deferral as in CSMA collisions detected within short time
colliding transmissions aborted reducing channel wastage
collision detection easy in wired LANs measure signal strengths compare
transmitted received signals
difficult in wireless LANs received signal strength overwhelmed by local transmission strength
human analogy the polite conversationalist
Link Layer 5-32
CSMACD (collision detection)
spatial layout of nodes
Link Layer 5-33
Ethernet CSMACD algorithm
1 NIC receives datagram from network layer creates frame
2 If NIC senses channel idle starts frame transmission If NIC senses channel busy waits until channel idle then transmits
3 If NIC transmits entire frame without detecting another transmission NIC is done with frame
4 If NIC detects another transmission while transmitting aborts and sends jam signal
5 After aborting NIC enters binary (exponential) backoff
after mth collision NIC chooses K at random from 012 hellip 2m-1 NIC waits K512 bit times returns to Step 2
longer backoff interval with more collisions
Link Layer 5-34
CSMACD efficiency
Tprop = max prop delay between 2 nodes in LAN
ttrans = time to transmit max-size frame
efficiency goes to 1
as tprop goes to 0
as ttrans goes to infinity
better performance than ALOHA and simple cheap decentralized
transpropttefficiency
51
1
Link Layer 5-35
ldquoTaking turnsrdquo MAC protocols
channel partitioning MAC protocols share channel efficiently and fairly at high load
inefficient at low load delay in channel access 1N bandwidth allocated even if only 1 active node
random access MAC protocols efficient at low load single node can fully utilize
channel
high load collision overhead
ldquotaking turnsrdquo protocols look for best of both worlds
Link Layer 5-36
polling master node ldquoinvitesrdquo
slave nodes to transmit in turn
typically used with ldquodumbrdquo slave devices
concerns
polling overhead
latency
single point of failure (master)
master
slaves
poll
data
data
ldquoTaking turnsrdquo MAC protocols
Link Layer 5-37
token passing control token passed
from one node to next sequentially
token message
concerns
token overhead
latency
single point of failure (token)
T
data
(nothing
to send)
T
ldquoTaking turnsrdquo MAC protocols
cable headend
CMTS
ISP
cable modem termination system
multiple 40Mbps downstream (broadcast) channels
single CMTS transmits into channels
multiple 30 Mbps upstream channels
multiple access all users contend for certain upstream channel time slots (others assigned)
Cable access network
cable modem
splitter
hellip
hellip
Internet framesTV channels control transmitted downstream at different frequencies
upstream Internet frames TV control transmitted upstream at different frequencies in time slots
Link Layer 5-39
DOCSIS data over cable service interface spec FDM over upstream downstream frequency channels
TDM upstream some slots assigned some have contention
downstream MAP frame assigns upstream slots
request for upstream slots (and data) transmitted random access (binary backoff) in selected slots
MAP frame for
Interval [t1 t2]
Residences with cable modems
Downstream channel i
Upstream channel j
t1 t2
Assigned minislots containing cable modem
upstream data frames
Minislots containing
minislots request frames
cable headend
CMTS
Cable access network
Link Layer 5-40
Summary of MAC protocols
channel partitioning by time frequency or code Time Division Frequency Division
random access (dynamic)
ALOHA S-ALOHA CSMA CSMACD
carrier sensing easy in some technologies (wire) hard in others (wireless)
CSMACD used in Ethernet
CSMACA used in 80211
taking turns
polling from central site token passing
bluetooth FDDI token ring
Link Layer 5-41
Link layer LANs outline
51 introduction services
52 error detection correction
53 multiple access protocols
54 LANs addressing ARP
Ethernet
switches
VLANS
55 link virtualization MPLS
56 data center networking
57 a day in the life of a web request
Link Layer 5-42
MAC addresses and ARP
32-bit IP address network-layer address for interface
used for layer 3 (network layer) forwarding
MAC (or LAN or physical or Ethernet) address function used lsquolocallyrdquo to get frame from one interface to
another physically-connected interface (same network in IP-addressing sense)
48 bit MAC address (for most LANs) burned in NIC ROM also sometimes software settable
eg 1A-2F-BB-76-09-AD
hexadecimal (base 16) notation
(each ldquonumberrdquo represents 4 bits)
Link Layer 5-43
LAN addresses and ARP
each adapter on LAN has unique LAN address
adapter
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN
(wired or
wireless)
Link Layer 5-44
LAN addresses (more)
MAC address allocation administered by IEEE
manufacturer buys portion of MAC address space (to assure uniqueness)
analogy MAC address like Social Security Number
IP address like postal address
MAC flat address portability can move LAN card from one LAN to another
IP hierarchical address not portable address depends on IP subnet to which node is
attached
Link Layer 5-45
ARP address resolution protocol
ARP table each IP node (host router) on LAN has table
IPMAC address mappings for some LAN nodes
lt IP address MAC address TTLgt
TTL (Time To Live) time after which address mapping will be forgotten (typically 20 min)
Question how to determine
interfacersquos MAC address
knowing its IP address
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN
137196723
137196778
137196714
137196788
Link Layer 5-46
ARP protocol same LAN
A wants to send datagram to B Brsquos MAC address not in
Arsquos ARP table
A broadcasts ARP query packet containing Bs IP address dest MAC address = FF-FF-
FF-FF-FF-FF
all nodes on LAN receive ARP query
B receives ARP packet replies to A with its (Bs) MAC address frame sent to Arsquos MAC
address (unicast)
A caches (saves) IP-to-MAC address pair in its ARP table until information becomes old (times out) soft state information that
times out (goes away) unless refreshed
ARP is ldquoplug-and-playrdquo nodes create their ARP
tables without intervention from net administrator
Link Layer 5-47
walkthrough send datagram from A to B via R
focus on addressing ndash at IP (datagram) and MAC layer (frame)
assume A knows Brsquos IP address
assume A knows IP address of first hop router R (how)
assume A knows Rrsquos MAC address (how)
Addressing routing to another LAN
R
1A-23-F9-CD-06-9B 222222222220
111111111110 E6-E9-00-17-BB-4B CC-49-DE-D0-AB-7D
111111111112
111111111111
74-29-9C-E8-FF-55
A
222222222222
49-BD-D2-C7-56-2A
222222222221 88-B2-2F-54-1A-0F
B
R
1A-23-F9-CD-06-9B 222222222220
111111111110 E6-E9-00-17-BB-4B CC-49-DE-D0-AB-7D
111111111112
111111111111
74-29-9C-E8-FF-55
A
222222222222
49-BD-D2-C7-56-2A
222222222221 88-B2-2F-54-1A-0F
B
Link Layer 5-48
Addressing routing to another LAN
IP
Eth
Phy
IP src 111111111111
IP dest 222222222222
A creates IP datagram with IP source A destination B
A creates link-layer frame with Rs MAC address as dest frame contains A-to-B IP datagram
MAC src 74-29-9C-E8-FF-55
MAC dest E6-E9-00-17-BB-4B
R
1A-23-F9-CD-06-9B 222222222220
111111111110 E6-E9-00-17-BB-4B CC-49-DE-D0-AB-7D
111111111112
111111111111
74-29-9C-E8-FF-55
A
222222222222
49-BD-D2-C7-56-2A
222222222221 88-B2-2F-54-1A-0F
B
Link Layer 5-49
Addressing routing to another LAN
IP
Eth
Phy
frame sent from A to R
IP
Eth
Phy
frame received at R datagram removed passed up to IP
MAC src 74-29-9C-E8-FF-55
MAC dest E6-E9-00-17-BB-4B
IP src 111111111111
IP dest 222222222222
IP src 111111111111
IP dest 222222222222
R
1A-23-F9-CD-06-9B 222222222220
111111111110 E6-E9-00-17-BB-4B CC-49-DE-D0-AB-7D
111111111112
111111111111
74-29-9C-E8-FF-55
A
222222222222
49-BD-D2-C7-56-2A
222222222221 88-B2-2F-54-1A-0F
B
Link Layer 5-50
Addressing routing to another LAN
IP src 111111111111
IP dest 222222222222
R forwards datagram with IP source A destination B
R creates link-layer frame with Bs MAC address as dest frame contains A-to-B IP datagram
MAC src 1A-23-F9-CD-06-9B
MAC dest 49-BD-D2-C7-56-2A
IP
Eth
Phy
IP
Eth
Phy
R
1A-23-F9-CD-06-9B 222222222220
111111111110 E6-E9-00-17-BB-4B CC-49-DE-D0-AB-7D
111111111112
111111111111
74-29-9C-E8-FF-55
A
222222222222
49-BD-D2-C7-56-2A
222222222221 88-B2-2F-54-1A-0F
B
Link Layer 5-51
Addressing routing to another LAN R forwards datagram with IP source A destination B
R creates link-layer frame with Bs MAC address as dest frame contains A-to-B IP datagram
IP src 111111111111
IP dest 222222222222
MAC src 1A-23-F9-CD-06-9B
MAC dest 49-BD-D2-C7-56-2A
IP
Eth
Phy
IP
Eth
Phy
R
1A-23-F9-CD-06-9B 222222222220
111111111110 E6-E9-00-17-BB-4B CC-49-DE-D0-AB-7D
111111111112
111111111111
74-29-9C-E8-FF-55
A
222222222222
49-BD-D2-C7-56-2A
222222222221 88-B2-2F-54-1A-0F
B
Link Layer 5-52
Addressing routing to another LAN R forwards datagram with IP source A destination B
R creates link-layer frame with Bs MAC address as dest frame contains A-to-B IP datagram
IP src 111111111111
IP dest 222222222222
MAC src 1A-23-F9-CD-06-9B
MAC dest 49-BD-D2-C7-56-2A
IP
Eth
Phy
Link Layer 5-53
Link layer LANs outline
51 introduction services
52 error detection correction
53 multiple access protocols
54 LANs addressing ARP
Ethernet
switches
VLANS
55 link virtualization MPLS
56 data center networking
57 a day in the life of a web request
Link Layer 5-54
Ethernet
ldquodominantrdquo wired LAN technology
cheap $20 for NIC
first widely used LAN technology
simpler cheaper than token LANs and ATM
kept up with speed race 10 Mbps ndash 10 Gbps
Metcalfersquos Ethernet sketch
Link Layer 5-55
Ethernet physical topology
bus popular through mid 90s all nodes in same collision domain (can collide with each
other)
star prevails today active switch in center
each ldquospokerdquo runs a (separate) Ethernet protocol (nodes
do not collide with each other)
switch
bus coaxial cable star
Link Layer 5-56
Ethernet frame structure
sending adapter encapsulates IP datagram (or other network layer protocol packet) in Ethernet frame
preamble
7 bytes with pattern 10101010 followed by one byte with pattern 10101011
used to synchronize receiver sender clock rates
dest address
source address
data (payload) CRC preamble
type
Link Layer 5-57
Ethernet frame structure (more)
addresses 6 byte source destination MAC addresses if adapter receives frame with matching destination
address or with broadcast address (eg ARP packet) it passes data in frame to network layer protocol
otherwise adapter discards frame
type indicates higher layer protocol (mostly IP but others possible eg Novell IPX AppleTalk)
CRC cyclic redundancy check at receiver error detected frame is dropped
dest address
source address
data (payload) CRC preamble
type
Link Layer 5-58
Ethernet unreliable connectionless
connectionless no handshaking between sending and receiving NICs
unreliable receiving NIC doesnt send acks or nacks to sending NIC
data in dropped frames recovered only if initial sender uses higher layer rdt (eg TCP) otherwise dropped data lost
Ethernetrsquos MAC protocol unslotted CSMACD wth binary backoff
Link Layer 5-59
8023 Ethernet standards link amp physical layers
many different Ethernet standards
common MAC protocol and frame format
different speeds 2 Mbps 10 Mbps 100 Mbps 1Gbps 10G bps
different physical layer media fiber cable
application
transport
network
link
physical
MAC protocol
and frame format
100BASE-TX
100BASE-T4
100BASE-FX 100BASE-T2
100BASE-SX 100BASE-BX
fiber physical layer copper (twister
pair) physical layer
Link Layer 5-60
Link layer LANs outline
51 introduction services
52 error detection correction
53 multiple access protocols
54 LANs addressing ARP
Ethernet
switches
VLANS
55 link virtualization MPLS
56 data center networking
57 a day in the life of a web request
Link Layer 5-61
Ethernet switch link-layer device takes an active role
store forward Ethernet frames
examine incoming framersquos MAC address selectively forward frame to one-or-more outgoing links when frame is to be forwarded on segment uses CSMACD to access segment
transparent
hosts are unaware of presence of switches
plug-and-play self-learning
switches do not need to be configured
Link Layer 5-62
Switch multiple simultaneous transmissions
hosts have dedicated direct connection to switch
switches buffer packets
Ethernet protocol used on each incoming link but no collisions full duplex
each link is its own collision domain
switching A-to-Arsquo and B-to-Brsquo can transmit simultaneously without collisions switch with six interfaces
(123456)
A
Arsquo
B
Brsquo C
Crsquo
1 2
3 4 5
6
Link Layer 5-63
Switch forwarding table
Q how does switch know Arsquo reachable via interface 4 Brsquo reachable via interface 5
switch with six interfaces
(123456)
A
Arsquo
B
Brsquo C
Crsquo
1 2
3 4 5
6 A each switch has a switch table each entry
(MAC address of host interface to
reach host time stamp)
looks like a routing table
Q how are entries created maintained in switch table
something like a routing protocol
A
Arsquo
B
Brsquo C
Crsquo
1 2
3 4 5
6
Link Layer 5-64
Switch self-learning
switch learns which hosts can be reached through which interfaces
when frame received switch ldquolearnsrdquo location of sender incoming LAN segment
records senderlocation pair in switch table
A Arsquo
Source A
Dest Arsquo
MAC addr interface TTL
Switch table
(initially empty) A 1 60
Link Layer 5-65
Switch frame filteringforwarding
when frame received at switch 1 record incoming link MAC address of sending host
2 index switch table using MAC destination address
3 if entry found for destination then
if destination on segment from which frame arrived then drop frame
else forward frame on interface indicated by entry
else flood forward on all interfaces except arriving
interface
A
Arsquo
B
Brsquo C
Crsquo
1 2
3 4 5
6
Link Layer 5-66
Self-learning forwarding example
A Arsquo
Source A
Dest Arsquo
MAC addr interface TTL
switch table
(initially empty) A 1 60
A Arsquo A Arsquo A Arsquo A Arsquo A Arsquo
frame destination Arsquo locaton unknown flood
Arsquo A
destination A location
known
Arsquo 4 60
selectively send
on just one link
Link Layer 5-67
Interconnecting switches
switches can be connected together
Q sending from A to G - how does S1 know to forward frame destined to F via S4 and S3
A self learning (works exactly the same as in single-switch case)
A
B
S1
C D
E
F
S2
S4
S3
H
I
G
Link Layer 5-68
Self-learning multi-switch example
Suppose C sends frame to I I responds to C
Q show switch tables and packet forwarding in S1 S2 S3 S4
A
B
S1
C D
E
F
S2
S4
S3
H
I
G
Link Layer 5-69
Institutional network
to external
network
router
IP subnet
mail server
web server
Link Layer 5-70
Switches vs routers
both are store-and-forward
routers network-layer devices (examine network-layer headers)
switches link-layer devices (examine link-layer headers)
both have forwarding tables
routers compute tables using routing algorithms IP addresses
switches learn forwarding table using flooding learning MAC addresses
application
transport
network
link
physical
network
link
physical
link
physical
switch
datagram
application
transport
network
link
physical
frame
frame
frame
datagram
Link Layer 5-71
VLANs motivation
consider CS user moves office to
EE but wants connect to CS switch
single broadcast domain
all layer-2 broadcast traffic (ARP DHCP unknown location of destination MAC address) must cross entire LAN
securityprivacy efficiency issues
Computer
Science Electrical
Engineering
Computer
Engineering
Link Layer 5-72
VLANs port-based VLAN switch ports
grouped (by switch management software) so that single physical switch helliphellip
switch(es) supporting
VLAN capabilities can
be configured to
define multiple virtual
LANS over single
physical LAN
infrastructure
Virtual Local
Area Network 1
8
9
16 10 2
7
hellip
Electrical Engineering
(VLAN ports 1-8)
Computer Science
(VLAN ports 9-15)
15
hellip
Electrical Engineering
(VLAN ports 1-8)
hellip
1
8 2
7 9
16 10
15
hellip
Computer Science
(VLAN ports 9-16)
hellip operates as multiple virtual switches
Link Layer 5-73
Port-based VLAN
1
8
9
16 10 2
7
hellip
Electrical Engineering
(VLAN ports 1-8)
Computer Science
(VLAN ports 9-15)
15
hellip
traffic isolation frames tofrom ports 1-8 can only reach ports 1-8 can also define VLAN based on
MAC addresses of endpoints rather than switch port
dynamic membership ports can be dynamically assigned among VLANs
router
forwarding between VLANS done via routing (just as with separate switches) in practice vendors sell combined
switches plus routers
Link Layer 5-74
VLANS spanning multiple switches
trunk port carries frames between VLANS defined over multiple physical switches frames forwarded within VLAN between switches canrsquot be vanilla
8021 frames (must carry VLAN ID info)
8021q protocol addsremoved additional header fields for frames forwarded between trunk ports
1
8
9
10 2
7
hellip
Electrical Engineering
(VLAN ports 1-8)
Computer Science
(VLAN ports 9-15)
15
hellip
2
7 3
Ports 235 belong to EE VLAN
Ports 4678 belong to CS VLAN
5
4 6 8 16
1
Link Layer 5-75
type
2-byte Tag Protocol Identifier
(value 81-00)
Tag Control Information (12 bit VLAN ID field
3 bit priority field like IP TOS)
Recomputed CRC
8021Q VLAN frame format
8021 frame
8021Q frame
dest address
source address
data (payload) CRC preamble
dest address
source address
preamble data (payload) CRC
type
Link Layer 5-76
Link layer LANs outline
51 introduction services
52 error detection correction
53 multiple access protocols
54 LANs addressing ARP
Ethernet
switches
VLANS
55 link virtualization MPLS
56 data center networking
57 a day in the life of a web request
Link Layer 5-77
Multiprotocol label switching (MPLS)
initial goal high-speed IP forwarding using fixed length label (instead of IP address) fast lookup using fixed length identifier (rather than
shortest prefix matching)
borrowing ideas from Virtual Circuit (VC) approach
but IP datagram still keeps IP address
PPP or Ethernet
header IP header remainder of link-layer frame MPLS header
label Exp S TTL
20 3 1 5
Link Layer 5-78
MPLS capable routers
aka label-switched router
forward packets to outgoing interface based only on label value (donrsquot inspect IP address) MPLS forwarding table distinct from IP forwarding tables
flexibility MPLS forwarding decisions can differ from those of IP use destination and source addresses to route flows to
same destination differently (traffic engineering)
re-route flows quickly if link fails pre-computed backup paths (useful for VoIP)
Link Layer 5-79
R2
D
R3 R5
A
R6
MPLS versus IP paths
IP router IP routing path to destination determined
by destination address alone
R4
Link Layer 5-80
R2
D
R3 R4
R5
A
R6
MPLS versus IP paths
IP-only router
IP routing path to destination determined by destination address alone
MPLS and IP router
MPLS routing path to destination can be based on source and dest address fast reroute precompute backup routes in
case of link failure
entry router (R4) can use different MPLS
routes to A based eg on source address
Link Layer 5-81
MPLS signaling
modify OSPF IS-IS link-state flooding protocols to carry info used by MPLS routing eg link bandwidth amount of ldquoreservedrdquo link bandwidth
D
R4
R5
A
R6
entry MPLS router uses RSVP-TE signaling protocol to set up MPLS forwarding at downstream routers
modified link state flooding
RSVP-TE
Link Layer 5-82
R1 R2
D
R3 R4 R5
0
1
0 0
A
R6
in out out
label label dest interface
6 - A 0
in out out
label label dest interface
10 6 A 1
12 9 D 0
in out out
label label dest interface
10 A 0
12 D 0
1
in out out
label label dest interface
8 6 A 0
0
8 A 1
MPLS forwarding tables
Link Layer 5-83
Link layer LANs outline
51 introduction services
52 error detection correction
53 multiple access protocols
54 LANs addressing ARP
Ethernet
switches
VLANS
55 link virtualization MPLS
56 data center networking
57 a day in the life of a web request
Link Layer 5-84
Data center networks
10rsquos to 100rsquos of thousands of hosts often closely coupled in close proximity e-business (eg Amazon)
content-servers (eg YouTube Akamai Apple Microsoft)
search engines data mining (eg Google)
challenges
multiple applications each serving massive numbers of clients
managingbalancing load avoiding processing networking data bottlenecks
Inside a 40-ft Microsoft container
Chicago data center
Link Layer 5-85
Server racks
TOR switches
Tier-1 switches
Tier-2 switches
Load balancer
Load balancer
B
1 2 3 4 5 6 7 8
A C
Border router
Access router
Internet
Data center networks
load balancer application-layer routing receives external client requests
directs workload within data center
returns results to external client (hiding data
center internals from client)
Server racks
TOR switches
Tier-1 switches
Tier-2 switches
1 2 3 4 5 6 7 8
Data center networks
rich interconnection among switches racks
increased throughput between racks (multiple routing
paths possible)
increased reliability via redundancy
Link Layer 5-87
Link layer LANs outline
51 introduction services
52 error detection correction
53 multiple access protocols
54 LANs addressing ARP
Ethernet
switches
VLANS
55 link virtualization MPLS
56 data center networking
57 a day in the life of a web request
Link Layer 5-88
Synthesis a day in the life of a web request
journey down protocol stack complete application transport network link
putting-it-all-together synthesis goal identify review understand protocols (at all
layers) involved in seemingly simple scenario requesting www page
scenario student attaches laptop to campus network requestsreceives wwwgooglecom
Link Layer 5-89
A day in the life scenario
Comcast network
68800013
Googlersquos network
64233160019 64233169105
web server
DNS server
school network
68802024
web page
browser
router
(runs DHCP)
Link Layer 5-90
A day in the lifehellip connecting to the Internet
connecting laptop needs to get its own IP address addr of first-hop router addr of DNS server use DHCP
DHCP
UDP
IP
Eth
Phy
DHCP
DHCP
DHCP
DHCP
DHCP
DHCP
UDP
IP
Eth
Phy
DHCP
DHCP
DHCP
DHCP DHCP
DHCP request encapsulated in UDP encapsulated in IP encapsulated in 8023 Ethernet
Ethernet frame broadcast
(dest FFFFFFFFFFFF) on LAN received at router running DHCP server
Ethernet demuxed to IP demuxed UDP demuxed to DHCP
router
(runs DHCP)
Link Layer 5-91
DHCP server formulates DHCP ACK containing clientrsquos IP address IP address of first-hop router for client name amp IP address of DNS server
DHCP
UDP
IP
Eth
Phy
DHCP
DHCP
DHCP
DHCP
DHCP
UDP
IP
Eth
Phy
DHCP
DHCP
DHCP
DHCP
DHCP
encapsulation at DHCP server frame forwarded (switch learning) through LAN demultiplexing at client
Client now has IP address knows name amp addr of DNS
server IP address of its first-hop router
DHCP client receives DHCP ACK reply
A day in the lifehellip connecting to the Internet
router
(runs DHCP)
Link Layer 5-92
A day in the lifehellip ARP (before DNS before HTTP)
before sending HTTP request need IP address of wwwgooglecom DNS
DNS
UDP
IP
Eth
Phy
DNS
DNS
DNS
DNS query created encapsulated in UDP encapsulated in IP encapsulated in Eth To send frame to router need MAC address of router interface ARP
ARP query broadcast received by
router which replies with ARP reply giving MAC address of router interface
client now knows MAC address of first hop router so can now send frame containing DNS query
ARP query
Eth
Phy
ARP
ARP
ARP reply
router
(runs DHCP)
Link Layer 5-93
DNS
UDP
IP
Eth
Phy
DNS
DNS
DNS
DNS
DNS
IP datagram containing DNS query forwarded via LAN switch from client to 1st hop router
IP datagram forwarded from campus network into comcast network routed (tables created by RIP OSPF IS-IS andor BGP routing protocols) to DNS server
demuxrsquoed to DNS server
DNS server replies to client with IP address of wwwgooglecom
Comcast network
68800013
DNS server
DNS
UDP
IP
Eth
Phy
DNS
DNS
DNS
DNS
A day in the lifehellip using DNS
router
(runs DHCP)
Link Layer 5-94
A day in the lifehellipTCP connection carrying HTTP
HTTP
TCP
IP
Eth
Phy
HTTP
to send HTTP request client first opens TCP socket to web server
TCP SYN segment (step 1 in 3-way handshake) inter-domain routed to web server
TCP connection established 64233169105
web server
SYN
SYN
SYN
SYN
TCP
IP
Eth
Phy
SYN
SYN
SYN
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
web server responds with TCP SYNACK (step 2 in 3-way handshake)
router
(runs DHCP)
Link Layer 5-95
A day in the lifehellip HTTP requestreply
HTTP
TCP
IP
Eth
Phy
HTTP
HTTP request sent into TCP socket
IP datagram containing HTTP request routed to wwwgooglecom
IP datagram containing HTTP reply routed back to client
64233169105
web server
HTTP
TCP
IP
Eth
Phy web server responds with
HTTP reply (containing web page)
HTTP
HTTP
HTTP HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
web page finally () displayed
Link Layer 5-96
Chapter 5 Summary
principles behind data link layer services error detection correction
sharing a broadcast channel multiple access
link layer addressing
instantiation and implementation of various link layer technologies Ethernet
switched LANS VLANs
virtualized networks as a link layer MPLS
synthesis a day in the life of a web request
Link Layer 5-97
Chapter 5 letrsquos take a breath
journey down protocol stack complete (except PHY)
solid understanding of networking principles practice
hellip could stop here hellip but lots of interesting topics wireless
multimedia
security
network management
Link Layer 5-6
Link layer services
framing link access encapsulate datagram into frame adding header trailer channel access if shared medium ldquoMACrdquo addresses used in frame headers to identify
source dest
bull different from IP address
reliable delivery between adjacent nodes we learned how to do this already (chapter 3) seldom used on low bit-error link (fiber some twisted
pair) wireless links high error rates
bull Q why both link-level and end-end reliability
Link Layer 5-7
flow control pacing between adjacent sending and receiving nodes
error detection errors caused by signal attenuation noise
receiver detects presence of errors
bull signals sender for retransmission or drops frame
error correction receiver identifies and corrects bit error(s) without resorting to
retransmission
half-duplex and full-duplex with half duplex nodes at both ends of link can transmit but not
at same time
Link layer services (more)
Link Layer 5-8
Where is the link layer implemented
in each and every host
link layer implemented in ldquoadaptorrdquo (aka network interface card NIC) or on a chip
Ethernet card 80211 card Ethernet chipset
implements link physical layer
attaches into hostrsquos system buses
combination of hardware software firmware
controller
physical
transmission
cpu memory
host
bus
(eg PCI)
network adapter
card
application
transport
network
link
link
physical
Link Layer 5-9
Adaptors communicating
sending side
encapsulates datagram in frame
adds error checking bits rdt flow control etc
receiving side
looks for errors rdt flow control etc
extracts datagram passes to upper layer at receiving side
controller controller
sending host receiving host
datagram datagram
datagram
frame
Link Layer 5-10
Link layer LANs outline
51 introduction services
52 error detection correction
53 multiple access protocols
54 LANs addressing ARP
Ethernet
switches
VLANS
55 link virtualization MPLS
56 data center networking
57 a day in the life of a web request
Link Layer 5-11
Error detection
EDC= Error Detection and Correction bits (redundancy)
D = Data protected by error checking may include header fields
bull Error detection not 100 reliable
bull protocol may miss some errors but rarely
bull larger EDC field yields better detection and correction
otherwise
Link Layer 5-12
Parity checking
single bit parity detect single bit
errors
two-dimensional bit parity detect and correct single bit errors
0 0
Link Layer 5-13
Internet checksum (review)
sender treat segment contents
as sequence of 16-bit integers
checksum addition (1rsquos complement sum) of segment contents
sender puts checksum value into UDP checksum field
receiver compute checksum of
received segment check if computed
checksum equals checksum field value NO - error detected YES - no error detected
But maybe errors nonetheless
goal detect ldquoerrorsrdquo (eg flipped bits) in transmitted packet (note used at transport layer only)
Link Layer 5-14
Cyclic redundancy check
more powerful error-detection coding
view data bits D as a binary number
choose r+1 bit pattern (generator) G
goal choose r CRC bits R such that ltDRgt exactly divisible by G (modulo 2)
receiver knows G divides ltDRgt by G If non-zero remainder error detected
can detect all burst errors less than r+1 bits
widely used in practice (Ethernet 80211 WiFi ATM)
Link Layer 5-15
CRC example
want D2r XOR R = nG
equivalently D2r = nG XOR R
equivalently if we divide D2r by
G want remainder R to satisfy
R = remainder[ ] D2r
G
1001 101110000 1001
1
101
01000
000 1010 1001
010 000
100 000 1000 0000 1000
D G
R
r = 3
Link Layer 5-16
Link layer LANs outline
51 introduction services
52 error detection correction
53 multiple access protocols
54 LANs addressing ARP
Ethernet
switches
VLANS
55 link virtualization MPLS
56 data center networking
57 a day in the life of a web request
Link Layer 5-17
Multiple access links protocols
two types of ldquolinksrdquo
point-to-point PPP for dial-up access
point-to-point link between Ethernet switch host
broadcast (shared wire or medium) old-fashioned Ethernet
upstream HFC
80211 wireless LAN
shared wire (eg cabled Ethernet)
shared RF (eg 80211 WiFi)
shared RF (satellite)
humans at a cocktail party
(shared air acoustical)
Link Layer 5-18
Multiple access protocols
single shared broadcast channel
two or more simultaneous transmissions by nodes interference
collision if node receives two or more signals at the same time
multiple access protocol distributed algorithm that determines how nodes share
channel ie determine when node can transmit
communication about channel sharing must use channel itself no out-of-band channel for coordination
Link Layer 5-19
An ideal multiple access protocol
given broadcast channel of rate R bps
desiderata 1 when one node wants to transmit it can send at rate R
2 when M nodes want to transmit each can send at average rate RM
3 fully decentralized
bull no special node to coordinate transmissions
bull no synchronization of clocks slots
4 simple
Link Layer 5-20
MAC protocols taxonomy
three broad classes
channel partitioning divide channel into smaller ldquopiecesrdquo (time slots frequency code)
allocate piece to node for exclusive use
random access channel not divided allow collisions
ldquorecoverrdquo from collisions
ldquotaking turnsrdquo nodes take turns but nodes with more to send can take longer
turns
Link Layer 5-21
Channel partitioning MAC protocols TDMA
TDMA time division multiple access access to channel in rounds each station gets fixed length slot (length = pkt
trans time) in each round unused slots go idle example 6-station LAN 134 have pkt slots
256 idle
1 3 4 1 3 4
6-slot
frame
6-slot
frame
Link Layer 5-22
FDMA frequency division multiple access channel spectrum divided into frequency bands
each station assigned fixed frequency band
unused transmission time in frequency bands go idle
example 6-station LAN 134 have pkt frequency bands 256 idle
frequency b
ands
FDM cable
Channel partitioning MAC protocols FDMA
Link Layer 5-23
Random access protocols
when node has packet to send transmit at full channel data rate R no a priori coordination among nodes
two or more transmitting nodes ldquocollisionrdquo random access MAC protocol specifies
how to detect collisions how to recover from collisions (eg via delayed
retransmissions)
examples of random access MAC protocols slotted ALOHA ALOHA CSMA CSMACD CSMACA
Link Layer 5-24
Slotted ALOHA
assumptions all frames same size
time divided into equal size slots (time to transmit 1 frame)
nodes start to transmit only slot beginning
nodes are synchronized
if 2 or more nodes transmit in slot all nodes detect collision
operation
when node obtains fresh frame transmits in next slot
if no collision node can send new frame in next slot
if collision node retransmits frame in each subsequent slot with prob p until success
Link Layer 5-25
Pros single active node can
continuously transmit at full rate of channel
highly decentralized only slots in nodes need to be in sync
simple
Cons
collisions wasting slots
idle slots
nodes may be able to detect collision in less than time to transmit packet
clock synchronization
Slotted ALOHA
1 1 1 1
2
3
2 2
3 3
node 1
node 2
node 3
C C C S S S E E E
Link Layer 5-26
suppose N nodes with many frames to send each transmits in slot with probability p
prob that given node has success in a slot = p(1-p)N-1
prob that any node has a success = Np(1-p)N-1
max efficiency find p that maximizes Np(1-p)N-1
for many nodes take limit of Np(1-p)N-1 as N goes to infinity gives
max efficiency = 1e = 37
efficiency long-run fraction of successful slots (many nodes all with many frames to send)
at best channel used for useful transmissions 37 of time
Slotted ALOHA efficiency
Link Layer 5-27
Pure (unslotted) ALOHA
unslotted Aloha simpler no synchronization
when frame first arrives
transmit immediately
collision probability increases
frame sent at t0 collides with other frames sent in [t0-1t0+1]
Link Layer 5-28
Pure ALOHA efficiency
P(success by given node) = P(node transmits)
P(no other node transmits in [t0-1t0]
P(no other node transmits in [t0-1t0]
= p (1-p)N-1 (1-p)N-1
= p (1-p)2(N-1)
hellip choosing optimum p and then letting n
= 1(2e) = 18
even worse than slotted Aloha
Link Layer 5-29
CSMA (carrier sense multiple access)
CSMA listen before transmit if channel sensed idle transmit entire frame
if channel sensed busy defer transmission
human analogy donrsquot interrupt others
Link Layer 5-30
CSMA collisions
collisions can still occur propagation delay means two nodes may not hear each otherrsquos transmission
collision entire packet transmission time wasted distance amp propagation
delay play role in in determining collision probability
spatial layout of nodes
Link Layer 5-31
CSMACD (collision detection)
CSMACD carrier sensing deferral as in CSMA collisions detected within short time
colliding transmissions aborted reducing channel wastage
collision detection easy in wired LANs measure signal strengths compare
transmitted received signals
difficult in wireless LANs received signal strength overwhelmed by local transmission strength
human analogy the polite conversationalist
Link Layer 5-32
CSMACD (collision detection)
spatial layout of nodes
Link Layer 5-33
Ethernet CSMACD algorithm
1 NIC receives datagram from network layer creates frame
2 If NIC senses channel idle starts frame transmission If NIC senses channel busy waits until channel idle then transmits
3 If NIC transmits entire frame without detecting another transmission NIC is done with frame
4 If NIC detects another transmission while transmitting aborts and sends jam signal
5 After aborting NIC enters binary (exponential) backoff
after mth collision NIC chooses K at random from 012 hellip 2m-1 NIC waits K512 bit times returns to Step 2
longer backoff interval with more collisions
Link Layer 5-34
CSMACD efficiency
Tprop = max prop delay between 2 nodes in LAN
ttrans = time to transmit max-size frame
efficiency goes to 1
as tprop goes to 0
as ttrans goes to infinity
better performance than ALOHA and simple cheap decentralized
transpropttefficiency
51
1
Link Layer 5-35
ldquoTaking turnsrdquo MAC protocols
channel partitioning MAC protocols share channel efficiently and fairly at high load
inefficient at low load delay in channel access 1N bandwidth allocated even if only 1 active node
random access MAC protocols efficient at low load single node can fully utilize
channel
high load collision overhead
ldquotaking turnsrdquo protocols look for best of both worlds
Link Layer 5-36
polling master node ldquoinvitesrdquo
slave nodes to transmit in turn
typically used with ldquodumbrdquo slave devices
concerns
polling overhead
latency
single point of failure (master)
master
slaves
poll
data
data
ldquoTaking turnsrdquo MAC protocols
Link Layer 5-37
token passing control token passed
from one node to next sequentially
token message
concerns
token overhead
latency
single point of failure (token)
T
data
(nothing
to send)
T
ldquoTaking turnsrdquo MAC protocols
cable headend
CMTS
ISP
cable modem termination system
multiple 40Mbps downstream (broadcast) channels
single CMTS transmits into channels
multiple 30 Mbps upstream channels
multiple access all users contend for certain upstream channel time slots (others assigned)
Cable access network
cable modem
splitter
hellip
hellip
Internet framesTV channels control transmitted downstream at different frequencies
upstream Internet frames TV control transmitted upstream at different frequencies in time slots
Link Layer 5-39
DOCSIS data over cable service interface spec FDM over upstream downstream frequency channels
TDM upstream some slots assigned some have contention
downstream MAP frame assigns upstream slots
request for upstream slots (and data) transmitted random access (binary backoff) in selected slots
MAP frame for
Interval [t1 t2]
Residences with cable modems
Downstream channel i
Upstream channel j
t1 t2
Assigned minislots containing cable modem
upstream data frames
Minislots containing
minislots request frames
cable headend
CMTS
Cable access network
Link Layer 5-40
Summary of MAC protocols
channel partitioning by time frequency or code Time Division Frequency Division
random access (dynamic)
ALOHA S-ALOHA CSMA CSMACD
carrier sensing easy in some technologies (wire) hard in others (wireless)
CSMACD used in Ethernet
CSMACA used in 80211
taking turns
polling from central site token passing
bluetooth FDDI token ring
Link Layer 5-41
Link layer LANs outline
51 introduction services
52 error detection correction
53 multiple access protocols
54 LANs addressing ARP
Ethernet
switches
VLANS
55 link virtualization MPLS
56 data center networking
57 a day in the life of a web request
Link Layer 5-42
MAC addresses and ARP
32-bit IP address network-layer address for interface
used for layer 3 (network layer) forwarding
MAC (or LAN or physical or Ethernet) address function used lsquolocallyrdquo to get frame from one interface to
another physically-connected interface (same network in IP-addressing sense)
48 bit MAC address (for most LANs) burned in NIC ROM also sometimes software settable
eg 1A-2F-BB-76-09-AD
hexadecimal (base 16) notation
(each ldquonumberrdquo represents 4 bits)
Link Layer 5-43
LAN addresses and ARP
each adapter on LAN has unique LAN address
adapter
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN
(wired or
wireless)
Link Layer 5-44
LAN addresses (more)
MAC address allocation administered by IEEE
manufacturer buys portion of MAC address space (to assure uniqueness)
analogy MAC address like Social Security Number
IP address like postal address
MAC flat address portability can move LAN card from one LAN to another
IP hierarchical address not portable address depends on IP subnet to which node is
attached
Link Layer 5-45
ARP address resolution protocol
ARP table each IP node (host router) on LAN has table
IPMAC address mappings for some LAN nodes
lt IP address MAC address TTLgt
TTL (Time To Live) time after which address mapping will be forgotten (typically 20 min)
Question how to determine
interfacersquos MAC address
knowing its IP address
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN
137196723
137196778
137196714
137196788
Link Layer 5-46
ARP protocol same LAN
A wants to send datagram to B Brsquos MAC address not in
Arsquos ARP table
A broadcasts ARP query packet containing Bs IP address dest MAC address = FF-FF-
FF-FF-FF-FF
all nodes on LAN receive ARP query
B receives ARP packet replies to A with its (Bs) MAC address frame sent to Arsquos MAC
address (unicast)
A caches (saves) IP-to-MAC address pair in its ARP table until information becomes old (times out) soft state information that
times out (goes away) unless refreshed
ARP is ldquoplug-and-playrdquo nodes create their ARP
tables without intervention from net administrator
Link Layer 5-47
walkthrough send datagram from A to B via R
focus on addressing ndash at IP (datagram) and MAC layer (frame)
assume A knows Brsquos IP address
assume A knows IP address of first hop router R (how)
assume A knows Rrsquos MAC address (how)
Addressing routing to another LAN
R
1A-23-F9-CD-06-9B 222222222220
111111111110 E6-E9-00-17-BB-4B CC-49-DE-D0-AB-7D
111111111112
111111111111
74-29-9C-E8-FF-55
A
222222222222
49-BD-D2-C7-56-2A
222222222221 88-B2-2F-54-1A-0F
B
R
1A-23-F9-CD-06-9B 222222222220
111111111110 E6-E9-00-17-BB-4B CC-49-DE-D0-AB-7D
111111111112
111111111111
74-29-9C-E8-FF-55
A
222222222222
49-BD-D2-C7-56-2A
222222222221 88-B2-2F-54-1A-0F
B
Link Layer 5-48
Addressing routing to another LAN
IP
Eth
Phy
IP src 111111111111
IP dest 222222222222
A creates IP datagram with IP source A destination B
A creates link-layer frame with Rs MAC address as dest frame contains A-to-B IP datagram
MAC src 74-29-9C-E8-FF-55
MAC dest E6-E9-00-17-BB-4B
R
1A-23-F9-CD-06-9B 222222222220
111111111110 E6-E9-00-17-BB-4B CC-49-DE-D0-AB-7D
111111111112
111111111111
74-29-9C-E8-FF-55
A
222222222222
49-BD-D2-C7-56-2A
222222222221 88-B2-2F-54-1A-0F
B
Link Layer 5-49
Addressing routing to another LAN
IP
Eth
Phy
frame sent from A to R
IP
Eth
Phy
frame received at R datagram removed passed up to IP
MAC src 74-29-9C-E8-FF-55
MAC dest E6-E9-00-17-BB-4B
IP src 111111111111
IP dest 222222222222
IP src 111111111111
IP dest 222222222222
R
1A-23-F9-CD-06-9B 222222222220
111111111110 E6-E9-00-17-BB-4B CC-49-DE-D0-AB-7D
111111111112
111111111111
74-29-9C-E8-FF-55
A
222222222222
49-BD-D2-C7-56-2A
222222222221 88-B2-2F-54-1A-0F
B
Link Layer 5-50
Addressing routing to another LAN
IP src 111111111111
IP dest 222222222222
R forwards datagram with IP source A destination B
R creates link-layer frame with Bs MAC address as dest frame contains A-to-B IP datagram
MAC src 1A-23-F9-CD-06-9B
MAC dest 49-BD-D2-C7-56-2A
IP
Eth
Phy
IP
Eth
Phy
R
1A-23-F9-CD-06-9B 222222222220
111111111110 E6-E9-00-17-BB-4B CC-49-DE-D0-AB-7D
111111111112
111111111111
74-29-9C-E8-FF-55
A
222222222222
49-BD-D2-C7-56-2A
222222222221 88-B2-2F-54-1A-0F
B
Link Layer 5-51
Addressing routing to another LAN R forwards datagram with IP source A destination B
R creates link-layer frame with Bs MAC address as dest frame contains A-to-B IP datagram
IP src 111111111111
IP dest 222222222222
MAC src 1A-23-F9-CD-06-9B
MAC dest 49-BD-D2-C7-56-2A
IP
Eth
Phy
IP
Eth
Phy
R
1A-23-F9-CD-06-9B 222222222220
111111111110 E6-E9-00-17-BB-4B CC-49-DE-D0-AB-7D
111111111112
111111111111
74-29-9C-E8-FF-55
A
222222222222
49-BD-D2-C7-56-2A
222222222221 88-B2-2F-54-1A-0F
B
Link Layer 5-52
Addressing routing to another LAN R forwards datagram with IP source A destination B
R creates link-layer frame with Bs MAC address as dest frame contains A-to-B IP datagram
IP src 111111111111
IP dest 222222222222
MAC src 1A-23-F9-CD-06-9B
MAC dest 49-BD-D2-C7-56-2A
IP
Eth
Phy
Link Layer 5-53
Link layer LANs outline
51 introduction services
52 error detection correction
53 multiple access protocols
54 LANs addressing ARP
Ethernet
switches
VLANS
55 link virtualization MPLS
56 data center networking
57 a day in the life of a web request
Link Layer 5-54
Ethernet
ldquodominantrdquo wired LAN technology
cheap $20 for NIC
first widely used LAN technology
simpler cheaper than token LANs and ATM
kept up with speed race 10 Mbps ndash 10 Gbps
Metcalfersquos Ethernet sketch
Link Layer 5-55
Ethernet physical topology
bus popular through mid 90s all nodes in same collision domain (can collide with each
other)
star prevails today active switch in center
each ldquospokerdquo runs a (separate) Ethernet protocol (nodes
do not collide with each other)
switch
bus coaxial cable star
Link Layer 5-56
Ethernet frame structure
sending adapter encapsulates IP datagram (or other network layer protocol packet) in Ethernet frame
preamble
7 bytes with pattern 10101010 followed by one byte with pattern 10101011
used to synchronize receiver sender clock rates
dest address
source address
data (payload) CRC preamble
type
Link Layer 5-57
Ethernet frame structure (more)
addresses 6 byte source destination MAC addresses if adapter receives frame with matching destination
address or with broadcast address (eg ARP packet) it passes data in frame to network layer protocol
otherwise adapter discards frame
type indicates higher layer protocol (mostly IP but others possible eg Novell IPX AppleTalk)
CRC cyclic redundancy check at receiver error detected frame is dropped
dest address
source address
data (payload) CRC preamble
type
Link Layer 5-58
Ethernet unreliable connectionless
connectionless no handshaking between sending and receiving NICs
unreliable receiving NIC doesnt send acks or nacks to sending NIC
data in dropped frames recovered only if initial sender uses higher layer rdt (eg TCP) otherwise dropped data lost
Ethernetrsquos MAC protocol unslotted CSMACD wth binary backoff
Link Layer 5-59
8023 Ethernet standards link amp physical layers
many different Ethernet standards
common MAC protocol and frame format
different speeds 2 Mbps 10 Mbps 100 Mbps 1Gbps 10G bps
different physical layer media fiber cable
application
transport
network
link
physical
MAC protocol
and frame format
100BASE-TX
100BASE-T4
100BASE-FX 100BASE-T2
100BASE-SX 100BASE-BX
fiber physical layer copper (twister
pair) physical layer
Link Layer 5-60
Link layer LANs outline
51 introduction services
52 error detection correction
53 multiple access protocols
54 LANs addressing ARP
Ethernet
switches
VLANS
55 link virtualization MPLS
56 data center networking
57 a day in the life of a web request
Link Layer 5-61
Ethernet switch link-layer device takes an active role
store forward Ethernet frames
examine incoming framersquos MAC address selectively forward frame to one-or-more outgoing links when frame is to be forwarded on segment uses CSMACD to access segment
transparent
hosts are unaware of presence of switches
plug-and-play self-learning
switches do not need to be configured
Link Layer 5-62
Switch multiple simultaneous transmissions
hosts have dedicated direct connection to switch
switches buffer packets
Ethernet protocol used on each incoming link but no collisions full duplex
each link is its own collision domain
switching A-to-Arsquo and B-to-Brsquo can transmit simultaneously without collisions switch with six interfaces
(123456)
A
Arsquo
B
Brsquo C
Crsquo
1 2
3 4 5
6
Link Layer 5-63
Switch forwarding table
Q how does switch know Arsquo reachable via interface 4 Brsquo reachable via interface 5
switch with six interfaces
(123456)
A
Arsquo
B
Brsquo C
Crsquo
1 2
3 4 5
6 A each switch has a switch table each entry
(MAC address of host interface to
reach host time stamp)
looks like a routing table
Q how are entries created maintained in switch table
something like a routing protocol
A
Arsquo
B
Brsquo C
Crsquo
1 2
3 4 5
6
Link Layer 5-64
Switch self-learning
switch learns which hosts can be reached through which interfaces
when frame received switch ldquolearnsrdquo location of sender incoming LAN segment
records senderlocation pair in switch table
A Arsquo
Source A
Dest Arsquo
MAC addr interface TTL
Switch table
(initially empty) A 1 60
Link Layer 5-65
Switch frame filteringforwarding
when frame received at switch 1 record incoming link MAC address of sending host
2 index switch table using MAC destination address
3 if entry found for destination then
if destination on segment from which frame arrived then drop frame
else forward frame on interface indicated by entry
else flood forward on all interfaces except arriving
interface
A
Arsquo
B
Brsquo C
Crsquo
1 2
3 4 5
6
Link Layer 5-66
Self-learning forwarding example
A Arsquo
Source A
Dest Arsquo
MAC addr interface TTL
switch table
(initially empty) A 1 60
A Arsquo A Arsquo A Arsquo A Arsquo A Arsquo
frame destination Arsquo locaton unknown flood
Arsquo A
destination A location
known
Arsquo 4 60
selectively send
on just one link
Link Layer 5-67
Interconnecting switches
switches can be connected together
Q sending from A to G - how does S1 know to forward frame destined to F via S4 and S3
A self learning (works exactly the same as in single-switch case)
A
B
S1
C D
E
F
S2
S4
S3
H
I
G
Link Layer 5-68
Self-learning multi-switch example
Suppose C sends frame to I I responds to C
Q show switch tables and packet forwarding in S1 S2 S3 S4
A
B
S1
C D
E
F
S2
S4
S3
H
I
G
Link Layer 5-69
Institutional network
to external
network
router
IP subnet
mail server
web server
Link Layer 5-70
Switches vs routers
both are store-and-forward
routers network-layer devices (examine network-layer headers)
switches link-layer devices (examine link-layer headers)
both have forwarding tables
routers compute tables using routing algorithms IP addresses
switches learn forwarding table using flooding learning MAC addresses
application
transport
network
link
physical
network
link
physical
link
physical
switch
datagram
application
transport
network
link
physical
frame
frame
frame
datagram
Link Layer 5-71
VLANs motivation
consider CS user moves office to
EE but wants connect to CS switch
single broadcast domain
all layer-2 broadcast traffic (ARP DHCP unknown location of destination MAC address) must cross entire LAN
securityprivacy efficiency issues
Computer
Science Electrical
Engineering
Computer
Engineering
Link Layer 5-72
VLANs port-based VLAN switch ports
grouped (by switch management software) so that single physical switch helliphellip
switch(es) supporting
VLAN capabilities can
be configured to
define multiple virtual
LANS over single
physical LAN
infrastructure
Virtual Local
Area Network 1
8
9
16 10 2
7
hellip
Electrical Engineering
(VLAN ports 1-8)
Computer Science
(VLAN ports 9-15)
15
hellip
Electrical Engineering
(VLAN ports 1-8)
hellip
1
8 2
7 9
16 10
15
hellip
Computer Science
(VLAN ports 9-16)
hellip operates as multiple virtual switches
Link Layer 5-73
Port-based VLAN
1
8
9
16 10 2
7
hellip
Electrical Engineering
(VLAN ports 1-8)
Computer Science
(VLAN ports 9-15)
15
hellip
traffic isolation frames tofrom ports 1-8 can only reach ports 1-8 can also define VLAN based on
MAC addresses of endpoints rather than switch port
dynamic membership ports can be dynamically assigned among VLANs
router
forwarding between VLANS done via routing (just as with separate switches) in practice vendors sell combined
switches plus routers
Link Layer 5-74
VLANS spanning multiple switches
trunk port carries frames between VLANS defined over multiple physical switches frames forwarded within VLAN between switches canrsquot be vanilla
8021 frames (must carry VLAN ID info)
8021q protocol addsremoved additional header fields for frames forwarded between trunk ports
1
8
9
10 2
7
hellip
Electrical Engineering
(VLAN ports 1-8)
Computer Science
(VLAN ports 9-15)
15
hellip
2
7 3
Ports 235 belong to EE VLAN
Ports 4678 belong to CS VLAN
5
4 6 8 16
1
Link Layer 5-75
type
2-byte Tag Protocol Identifier
(value 81-00)
Tag Control Information (12 bit VLAN ID field
3 bit priority field like IP TOS)
Recomputed CRC
8021Q VLAN frame format
8021 frame
8021Q frame
dest address
source address
data (payload) CRC preamble
dest address
source address
preamble data (payload) CRC
type
Link Layer 5-76
Link layer LANs outline
51 introduction services
52 error detection correction
53 multiple access protocols
54 LANs addressing ARP
Ethernet
switches
VLANS
55 link virtualization MPLS
56 data center networking
57 a day in the life of a web request
Link Layer 5-77
Multiprotocol label switching (MPLS)
initial goal high-speed IP forwarding using fixed length label (instead of IP address) fast lookup using fixed length identifier (rather than
shortest prefix matching)
borrowing ideas from Virtual Circuit (VC) approach
but IP datagram still keeps IP address
PPP or Ethernet
header IP header remainder of link-layer frame MPLS header
label Exp S TTL
20 3 1 5
Link Layer 5-78
MPLS capable routers
aka label-switched router
forward packets to outgoing interface based only on label value (donrsquot inspect IP address) MPLS forwarding table distinct from IP forwarding tables
flexibility MPLS forwarding decisions can differ from those of IP use destination and source addresses to route flows to
same destination differently (traffic engineering)
re-route flows quickly if link fails pre-computed backup paths (useful for VoIP)
Link Layer 5-79
R2
D
R3 R5
A
R6
MPLS versus IP paths
IP router IP routing path to destination determined
by destination address alone
R4
Link Layer 5-80
R2
D
R3 R4
R5
A
R6
MPLS versus IP paths
IP-only router
IP routing path to destination determined by destination address alone
MPLS and IP router
MPLS routing path to destination can be based on source and dest address fast reroute precompute backup routes in
case of link failure
entry router (R4) can use different MPLS
routes to A based eg on source address
Link Layer 5-81
MPLS signaling
modify OSPF IS-IS link-state flooding protocols to carry info used by MPLS routing eg link bandwidth amount of ldquoreservedrdquo link bandwidth
D
R4
R5
A
R6
entry MPLS router uses RSVP-TE signaling protocol to set up MPLS forwarding at downstream routers
modified link state flooding
RSVP-TE
Link Layer 5-82
R1 R2
D
R3 R4 R5
0
1
0 0
A
R6
in out out
label label dest interface
6 - A 0
in out out
label label dest interface
10 6 A 1
12 9 D 0
in out out
label label dest interface
10 A 0
12 D 0
1
in out out
label label dest interface
8 6 A 0
0
8 A 1
MPLS forwarding tables
Link Layer 5-83
Link layer LANs outline
51 introduction services
52 error detection correction
53 multiple access protocols
54 LANs addressing ARP
Ethernet
switches
VLANS
55 link virtualization MPLS
56 data center networking
57 a day in the life of a web request
Link Layer 5-84
Data center networks
10rsquos to 100rsquos of thousands of hosts often closely coupled in close proximity e-business (eg Amazon)
content-servers (eg YouTube Akamai Apple Microsoft)
search engines data mining (eg Google)
challenges
multiple applications each serving massive numbers of clients
managingbalancing load avoiding processing networking data bottlenecks
Inside a 40-ft Microsoft container
Chicago data center
Link Layer 5-85
Server racks
TOR switches
Tier-1 switches
Tier-2 switches
Load balancer
Load balancer
B
1 2 3 4 5 6 7 8
A C
Border router
Access router
Internet
Data center networks
load balancer application-layer routing receives external client requests
directs workload within data center
returns results to external client (hiding data
center internals from client)
Server racks
TOR switches
Tier-1 switches
Tier-2 switches
1 2 3 4 5 6 7 8
Data center networks
rich interconnection among switches racks
increased throughput between racks (multiple routing
paths possible)
increased reliability via redundancy
Link Layer 5-87
Link layer LANs outline
51 introduction services
52 error detection correction
53 multiple access protocols
54 LANs addressing ARP
Ethernet
switches
VLANS
55 link virtualization MPLS
56 data center networking
57 a day in the life of a web request
Link Layer 5-88
Synthesis a day in the life of a web request
journey down protocol stack complete application transport network link
putting-it-all-together synthesis goal identify review understand protocols (at all
layers) involved in seemingly simple scenario requesting www page
scenario student attaches laptop to campus network requestsreceives wwwgooglecom
Link Layer 5-89
A day in the life scenario
Comcast network
68800013
Googlersquos network
64233160019 64233169105
web server
DNS server
school network
68802024
web page
browser
router
(runs DHCP)
Link Layer 5-90
A day in the lifehellip connecting to the Internet
connecting laptop needs to get its own IP address addr of first-hop router addr of DNS server use DHCP
DHCP
UDP
IP
Eth
Phy
DHCP
DHCP
DHCP
DHCP
DHCP
DHCP
UDP
IP
Eth
Phy
DHCP
DHCP
DHCP
DHCP DHCP
DHCP request encapsulated in UDP encapsulated in IP encapsulated in 8023 Ethernet
Ethernet frame broadcast
(dest FFFFFFFFFFFF) on LAN received at router running DHCP server
Ethernet demuxed to IP demuxed UDP demuxed to DHCP
router
(runs DHCP)
Link Layer 5-91
DHCP server formulates DHCP ACK containing clientrsquos IP address IP address of first-hop router for client name amp IP address of DNS server
DHCP
UDP
IP
Eth
Phy
DHCP
DHCP
DHCP
DHCP
DHCP
UDP
IP
Eth
Phy
DHCP
DHCP
DHCP
DHCP
DHCP
encapsulation at DHCP server frame forwarded (switch learning) through LAN demultiplexing at client
Client now has IP address knows name amp addr of DNS
server IP address of its first-hop router
DHCP client receives DHCP ACK reply
A day in the lifehellip connecting to the Internet
router
(runs DHCP)
Link Layer 5-92
A day in the lifehellip ARP (before DNS before HTTP)
before sending HTTP request need IP address of wwwgooglecom DNS
DNS
UDP
IP
Eth
Phy
DNS
DNS
DNS
DNS query created encapsulated in UDP encapsulated in IP encapsulated in Eth To send frame to router need MAC address of router interface ARP
ARP query broadcast received by
router which replies with ARP reply giving MAC address of router interface
client now knows MAC address of first hop router so can now send frame containing DNS query
ARP query
Eth
Phy
ARP
ARP
ARP reply
router
(runs DHCP)
Link Layer 5-93
DNS
UDP
IP
Eth
Phy
DNS
DNS
DNS
DNS
DNS
IP datagram containing DNS query forwarded via LAN switch from client to 1st hop router
IP datagram forwarded from campus network into comcast network routed (tables created by RIP OSPF IS-IS andor BGP routing protocols) to DNS server
demuxrsquoed to DNS server
DNS server replies to client with IP address of wwwgooglecom
Comcast network
68800013
DNS server
DNS
UDP
IP
Eth
Phy
DNS
DNS
DNS
DNS
A day in the lifehellip using DNS
router
(runs DHCP)
Link Layer 5-94
A day in the lifehellipTCP connection carrying HTTP
HTTP
TCP
IP
Eth
Phy
HTTP
to send HTTP request client first opens TCP socket to web server
TCP SYN segment (step 1 in 3-way handshake) inter-domain routed to web server
TCP connection established 64233169105
web server
SYN
SYN
SYN
SYN
TCP
IP
Eth
Phy
SYN
SYN
SYN
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
web server responds with TCP SYNACK (step 2 in 3-way handshake)
router
(runs DHCP)
Link Layer 5-95
A day in the lifehellip HTTP requestreply
HTTP
TCP
IP
Eth
Phy
HTTP
HTTP request sent into TCP socket
IP datagram containing HTTP request routed to wwwgooglecom
IP datagram containing HTTP reply routed back to client
64233169105
web server
HTTP
TCP
IP
Eth
Phy web server responds with
HTTP reply (containing web page)
HTTP
HTTP
HTTP HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
web page finally () displayed
Link Layer 5-96
Chapter 5 Summary
principles behind data link layer services error detection correction
sharing a broadcast channel multiple access
link layer addressing
instantiation and implementation of various link layer technologies Ethernet
switched LANS VLANs
virtualized networks as a link layer MPLS
synthesis a day in the life of a web request
Link Layer 5-97
Chapter 5 letrsquos take a breath
journey down protocol stack complete (except PHY)
solid understanding of networking principles practice
hellip could stop here hellip but lots of interesting topics wireless
multimedia
security
network management
Link Layer 5-7
flow control pacing between adjacent sending and receiving nodes
error detection errors caused by signal attenuation noise
receiver detects presence of errors
bull signals sender for retransmission or drops frame
error correction receiver identifies and corrects bit error(s) without resorting to
retransmission
half-duplex and full-duplex with half duplex nodes at both ends of link can transmit but not
at same time
Link layer services (more)
Link Layer 5-8
Where is the link layer implemented
in each and every host
link layer implemented in ldquoadaptorrdquo (aka network interface card NIC) or on a chip
Ethernet card 80211 card Ethernet chipset
implements link physical layer
attaches into hostrsquos system buses
combination of hardware software firmware
controller
physical
transmission
cpu memory
host
bus
(eg PCI)
network adapter
card
application
transport
network
link
link
physical
Link Layer 5-9
Adaptors communicating
sending side
encapsulates datagram in frame
adds error checking bits rdt flow control etc
receiving side
looks for errors rdt flow control etc
extracts datagram passes to upper layer at receiving side
controller controller
sending host receiving host
datagram datagram
datagram
frame
Link Layer 5-10
Link layer LANs outline
51 introduction services
52 error detection correction
53 multiple access protocols
54 LANs addressing ARP
Ethernet
switches
VLANS
55 link virtualization MPLS
56 data center networking
57 a day in the life of a web request
Link Layer 5-11
Error detection
EDC= Error Detection and Correction bits (redundancy)
D = Data protected by error checking may include header fields
bull Error detection not 100 reliable
bull protocol may miss some errors but rarely
bull larger EDC field yields better detection and correction
otherwise
Link Layer 5-12
Parity checking
single bit parity detect single bit
errors
two-dimensional bit parity detect and correct single bit errors
0 0
Link Layer 5-13
Internet checksum (review)
sender treat segment contents
as sequence of 16-bit integers
checksum addition (1rsquos complement sum) of segment contents
sender puts checksum value into UDP checksum field
receiver compute checksum of
received segment check if computed
checksum equals checksum field value NO - error detected YES - no error detected
But maybe errors nonetheless
goal detect ldquoerrorsrdquo (eg flipped bits) in transmitted packet (note used at transport layer only)
Link Layer 5-14
Cyclic redundancy check
more powerful error-detection coding
view data bits D as a binary number
choose r+1 bit pattern (generator) G
goal choose r CRC bits R such that ltDRgt exactly divisible by G (modulo 2)
receiver knows G divides ltDRgt by G If non-zero remainder error detected
can detect all burst errors less than r+1 bits
widely used in practice (Ethernet 80211 WiFi ATM)
Link Layer 5-15
CRC example
want D2r XOR R = nG
equivalently D2r = nG XOR R
equivalently if we divide D2r by
G want remainder R to satisfy
R = remainder[ ] D2r
G
1001 101110000 1001
1
101
01000
000 1010 1001
010 000
100 000 1000 0000 1000
D G
R
r = 3
Link Layer 5-16
Link layer LANs outline
51 introduction services
52 error detection correction
53 multiple access protocols
54 LANs addressing ARP
Ethernet
switches
VLANS
55 link virtualization MPLS
56 data center networking
57 a day in the life of a web request
Link Layer 5-17
Multiple access links protocols
two types of ldquolinksrdquo
point-to-point PPP for dial-up access
point-to-point link between Ethernet switch host
broadcast (shared wire or medium) old-fashioned Ethernet
upstream HFC
80211 wireless LAN
shared wire (eg cabled Ethernet)
shared RF (eg 80211 WiFi)
shared RF (satellite)
humans at a cocktail party
(shared air acoustical)
Link Layer 5-18
Multiple access protocols
single shared broadcast channel
two or more simultaneous transmissions by nodes interference
collision if node receives two or more signals at the same time
multiple access protocol distributed algorithm that determines how nodes share
channel ie determine when node can transmit
communication about channel sharing must use channel itself no out-of-band channel for coordination
Link Layer 5-19
An ideal multiple access protocol
given broadcast channel of rate R bps
desiderata 1 when one node wants to transmit it can send at rate R
2 when M nodes want to transmit each can send at average rate RM
3 fully decentralized
bull no special node to coordinate transmissions
bull no synchronization of clocks slots
4 simple
Link Layer 5-20
MAC protocols taxonomy
three broad classes
channel partitioning divide channel into smaller ldquopiecesrdquo (time slots frequency code)
allocate piece to node for exclusive use
random access channel not divided allow collisions
ldquorecoverrdquo from collisions
ldquotaking turnsrdquo nodes take turns but nodes with more to send can take longer
turns
Link Layer 5-21
Channel partitioning MAC protocols TDMA
TDMA time division multiple access access to channel in rounds each station gets fixed length slot (length = pkt
trans time) in each round unused slots go idle example 6-station LAN 134 have pkt slots
256 idle
1 3 4 1 3 4
6-slot
frame
6-slot
frame
Link Layer 5-22
FDMA frequency division multiple access channel spectrum divided into frequency bands
each station assigned fixed frequency band
unused transmission time in frequency bands go idle
example 6-station LAN 134 have pkt frequency bands 256 idle
frequency b
ands
FDM cable
Channel partitioning MAC protocols FDMA
Link Layer 5-23
Random access protocols
when node has packet to send transmit at full channel data rate R no a priori coordination among nodes
two or more transmitting nodes ldquocollisionrdquo random access MAC protocol specifies
how to detect collisions how to recover from collisions (eg via delayed
retransmissions)
examples of random access MAC protocols slotted ALOHA ALOHA CSMA CSMACD CSMACA
Link Layer 5-24
Slotted ALOHA
assumptions all frames same size
time divided into equal size slots (time to transmit 1 frame)
nodes start to transmit only slot beginning
nodes are synchronized
if 2 or more nodes transmit in slot all nodes detect collision
operation
when node obtains fresh frame transmits in next slot
if no collision node can send new frame in next slot
if collision node retransmits frame in each subsequent slot with prob p until success
Link Layer 5-25
Pros single active node can
continuously transmit at full rate of channel
highly decentralized only slots in nodes need to be in sync
simple
Cons
collisions wasting slots
idle slots
nodes may be able to detect collision in less than time to transmit packet
clock synchronization
Slotted ALOHA
1 1 1 1
2
3
2 2
3 3
node 1
node 2
node 3
C C C S S S E E E
Link Layer 5-26
suppose N nodes with many frames to send each transmits in slot with probability p
prob that given node has success in a slot = p(1-p)N-1
prob that any node has a success = Np(1-p)N-1
max efficiency find p that maximizes Np(1-p)N-1
for many nodes take limit of Np(1-p)N-1 as N goes to infinity gives
max efficiency = 1e = 37
efficiency long-run fraction of successful slots (many nodes all with many frames to send)
at best channel used for useful transmissions 37 of time
Slotted ALOHA efficiency
Link Layer 5-27
Pure (unslotted) ALOHA
unslotted Aloha simpler no synchronization
when frame first arrives
transmit immediately
collision probability increases
frame sent at t0 collides with other frames sent in [t0-1t0+1]
Link Layer 5-28
Pure ALOHA efficiency
P(success by given node) = P(node transmits)
P(no other node transmits in [t0-1t0]
P(no other node transmits in [t0-1t0]
= p (1-p)N-1 (1-p)N-1
= p (1-p)2(N-1)
hellip choosing optimum p and then letting n
= 1(2e) = 18
even worse than slotted Aloha
Link Layer 5-29
CSMA (carrier sense multiple access)
CSMA listen before transmit if channel sensed idle transmit entire frame
if channel sensed busy defer transmission
human analogy donrsquot interrupt others
Link Layer 5-30
CSMA collisions
collisions can still occur propagation delay means two nodes may not hear each otherrsquos transmission
collision entire packet transmission time wasted distance amp propagation
delay play role in in determining collision probability
spatial layout of nodes
Link Layer 5-31
CSMACD (collision detection)
CSMACD carrier sensing deferral as in CSMA collisions detected within short time
colliding transmissions aborted reducing channel wastage
collision detection easy in wired LANs measure signal strengths compare
transmitted received signals
difficult in wireless LANs received signal strength overwhelmed by local transmission strength
human analogy the polite conversationalist
Link Layer 5-32
CSMACD (collision detection)
spatial layout of nodes
Link Layer 5-33
Ethernet CSMACD algorithm
1 NIC receives datagram from network layer creates frame
2 If NIC senses channel idle starts frame transmission If NIC senses channel busy waits until channel idle then transmits
3 If NIC transmits entire frame without detecting another transmission NIC is done with frame
4 If NIC detects another transmission while transmitting aborts and sends jam signal
5 After aborting NIC enters binary (exponential) backoff
after mth collision NIC chooses K at random from 012 hellip 2m-1 NIC waits K512 bit times returns to Step 2
longer backoff interval with more collisions
Link Layer 5-34
CSMACD efficiency
Tprop = max prop delay between 2 nodes in LAN
ttrans = time to transmit max-size frame
efficiency goes to 1
as tprop goes to 0
as ttrans goes to infinity
better performance than ALOHA and simple cheap decentralized
transpropttefficiency
51
1
Link Layer 5-35
ldquoTaking turnsrdquo MAC protocols
channel partitioning MAC protocols share channel efficiently and fairly at high load
inefficient at low load delay in channel access 1N bandwidth allocated even if only 1 active node
random access MAC protocols efficient at low load single node can fully utilize
channel
high load collision overhead
ldquotaking turnsrdquo protocols look for best of both worlds
Link Layer 5-36
polling master node ldquoinvitesrdquo
slave nodes to transmit in turn
typically used with ldquodumbrdquo slave devices
concerns
polling overhead
latency
single point of failure (master)
master
slaves
poll
data
data
ldquoTaking turnsrdquo MAC protocols
Link Layer 5-37
token passing control token passed
from one node to next sequentially
token message
concerns
token overhead
latency
single point of failure (token)
T
data
(nothing
to send)
T
ldquoTaking turnsrdquo MAC protocols
cable headend
CMTS
ISP
cable modem termination system
multiple 40Mbps downstream (broadcast) channels
single CMTS transmits into channels
multiple 30 Mbps upstream channels
multiple access all users contend for certain upstream channel time slots (others assigned)
Cable access network
cable modem
splitter
hellip
hellip
Internet framesTV channels control transmitted downstream at different frequencies
upstream Internet frames TV control transmitted upstream at different frequencies in time slots
Link Layer 5-39
DOCSIS data over cable service interface spec FDM over upstream downstream frequency channels
TDM upstream some slots assigned some have contention
downstream MAP frame assigns upstream slots
request for upstream slots (and data) transmitted random access (binary backoff) in selected slots
MAP frame for
Interval [t1 t2]
Residences with cable modems
Downstream channel i
Upstream channel j
t1 t2
Assigned minislots containing cable modem
upstream data frames
Minislots containing
minislots request frames
cable headend
CMTS
Cable access network
Link Layer 5-40
Summary of MAC protocols
channel partitioning by time frequency or code Time Division Frequency Division
random access (dynamic)
ALOHA S-ALOHA CSMA CSMACD
carrier sensing easy in some technologies (wire) hard in others (wireless)
CSMACD used in Ethernet
CSMACA used in 80211
taking turns
polling from central site token passing
bluetooth FDDI token ring
Link Layer 5-41
Link layer LANs outline
51 introduction services
52 error detection correction
53 multiple access protocols
54 LANs addressing ARP
Ethernet
switches
VLANS
55 link virtualization MPLS
56 data center networking
57 a day in the life of a web request
Link Layer 5-42
MAC addresses and ARP
32-bit IP address network-layer address for interface
used for layer 3 (network layer) forwarding
MAC (or LAN or physical or Ethernet) address function used lsquolocallyrdquo to get frame from one interface to
another physically-connected interface (same network in IP-addressing sense)
48 bit MAC address (for most LANs) burned in NIC ROM also sometimes software settable
eg 1A-2F-BB-76-09-AD
hexadecimal (base 16) notation
(each ldquonumberrdquo represents 4 bits)
Link Layer 5-43
LAN addresses and ARP
each adapter on LAN has unique LAN address
adapter
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN
(wired or
wireless)
Link Layer 5-44
LAN addresses (more)
MAC address allocation administered by IEEE
manufacturer buys portion of MAC address space (to assure uniqueness)
analogy MAC address like Social Security Number
IP address like postal address
MAC flat address portability can move LAN card from one LAN to another
IP hierarchical address not portable address depends on IP subnet to which node is
attached
Link Layer 5-45
ARP address resolution protocol
ARP table each IP node (host router) on LAN has table
IPMAC address mappings for some LAN nodes
lt IP address MAC address TTLgt
TTL (Time To Live) time after which address mapping will be forgotten (typically 20 min)
Question how to determine
interfacersquos MAC address
knowing its IP address
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN
137196723
137196778
137196714
137196788
Link Layer 5-46
ARP protocol same LAN
A wants to send datagram to B Brsquos MAC address not in
Arsquos ARP table
A broadcasts ARP query packet containing Bs IP address dest MAC address = FF-FF-
FF-FF-FF-FF
all nodes on LAN receive ARP query
B receives ARP packet replies to A with its (Bs) MAC address frame sent to Arsquos MAC
address (unicast)
A caches (saves) IP-to-MAC address pair in its ARP table until information becomes old (times out) soft state information that
times out (goes away) unless refreshed
ARP is ldquoplug-and-playrdquo nodes create their ARP
tables without intervention from net administrator
Link Layer 5-47
walkthrough send datagram from A to B via R
focus on addressing ndash at IP (datagram) and MAC layer (frame)
assume A knows Brsquos IP address
assume A knows IP address of first hop router R (how)
assume A knows Rrsquos MAC address (how)
Addressing routing to another LAN
R
1A-23-F9-CD-06-9B 222222222220
111111111110 E6-E9-00-17-BB-4B CC-49-DE-D0-AB-7D
111111111112
111111111111
74-29-9C-E8-FF-55
A
222222222222
49-BD-D2-C7-56-2A
222222222221 88-B2-2F-54-1A-0F
B
R
1A-23-F9-CD-06-9B 222222222220
111111111110 E6-E9-00-17-BB-4B CC-49-DE-D0-AB-7D
111111111112
111111111111
74-29-9C-E8-FF-55
A
222222222222
49-BD-D2-C7-56-2A
222222222221 88-B2-2F-54-1A-0F
B
Link Layer 5-48
Addressing routing to another LAN
IP
Eth
Phy
IP src 111111111111
IP dest 222222222222
A creates IP datagram with IP source A destination B
A creates link-layer frame with Rs MAC address as dest frame contains A-to-B IP datagram
MAC src 74-29-9C-E8-FF-55
MAC dest E6-E9-00-17-BB-4B
R
1A-23-F9-CD-06-9B 222222222220
111111111110 E6-E9-00-17-BB-4B CC-49-DE-D0-AB-7D
111111111112
111111111111
74-29-9C-E8-FF-55
A
222222222222
49-BD-D2-C7-56-2A
222222222221 88-B2-2F-54-1A-0F
B
Link Layer 5-49
Addressing routing to another LAN
IP
Eth
Phy
frame sent from A to R
IP
Eth
Phy
frame received at R datagram removed passed up to IP
MAC src 74-29-9C-E8-FF-55
MAC dest E6-E9-00-17-BB-4B
IP src 111111111111
IP dest 222222222222
IP src 111111111111
IP dest 222222222222
R
1A-23-F9-CD-06-9B 222222222220
111111111110 E6-E9-00-17-BB-4B CC-49-DE-D0-AB-7D
111111111112
111111111111
74-29-9C-E8-FF-55
A
222222222222
49-BD-D2-C7-56-2A
222222222221 88-B2-2F-54-1A-0F
B
Link Layer 5-50
Addressing routing to another LAN
IP src 111111111111
IP dest 222222222222
R forwards datagram with IP source A destination B
R creates link-layer frame with Bs MAC address as dest frame contains A-to-B IP datagram
MAC src 1A-23-F9-CD-06-9B
MAC dest 49-BD-D2-C7-56-2A
IP
Eth
Phy
IP
Eth
Phy
R
1A-23-F9-CD-06-9B 222222222220
111111111110 E6-E9-00-17-BB-4B CC-49-DE-D0-AB-7D
111111111112
111111111111
74-29-9C-E8-FF-55
A
222222222222
49-BD-D2-C7-56-2A
222222222221 88-B2-2F-54-1A-0F
B
Link Layer 5-51
Addressing routing to another LAN R forwards datagram with IP source A destination B
R creates link-layer frame with Bs MAC address as dest frame contains A-to-B IP datagram
IP src 111111111111
IP dest 222222222222
MAC src 1A-23-F9-CD-06-9B
MAC dest 49-BD-D2-C7-56-2A
IP
Eth
Phy
IP
Eth
Phy
R
1A-23-F9-CD-06-9B 222222222220
111111111110 E6-E9-00-17-BB-4B CC-49-DE-D0-AB-7D
111111111112
111111111111
74-29-9C-E8-FF-55
A
222222222222
49-BD-D2-C7-56-2A
222222222221 88-B2-2F-54-1A-0F
B
Link Layer 5-52
Addressing routing to another LAN R forwards datagram with IP source A destination B
R creates link-layer frame with Bs MAC address as dest frame contains A-to-B IP datagram
IP src 111111111111
IP dest 222222222222
MAC src 1A-23-F9-CD-06-9B
MAC dest 49-BD-D2-C7-56-2A
IP
Eth
Phy
Link Layer 5-53
Link layer LANs outline
51 introduction services
52 error detection correction
53 multiple access protocols
54 LANs addressing ARP
Ethernet
switches
VLANS
55 link virtualization MPLS
56 data center networking
57 a day in the life of a web request
Link Layer 5-54
Ethernet
ldquodominantrdquo wired LAN technology
cheap $20 for NIC
first widely used LAN technology
simpler cheaper than token LANs and ATM
kept up with speed race 10 Mbps ndash 10 Gbps
Metcalfersquos Ethernet sketch
Link Layer 5-55
Ethernet physical topology
bus popular through mid 90s all nodes in same collision domain (can collide with each
other)
star prevails today active switch in center
each ldquospokerdquo runs a (separate) Ethernet protocol (nodes
do not collide with each other)
switch
bus coaxial cable star
Link Layer 5-56
Ethernet frame structure
sending adapter encapsulates IP datagram (or other network layer protocol packet) in Ethernet frame
preamble
7 bytes with pattern 10101010 followed by one byte with pattern 10101011
used to synchronize receiver sender clock rates
dest address
source address
data (payload) CRC preamble
type
Link Layer 5-57
Ethernet frame structure (more)
addresses 6 byte source destination MAC addresses if adapter receives frame with matching destination
address or with broadcast address (eg ARP packet) it passes data in frame to network layer protocol
otherwise adapter discards frame
type indicates higher layer protocol (mostly IP but others possible eg Novell IPX AppleTalk)
CRC cyclic redundancy check at receiver error detected frame is dropped
dest address
source address
data (payload) CRC preamble
type
Link Layer 5-58
Ethernet unreliable connectionless
connectionless no handshaking between sending and receiving NICs
unreliable receiving NIC doesnt send acks or nacks to sending NIC
data in dropped frames recovered only if initial sender uses higher layer rdt (eg TCP) otherwise dropped data lost
Ethernetrsquos MAC protocol unslotted CSMACD wth binary backoff
Link Layer 5-59
8023 Ethernet standards link amp physical layers
many different Ethernet standards
common MAC protocol and frame format
different speeds 2 Mbps 10 Mbps 100 Mbps 1Gbps 10G bps
different physical layer media fiber cable
application
transport
network
link
physical
MAC protocol
and frame format
100BASE-TX
100BASE-T4
100BASE-FX 100BASE-T2
100BASE-SX 100BASE-BX
fiber physical layer copper (twister
pair) physical layer
Link Layer 5-60
Link layer LANs outline
51 introduction services
52 error detection correction
53 multiple access protocols
54 LANs addressing ARP
Ethernet
switches
VLANS
55 link virtualization MPLS
56 data center networking
57 a day in the life of a web request
Link Layer 5-61
Ethernet switch link-layer device takes an active role
store forward Ethernet frames
examine incoming framersquos MAC address selectively forward frame to one-or-more outgoing links when frame is to be forwarded on segment uses CSMACD to access segment
transparent
hosts are unaware of presence of switches
plug-and-play self-learning
switches do not need to be configured
Link Layer 5-62
Switch multiple simultaneous transmissions
hosts have dedicated direct connection to switch
switches buffer packets
Ethernet protocol used on each incoming link but no collisions full duplex
each link is its own collision domain
switching A-to-Arsquo and B-to-Brsquo can transmit simultaneously without collisions switch with six interfaces
(123456)
A
Arsquo
B
Brsquo C
Crsquo
1 2
3 4 5
6
Link Layer 5-63
Switch forwarding table
Q how does switch know Arsquo reachable via interface 4 Brsquo reachable via interface 5
switch with six interfaces
(123456)
A
Arsquo
B
Brsquo C
Crsquo
1 2
3 4 5
6 A each switch has a switch table each entry
(MAC address of host interface to
reach host time stamp)
looks like a routing table
Q how are entries created maintained in switch table
something like a routing protocol
A
Arsquo
B
Brsquo C
Crsquo
1 2
3 4 5
6
Link Layer 5-64
Switch self-learning
switch learns which hosts can be reached through which interfaces
when frame received switch ldquolearnsrdquo location of sender incoming LAN segment
records senderlocation pair in switch table
A Arsquo
Source A
Dest Arsquo
MAC addr interface TTL
Switch table
(initially empty) A 1 60
Link Layer 5-65
Switch frame filteringforwarding
when frame received at switch 1 record incoming link MAC address of sending host
2 index switch table using MAC destination address
3 if entry found for destination then
if destination on segment from which frame arrived then drop frame
else forward frame on interface indicated by entry
else flood forward on all interfaces except arriving
interface
A
Arsquo
B
Brsquo C
Crsquo
1 2
3 4 5
6
Link Layer 5-66
Self-learning forwarding example
A Arsquo
Source A
Dest Arsquo
MAC addr interface TTL
switch table
(initially empty) A 1 60
A Arsquo A Arsquo A Arsquo A Arsquo A Arsquo
frame destination Arsquo locaton unknown flood
Arsquo A
destination A location
known
Arsquo 4 60
selectively send
on just one link
Link Layer 5-67
Interconnecting switches
switches can be connected together
Q sending from A to G - how does S1 know to forward frame destined to F via S4 and S3
A self learning (works exactly the same as in single-switch case)
A
B
S1
C D
E
F
S2
S4
S3
H
I
G
Link Layer 5-68
Self-learning multi-switch example
Suppose C sends frame to I I responds to C
Q show switch tables and packet forwarding in S1 S2 S3 S4
A
B
S1
C D
E
F
S2
S4
S3
H
I
G
Link Layer 5-69
Institutional network
to external
network
router
IP subnet
mail server
web server
Link Layer 5-70
Switches vs routers
both are store-and-forward
routers network-layer devices (examine network-layer headers)
switches link-layer devices (examine link-layer headers)
both have forwarding tables
routers compute tables using routing algorithms IP addresses
switches learn forwarding table using flooding learning MAC addresses
application
transport
network
link
physical
network
link
physical
link
physical
switch
datagram
application
transport
network
link
physical
frame
frame
frame
datagram
Link Layer 5-71
VLANs motivation
consider CS user moves office to
EE but wants connect to CS switch
single broadcast domain
all layer-2 broadcast traffic (ARP DHCP unknown location of destination MAC address) must cross entire LAN
securityprivacy efficiency issues
Computer
Science Electrical
Engineering
Computer
Engineering
Link Layer 5-72
VLANs port-based VLAN switch ports
grouped (by switch management software) so that single physical switch helliphellip
switch(es) supporting
VLAN capabilities can
be configured to
define multiple virtual
LANS over single
physical LAN
infrastructure
Virtual Local
Area Network 1
8
9
16 10 2
7
hellip
Electrical Engineering
(VLAN ports 1-8)
Computer Science
(VLAN ports 9-15)
15
hellip
Electrical Engineering
(VLAN ports 1-8)
hellip
1
8 2
7 9
16 10
15
hellip
Computer Science
(VLAN ports 9-16)
hellip operates as multiple virtual switches
Link Layer 5-73
Port-based VLAN
1
8
9
16 10 2
7
hellip
Electrical Engineering
(VLAN ports 1-8)
Computer Science
(VLAN ports 9-15)
15
hellip
traffic isolation frames tofrom ports 1-8 can only reach ports 1-8 can also define VLAN based on
MAC addresses of endpoints rather than switch port
dynamic membership ports can be dynamically assigned among VLANs
router
forwarding between VLANS done via routing (just as with separate switches) in practice vendors sell combined
switches plus routers
Link Layer 5-74
VLANS spanning multiple switches
trunk port carries frames between VLANS defined over multiple physical switches frames forwarded within VLAN between switches canrsquot be vanilla
8021 frames (must carry VLAN ID info)
8021q protocol addsremoved additional header fields for frames forwarded between trunk ports
1
8
9
10 2
7
hellip
Electrical Engineering
(VLAN ports 1-8)
Computer Science
(VLAN ports 9-15)
15
hellip
2
7 3
Ports 235 belong to EE VLAN
Ports 4678 belong to CS VLAN
5
4 6 8 16
1
Link Layer 5-75
type
2-byte Tag Protocol Identifier
(value 81-00)
Tag Control Information (12 bit VLAN ID field
3 bit priority field like IP TOS)
Recomputed CRC
8021Q VLAN frame format
8021 frame
8021Q frame
dest address
source address
data (payload) CRC preamble
dest address
source address
preamble data (payload) CRC
type
Link Layer 5-76
Link layer LANs outline
51 introduction services
52 error detection correction
53 multiple access protocols
54 LANs addressing ARP
Ethernet
switches
VLANS
55 link virtualization MPLS
56 data center networking
57 a day in the life of a web request
Link Layer 5-77
Multiprotocol label switching (MPLS)
initial goal high-speed IP forwarding using fixed length label (instead of IP address) fast lookup using fixed length identifier (rather than
shortest prefix matching)
borrowing ideas from Virtual Circuit (VC) approach
but IP datagram still keeps IP address
PPP or Ethernet
header IP header remainder of link-layer frame MPLS header
label Exp S TTL
20 3 1 5
Link Layer 5-78
MPLS capable routers
aka label-switched router
forward packets to outgoing interface based only on label value (donrsquot inspect IP address) MPLS forwarding table distinct from IP forwarding tables
flexibility MPLS forwarding decisions can differ from those of IP use destination and source addresses to route flows to
same destination differently (traffic engineering)
re-route flows quickly if link fails pre-computed backup paths (useful for VoIP)
Link Layer 5-79
R2
D
R3 R5
A
R6
MPLS versus IP paths
IP router IP routing path to destination determined
by destination address alone
R4
Link Layer 5-80
R2
D
R3 R4
R5
A
R6
MPLS versus IP paths
IP-only router
IP routing path to destination determined by destination address alone
MPLS and IP router
MPLS routing path to destination can be based on source and dest address fast reroute precompute backup routes in
case of link failure
entry router (R4) can use different MPLS
routes to A based eg on source address
Link Layer 5-81
MPLS signaling
modify OSPF IS-IS link-state flooding protocols to carry info used by MPLS routing eg link bandwidth amount of ldquoreservedrdquo link bandwidth
D
R4
R5
A
R6
entry MPLS router uses RSVP-TE signaling protocol to set up MPLS forwarding at downstream routers
modified link state flooding
RSVP-TE
Link Layer 5-82
R1 R2
D
R3 R4 R5
0
1
0 0
A
R6
in out out
label label dest interface
6 - A 0
in out out
label label dest interface
10 6 A 1
12 9 D 0
in out out
label label dest interface
10 A 0
12 D 0
1
in out out
label label dest interface
8 6 A 0
0
8 A 1
MPLS forwarding tables
Link Layer 5-83
Link layer LANs outline
51 introduction services
52 error detection correction
53 multiple access protocols
54 LANs addressing ARP
Ethernet
switches
VLANS
55 link virtualization MPLS
56 data center networking
57 a day in the life of a web request
Link Layer 5-84
Data center networks
10rsquos to 100rsquos of thousands of hosts often closely coupled in close proximity e-business (eg Amazon)
content-servers (eg YouTube Akamai Apple Microsoft)
search engines data mining (eg Google)
challenges
multiple applications each serving massive numbers of clients
managingbalancing load avoiding processing networking data bottlenecks
Inside a 40-ft Microsoft container
Chicago data center
Link Layer 5-85
Server racks
TOR switches
Tier-1 switches
Tier-2 switches
Load balancer
Load balancer
B
1 2 3 4 5 6 7 8
A C
Border router
Access router
Internet
Data center networks
load balancer application-layer routing receives external client requests
directs workload within data center
returns results to external client (hiding data
center internals from client)
Server racks
TOR switches
Tier-1 switches
Tier-2 switches
1 2 3 4 5 6 7 8
Data center networks
rich interconnection among switches racks
increased throughput between racks (multiple routing
paths possible)
increased reliability via redundancy
Link Layer 5-87
Link layer LANs outline
51 introduction services
52 error detection correction
53 multiple access protocols
54 LANs addressing ARP
Ethernet
switches
VLANS
55 link virtualization MPLS
56 data center networking
57 a day in the life of a web request
Link Layer 5-88
Synthesis a day in the life of a web request
journey down protocol stack complete application transport network link
putting-it-all-together synthesis goal identify review understand protocols (at all
layers) involved in seemingly simple scenario requesting www page
scenario student attaches laptop to campus network requestsreceives wwwgooglecom
Link Layer 5-89
A day in the life scenario
Comcast network
68800013
Googlersquos network
64233160019 64233169105
web server
DNS server
school network
68802024
web page
browser
router
(runs DHCP)
Link Layer 5-90
A day in the lifehellip connecting to the Internet
connecting laptop needs to get its own IP address addr of first-hop router addr of DNS server use DHCP
DHCP
UDP
IP
Eth
Phy
DHCP
DHCP
DHCP
DHCP
DHCP
DHCP
UDP
IP
Eth
Phy
DHCP
DHCP
DHCP
DHCP DHCP
DHCP request encapsulated in UDP encapsulated in IP encapsulated in 8023 Ethernet
Ethernet frame broadcast
(dest FFFFFFFFFFFF) on LAN received at router running DHCP server
Ethernet demuxed to IP demuxed UDP demuxed to DHCP
router
(runs DHCP)
Link Layer 5-91
DHCP server formulates DHCP ACK containing clientrsquos IP address IP address of first-hop router for client name amp IP address of DNS server
DHCP
UDP
IP
Eth
Phy
DHCP
DHCP
DHCP
DHCP
DHCP
UDP
IP
Eth
Phy
DHCP
DHCP
DHCP
DHCP
DHCP
encapsulation at DHCP server frame forwarded (switch learning) through LAN demultiplexing at client
Client now has IP address knows name amp addr of DNS
server IP address of its first-hop router
DHCP client receives DHCP ACK reply
A day in the lifehellip connecting to the Internet
router
(runs DHCP)
Link Layer 5-92
A day in the lifehellip ARP (before DNS before HTTP)
before sending HTTP request need IP address of wwwgooglecom DNS
DNS
UDP
IP
Eth
Phy
DNS
DNS
DNS
DNS query created encapsulated in UDP encapsulated in IP encapsulated in Eth To send frame to router need MAC address of router interface ARP
ARP query broadcast received by
router which replies with ARP reply giving MAC address of router interface
client now knows MAC address of first hop router so can now send frame containing DNS query
ARP query
Eth
Phy
ARP
ARP
ARP reply
router
(runs DHCP)
Link Layer 5-93
DNS
UDP
IP
Eth
Phy
DNS
DNS
DNS
DNS
DNS
IP datagram containing DNS query forwarded via LAN switch from client to 1st hop router
IP datagram forwarded from campus network into comcast network routed (tables created by RIP OSPF IS-IS andor BGP routing protocols) to DNS server
demuxrsquoed to DNS server
DNS server replies to client with IP address of wwwgooglecom
Comcast network
68800013
DNS server
DNS
UDP
IP
Eth
Phy
DNS
DNS
DNS
DNS
A day in the lifehellip using DNS
router
(runs DHCP)
Link Layer 5-94
A day in the lifehellipTCP connection carrying HTTP
HTTP
TCP
IP
Eth
Phy
HTTP
to send HTTP request client first opens TCP socket to web server
TCP SYN segment (step 1 in 3-way handshake) inter-domain routed to web server
TCP connection established 64233169105
web server
SYN
SYN
SYN
SYN
TCP
IP
Eth
Phy
SYN
SYN
SYN
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
web server responds with TCP SYNACK (step 2 in 3-way handshake)
router
(runs DHCP)
Link Layer 5-95
A day in the lifehellip HTTP requestreply
HTTP
TCP
IP
Eth
Phy
HTTP
HTTP request sent into TCP socket
IP datagram containing HTTP request routed to wwwgooglecom
IP datagram containing HTTP reply routed back to client
64233169105
web server
HTTP
TCP
IP
Eth
Phy web server responds with
HTTP reply (containing web page)
HTTP
HTTP
HTTP HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
web page finally () displayed
Link Layer 5-96
Chapter 5 Summary
principles behind data link layer services error detection correction
sharing a broadcast channel multiple access
link layer addressing
instantiation and implementation of various link layer technologies Ethernet
switched LANS VLANs
virtualized networks as a link layer MPLS
synthesis a day in the life of a web request
Link Layer 5-97
Chapter 5 letrsquos take a breath
journey down protocol stack complete (except PHY)
solid understanding of networking principles practice
hellip could stop here hellip but lots of interesting topics wireless
multimedia
security
network management
Link Layer 5-8
Where is the link layer implemented
in each and every host
link layer implemented in ldquoadaptorrdquo (aka network interface card NIC) or on a chip
Ethernet card 80211 card Ethernet chipset
implements link physical layer
attaches into hostrsquos system buses
combination of hardware software firmware
controller
physical
transmission
cpu memory
host
bus
(eg PCI)
network adapter
card
application
transport
network
link
link
physical
Link Layer 5-9
Adaptors communicating
sending side
encapsulates datagram in frame
adds error checking bits rdt flow control etc
receiving side
looks for errors rdt flow control etc
extracts datagram passes to upper layer at receiving side
controller controller
sending host receiving host
datagram datagram
datagram
frame
Link Layer 5-10
Link layer LANs outline
51 introduction services
52 error detection correction
53 multiple access protocols
54 LANs addressing ARP
Ethernet
switches
VLANS
55 link virtualization MPLS
56 data center networking
57 a day in the life of a web request
Link Layer 5-11
Error detection
EDC= Error Detection and Correction bits (redundancy)
D = Data protected by error checking may include header fields
bull Error detection not 100 reliable
bull protocol may miss some errors but rarely
bull larger EDC field yields better detection and correction
otherwise
Link Layer 5-12
Parity checking
single bit parity detect single bit
errors
two-dimensional bit parity detect and correct single bit errors
0 0
Link Layer 5-13
Internet checksum (review)
sender treat segment contents
as sequence of 16-bit integers
checksum addition (1rsquos complement sum) of segment contents
sender puts checksum value into UDP checksum field
receiver compute checksum of
received segment check if computed
checksum equals checksum field value NO - error detected YES - no error detected
But maybe errors nonetheless
goal detect ldquoerrorsrdquo (eg flipped bits) in transmitted packet (note used at transport layer only)
Link Layer 5-14
Cyclic redundancy check
more powerful error-detection coding
view data bits D as a binary number
choose r+1 bit pattern (generator) G
goal choose r CRC bits R such that ltDRgt exactly divisible by G (modulo 2)
receiver knows G divides ltDRgt by G If non-zero remainder error detected
can detect all burst errors less than r+1 bits
widely used in practice (Ethernet 80211 WiFi ATM)
Link Layer 5-15
CRC example
want D2r XOR R = nG
equivalently D2r = nG XOR R
equivalently if we divide D2r by
G want remainder R to satisfy
R = remainder[ ] D2r
G
1001 101110000 1001
1
101
01000
000 1010 1001
010 000
100 000 1000 0000 1000
D G
R
r = 3
Link Layer 5-16
Link layer LANs outline
51 introduction services
52 error detection correction
53 multiple access protocols
54 LANs addressing ARP
Ethernet
switches
VLANS
55 link virtualization MPLS
56 data center networking
57 a day in the life of a web request
Link Layer 5-17
Multiple access links protocols
two types of ldquolinksrdquo
point-to-point PPP for dial-up access
point-to-point link between Ethernet switch host
broadcast (shared wire or medium) old-fashioned Ethernet
upstream HFC
80211 wireless LAN
shared wire (eg cabled Ethernet)
shared RF (eg 80211 WiFi)
shared RF (satellite)
humans at a cocktail party
(shared air acoustical)
Link Layer 5-18
Multiple access protocols
single shared broadcast channel
two or more simultaneous transmissions by nodes interference
collision if node receives two or more signals at the same time
multiple access protocol distributed algorithm that determines how nodes share
channel ie determine when node can transmit
communication about channel sharing must use channel itself no out-of-band channel for coordination
Link Layer 5-19
An ideal multiple access protocol
given broadcast channel of rate R bps
desiderata 1 when one node wants to transmit it can send at rate R
2 when M nodes want to transmit each can send at average rate RM
3 fully decentralized
bull no special node to coordinate transmissions
bull no synchronization of clocks slots
4 simple
Link Layer 5-20
MAC protocols taxonomy
three broad classes
channel partitioning divide channel into smaller ldquopiecesrdquo (time slots frequency code)
allocate piece to node for exclusive use
random access channel not divided allow collisions
ldquorecoverrdquo from collisions
ldquotaking turnsrdquo nodes take turns but nodes with more to send can take longer
turns
Link Layer 5-21
Channel partitioning MAC protocols TDMA
TDMA time division multiple access access to channel in rounds each station gets fixed length slot (length = pkt
trans time) in each round unused slots go idle example 6-station LAN 134 have pkt slots
256 idle
1 3 4 1 3 4
6-slot
frame
6-slot
frame
Link Layer 5-22
FDMA frequency division multiple access channel spectrum divided into frequency bands
each station assigned fixed frequency band
unused transmission time in frequency bands go idle
example 6-station LAN 134 have pkt frequency bands 256 idle
frequency b
ands
FDM cable
Channel partitioning MAC protocols FDMA
Link Layer 5-23
Random access protocols
when node has packet to send transmit at full channel data rate R no a priori coordination among nodes
two or more transmitting nodes ldquocollisionrdquo random access MAC protocol specifies
how to detect collisions how to recover from collisions (eg via delayed
retransmissions)
examples of random access MAC protocols slotted ALOHA ALOHA CSMA CSMACD CSMACA
Link Layer 5-24
Slotted ALOHA
assumptions all frames same size
time divided into equal size slots (time to transmit 1 frame)
nodes start to transmit only slot beginning
nodes are synchronized
if 2 or more nodes transmit in slot all nodes detect collision
operation
when node obtains fresh frame transmits in next slot
if no collision node can send new frame in next slot
if collision node retransmits frame in each subsequent slot with prob p until success
Link Layer 5-25
Pros single active node can
continuously transmit at full rate of channel
highly decentralized only slots in nodes need to be in sync
simple
Cons
collisions wasting slots
idle slots
nodes may be able to detect collision in less than time to transmit packet
clock synchronization
Slotted ALOHA
1 1 1 1
2
3
2 2
3 3
node 1
node 2
node 3
C C C S S S E E E
Link Layer 5-26
suppose N nodes with many frames to send each transmits in slot with probability p
prob that given node has success in a slot = p(1-p)N-1
prob that any node has a success = Np(1-p)N-1
max efficiency find p that maximizes Np(1-p)N-1
for many nodes take limit of Np(1-p)N-1 as N goes to infinity gives
max efficiency = 1e = 37
efficiency long-run fraction of successful slots (many nodes all with many frames to send)
at best channel used for useful transmissions 37 of time
Slotted ALOHA efficiency
Link Layer 5-27
Pure (unslotted) ALOHA
unslotted Aloha simpler no synchronization
when frame first arrives
transmit immediately
collision probability increases
frame sent at t0 collides with other frames sent in [t0-1t0+1]
Link Layer 5-28
Pure ALOHA efficiency
P(success by given node) = P(node transmits)
P(no other node transmits in [t0-1t0]
P(no other node transmits in [t0-1t0]
= p (1-p)N-1 (1-p)N-1
= p (1-p)2(N-1)
hellip choosing optimum p and then letting n
= 1(2e) = 18
even worse than slotted Aloha
Link Layer 5-29
CSMA (carrier sense multiple access)
CSMA listen before transmit if channel sensed idle transmit entire frame
if channel sensed busy defer transmission
human analogy donrsquot interrupt others
Link Layer 5-30
CSMA collisions
collisions can still occur propagation delay means two nodes may not hear each otherrsquos transmission
collision entire packet transmission time wasted distance amp propagation
delay play role in in determining collision probability
spatial layout of nodes
Link Layer 5-31
CSMACD (collision detection)
CSMACD carrier sensing deferral as in CSMA collisions detected within short time
colliding transmissions aborted reducing channel wastage
collision detection easy in wired LANs measure signal strengths compare
transmitted received signals
difficult in wireless LANs received signal strength overwhelmed by local transmission strength
human analogy the polite conversationalist
Link Layer 5-32
CSMACD (collision detection)
spatial layout of nodes
Link Layer 5-33
Ethernet CSMACD algorithm
1 NIC receives datagram from network layer creates frame
2 If NIC senses channel idle starts frame transmission If NIC senses channel busy waits until channel idle then transmits
3 If NIC transmits entire frame without detecting another transmission NIC is done with frame
4 If NIC detects another transmission while transmitting aborts and sends jam signal
5 After aborting NIC enters binary (exponential) backoff
after mth collision NIC chooses K at random from 012 hellip 2m-1 NIC waits K512 bit times returns to Step 2
longer backoff interval with more collisions
Link Layer 5-34
CSMACD efficiency
Tprop = max prop delay between 2 nodes in LAN
ttrans = time to transmit max-size frame
efficiency goes to 1
as tprop goes to 0
as ttrans goes to infinity
better performance than ALOHA and simple cheap decentralized
transpropttefficiency
51
1
Link Layer 5-35
ldquoTaking turnsrdquo MAC protocols
channel partitioning MAC protocols share channel efficiently and fairly at high load
inefficient at low load delay in channel access 1N bandwidth allocated even if only 1 active node
random access MAC protocols efficient at low load single node can fully utilize
channel
high load collision overhead
ldquotaking turnsrdquo protocols look for best of both worlds
Link Layer 5-36
polling master node ldquoinvitesrdquo
slave nodes to transmit in turn
typically used with ldquodumbrdquo slave devices
concerns
polling overhead
latency
single point of failure (master)
master
slaves
poll
data
data
ldquoTaking turnsrdquo MAC protocols
Link Layer 5-37
token passing control token passed
from one node to next sequentially
token message
concerns
token overhead
latency
single point of failure (token)
T
data
(nothing
to send)
T
ldquoTaking turnsrdquo MAC protocols
cable headend
CMTS
ISP
cable modem termination system
multiple 40Mbps downstream (broadcast) channels
single CMTS transmits into channels
multiple 30 Mbps upstream channels
multiple access all users contend for certain upstream channel time slots (others assigned)
Cable access network
cable modem
splitter
hellip
hellip
Internet framesTV channels control transmitted downstream at different frequencies
upstream Internet frames TV control transmitted upstream at different frequencies in time slots
Link Layer 5-39
DOCSIS data over cable service interface spec FDM over upstream downstream frequency channels
TDM upstream some slots assigned some have contention
downstream MAP frame assigns upstream slots
request for upstream slots (and data) transmitted random access (binary backoff) in selected slots
MAP frame for
Interval [t1 t2]
Residences with cable modems
Downstream channel i
Upstream channel j
t1 t2
Assigned minislots containing cable modem
upstream data frames
Minislots containing
minislots request frames
cable headend
CMTS
Cable access network
Link Layer 5-40
Summary of MAC protocols
channel partitioning by time frequency or code Time Division Frequency Division
random access (dynamic)
ALOHA S-ALOHA CSMA CSMACD
carrier sensing easy in some technologies (wire) hard in others (wireless)
CSMACD used in Ethernet
CSMACA used in 80211
taking turns
polling from central site token passing
bluetooth FDDI token ring
Link Layer 5-41
Link layer LANs outline
51 introduction services
52 error detection correction
53 multiple access protocols
54 LANs addressing ARP
Ethernet
switches
VLANS
55 link virtualization MPLS
56 data center networking
57 a day in the life of a web request
Link Layer 5-42
MAC addresses and ARP
32-bit IP address network-layer address for interface
used for layer 3 (network layer) forwarding
MAC (or LAN or physical or Ethernet) address function used lsquolocallyrdquo to get frame from one interface to
another physically-connected interface (same network in IP-addressing sense)
48 bit MAC address (for most LANs) burned in NIC ROM also sometimes software settable
eg 1A-2F-BB-76-09-AD
hexadecimal (base 16) notation
(each ldquonumberrdquo represents 4 bits)
Link Layer 5-43
LAN addresses and ARP
each adapter on LAN has unique LAN address
adapter
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN
(wired or
wireless)
Link Layer 5-44
LAN addresses (more)
MAC address allocation administered by IEEE
manufacturer buys portion of MAC address space (to assure uniqueness)
analogy MAC address like Social Security Number
IP address like postal address
MAC flat address portability can move LAN card from one LAN to another
IP hierarchical address not portable address depends on IP subnet to which node is
attached
Link Layer 5-45
ARP address resolution protocol
ARP table each IP node (host router) on LAN has table
IPMAC address mappings for some LAN nodes
lt IP address MAC address TTLgt
TTL (Time To Live) time after which address mapping will be forgotten (typically 20 min)
Question how to determine
interfacersquos MAC address
knowing its IP address
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN
137196723
137196778
137196714
137196788
Link Layer 5-46
ARP protocol same LAN
A wants to send datagram to B Brsquos MAC address not in
Arsquos ARP table
A broadcasts ARP query packet containing Bs IP address dest MAC address = FF-FF-
FF-FF-FF-FF
all nodes on LAN receive ARP query
B receives ARP packet replies to A with its (Bs) MAC address frame sent to Arsquos MAC
address (unicast)
A caches (saves) IP-to-MAC address pair in its ARP table until information becomes old (times out) soft state information that
times out (goes away) unless refreshed
ARP is ldquoplug-and-playrdquo nodes create their ARP
tables without intervention from net administrator
Link Layer 5-47
walkthrough send datagram from A to B via R
focus on addressing ndash at IP (datagram) and MAC layer (frame)
assume A knows Brsquos IP address
assume A knows IP address of first hop router R (how)
assume A knows Rrsquos MAC address (how)
Addressing routing to another LAN
R
1A-23-F9-CD-06-9B 222222222220
111111111110 E6-E9-00-17-BB-4B CC-49-DE-D0-AB-7D
111111111112
111111111111
74-29-9C-E8-FF-55
A
222222222222
49-BD-D2-C7-56-2A
222222222221 88-B2-2F-54-1A-0F
B
R
1A-23-F9-CD-06-9B 222222222220
111111111110 E6-E9-00-17-BB-4B CC-49-DE-D0-AB-7D
111111111112
111111111111
74-29-9C-E8-FF-55
A
222222222222
49-BD-D2-C7-56-2A
222222222221 88-B2-2F-54-1A-0F
B
Link Layer 5-48
Addressing routing to another LAN
IP
Eth
Phy
IP src 111111111111
IP dest 222222222222
A creates IP datagram with IP source A destination B
A creates link-layer frame with Rs MAC address as dest frame contains A-to-B IP datagram
MAC src 74-29-9C-E8-FF-55
MAC dest E6-E9-00-17-BB-4B
R
1A-23-F9-CD-06-9B 222222222220
111111111110 E6-E9-00-17-BB-4B CC-49-DE-D0-AB-7D
111111111112
111111111111
74-29-9C-E8-FF-55
A
222222222222
49-BD-D2-C7-56-2A
222222222221 88-B2-2F-54-1A-0F
B
Link Layer 5-49
Addressing routing to another LAN
IP
Eth
Phy
frame sent from A to R
IP
Eth
Phy
frame received at R datagram removed passed up to IP
MAC src 74-29-9C-E8-FF-55
MAC dest E6-E9-00-17-BB-4B
IP src 111111111111
IP dest 222222222222
IP src 111111111111
IP dest 222222222222
R
1A-23-F9-CD-06-9B 222222222220
111111111110 E6-E9-00-17-BB-4B CC-49-DE-D0-AB-7D
111111111112
111111111111
74-29-9C-E8-FF-55
A
222222222222
49-BD-D2-C7-56-2A
222222222221 88-B2-2F-54-1A-0F
B
Link Layer 5-50
Addressing routing to another LAN
IP src 111111111111
IP dest 222222222222
R forwards datagram with IP source A destination B
R creates link-layer frame with Bs MAC address as dest frame contains A-to-B IP datagram
MAC src 1A-23-F9-CD-06-9B
MAC dest 49-BD-D2-C7-56-2A
IP
Eth
Phy
IP
Eth
Phy
R
1A-23-F9-CD-06-9B 222222222220
111111111110 E6-E9-00-17-BB-4B CC-49-DE-D0-AB-7D
111111111112
111111111111
74-29-9C-E8-FF-55
A
222222222222
49-BD-D2-C7-56-2A
222222222221 88-B2-2F-54-1A-0F
B
Link Layer 5-51
Addressing routing to another LAN R forwards datagram with IP source A destination B
R creates link-layer frame with Bs MAC address as dest frame contains A-to-B IP datagram
IP src 111111111111
IP dest 222222222222
MAC src 1A-23-F9-CD-06-9B
MAC dest 49-BD-D2-C7-56-2A
IP
Eth
Phy
IP
Eth
Phy
R
1A-23-F9-CD-06-9B 222222222220
111111111110 E6-E9-00-17-BB-4B CC-49-DE-D0-AB-7D
111111111112
111111111111
74-29-9C-E8-FF-55
A
222222222222
49-BD-D2-C7-56-2A
222222222221 88-B2-2F-54-1A-0F
B
Link Layer 5-52
Addressing routing to another LAN R forwards datagram with IP source A destination B
R creates link-layer frame with Bs MAC address as dest frame contains A-to-B IP datagram
IP src 111111111111
IP dest 222222222222
MAC src 1A-23-F9-CD-06-9B
MAC dest 49-BD-D2-C7-56-2A
IP
Eth
Phy
Link Layer 5-53
Link layer LANs outline
51 introduction services
52 error detection correction
53 multiple access protocols
54 LANs addressing ARP
Ethernet
switches
VLANS
55 link virtualization MPLS
56 data center networking
57 a day in the life of a web request
Link Layer 5-54
Ethernet
ldquodominantrdquo wired LAN technology
cheap $20 for NIC
first widely used LAN technology
simpler cheaper than token LANs and ATM
kept up with speed race 10 Mbps ndash 10 Gbps
Metcalfersquos Ethernet sketch
Link Layer 5-55
Ethernet physical topology
bus popular through mid 90s all nodes in same collision domain (can collide with each
other)
star prevails today active switch in center
each ldquospokerdquo runs a (separate) Ethernet protocol (nodes
do not collide with each other)
switch
bus coaxial cable star
Link Layer 5-56
Ethernet frame structure
sending adapter encapsulates IP datagram (or other network layer protocol packet) in Ethernet frame
preamble
7 bytes with pattern 10101010 followed by one byte with pattern 10101011
used to synchronize receiver sender clock rates
dest address
source address
data (payload) CRC preamble
type
Link Layer 5-57
Ethernet frame structure (more)
addresses 6 byte source destination MAC addresses if adapter receives frame with matching destination
address or with broadcast address (eg ARP packet) it passes data in frame to network layer protocol
otherwise adapter discards frame
type indicates higher layer protocol (mostly IP but others possible eg Novell IPX AppleTalk)
CRC cyclic redundancy check at receiver error detected frame is dropped
dest address
source address
data (payload) CRC preamble
type
Link Layer 5-58
Ethernet unreliable connectionless
connectionless no handshaking between sending and receiving NICs
unreliable receiving NIC doesnt send acks or nacks to sending NIC
data in dropped frames recovered only if initial sender uses higher layer rdt (eg TCP) otherwise dropped data lost
Ethernetrsquos MAC protocol unslotted CSMACD wth binary backoff
Link Layer 5-59
8023 Ethernet standards link amp physical layers
many different Ethernet standards
common MAC protocol and frame format
different speeds 2 Mbps 10 Mbps 100 Mbps 1Gbps 10G bps
different physical layer media fiber cable
application
transport
network
link
physical
MAC protocol
and frame format
100BASE-TX
100BASE-T4
100BASE-FX 100BASE-T2
100BASE-SX 100BASE-BX
fiber physical layer copper (twister
pair) physical layer
Link Layer 5-60
Link layer LANs outline
51 introduction services
52 error detection correction
53 multiple access protocols
54 LANs addressing ARP
Ethernet
switches
VLANS
55 link virtualization MPLS
56 data center networking
57 a day in the life of a web request
Link Layer 5-61
Ethernet switch link-layer device takes an active role
store forward Ethernet frames
examine incoming framersquos MAC address selectively forward frame to one-or-more outgoing links when frame is to be forwarded on segment uses CSMACD to access segment
transparent
hosts are unaware of presence of switches
plug-and-play self-learning
switches do not need to be configured
Link Layer 5-62
Switch multiple simultaneous transmissions
hosts have dedicated direct connection to switch
switches buffer packets
Ethernet protocol used on each incoming link but no collisions full duplex
each link is its own collision domain
switching A-to-Arsquo and B-to-Brsquo can transmit simultaneously without collisions switch with six interfaces
(123456)
A
Arsquo
B
Brsquo C
Crsquo
1 2
3 4 5
6
Link Layer 5-63
Switch forwarding table
Q how does switch know Arsquo reachable via interface 4 Brsquo reachable via interface 5
switch with six interfaces
(123456)
A
Arsquo
B
Brsquo C
Crsquo
1 2
3 4 5
6 A each switch has a switch table each entry
(MAC address of host interface to
reach host time stamp)
looks like a routing table
Q how are entries created maintained in switch table
something like a routing protocol
A
Arsquo
B
Brsquo C
Crsquo
1 2
3 4 5
6
Link Layer 5-64
Switch self-learning
switch learns which hosts can be reached through which interfaces
when frame received switch ldquolearnsrdquo location of sender incoming LAN segment
records senderlocation pair in switch table
A Arsquo
Source A
Dest Arsquo
MAC addr interface TTL
Switch table
(initially empty) A 1 60
Link Layer 5-65
Switch frame filteringforwarding
when frame received at switch 1 record incoming link MAC address of sending host
2 index switch table using MAC destination address
3 if entry found for destination then
if destination on segment from which frame arrived then drop frame
else forward frame on interface indicated by entry
else flood forward on all interfaces except arriving
interface
A
Arsquo
B
Brsquo C
Crsquo
1 2
3 4 5
6
Link Layer 5-66
Self-learning forwarding example
A Arsquo
Source A
Dest Arsquo
MAC addr interface TTL
switch table
(initially empty) A 1 60
A Arsquo A Arsquo A Arsquo A Arsquo A Arsquo
frame destination Arsquo locaton unknown flood
Arsquo A
destination A location
known
Arsquo 4 60
selectively send
on just one link
Link Layer 5-67
Interconnecting switches
switches can be connected together
Q sending from A to G - how does S1 know to forward frame destined to F via S4 and S3
A self learning (works exactly the same as in single-switch case)
A
B
S1
C D
E
F
S2
S4
S3
H
I
G
Link Layer 5-68
Self-learning multi-switch example
Suppose C sends frame to I I responds to C
Q show switch tables and packet forwarding in S1 S2 S3 S4
A
B
S1
C D
E
F
S2
S4
S3
H
I
G
Link Layer 5-69
Institutional network
to external
network
router
IP subnet
mail server
web server
Link Layer 5-70
Switches vs routers
both are store-and-forward
routers network-layer devices (examine network-layer headers)
switches link-layer devices (examine link-layer headers)
both have forwarding tables
routers compute tables using routing algorithms IP addresses
switches learn forwarding table using flooding learning MAC addresses
application
transport
network
link
physical
network
link
physical
link
physical
switch
datagram
application
transport
network
link
physical
frame
frame
frame
datagram
Link Layer 5-71
VLANs motivation
consider CS user moves office to
EE but wants connect to CS switch
single broadcast domain
all layer-2 broadcast traffic (ARP DHCP unknown location of destination MAC address) must cross entire LAN
securityprivacy efficiency issues
Computer
Science Electrical
Engineering
Computer
Engineering
Link Layer 5-72
VLANs port-based VLAN switch ports
grouped (by switch management software) so that single physical switch helliphellip
switch(es) supporting
VLAN capabilities can
be configured to
define multiple virtual
LANS over single
physical LAN
infrastructure
Virtual Local
Area Network 1
8
9
16 10 2
7
hellip
Electrical Engineering
(VLAN ports 1-8)
Computer Science
(VLAN ports 9-15)
15
hellip
Electrical Engineering
(VLAN ports 1-8)
hellip
1
8 2
7 9
16 10
15
hellip
Computer Science
(VLAN ports 9-16)
hellip operates as multiple virtual switches
Link Layer 5-73
Port-based VLAN
1
8
9
16 10 2
7
hellip
Electrical Engineering
(VLAN ports 1-8)
Computer Science
(VLAN ports 9-15)
15
hellip
traffic isolation frames tofrom ports 1-8 can only reach ports 1-8 can also define VLAN based on
MAC addresses of endpoints rather than switch port
dynamic membership ports can be dynamically assigned among VLANs
router
forwarding between VLANS done via routing (just as with separate switches) in practice vendors sell combined
switches plus routers
Link Layer 5-74
VLANS spanning multiple switches
trunk port carries frames between VLANS defined over multiple physical switches frames forwarded within VLAN between switches canrsquot be vanilla
8021 frames (must carry VLAN ID info)
8021q protocol addsremoved additional header fields for frames forwarded between trunk ports
1
8
9
10 2
7
hellip
Electrical Engineering
(VLAN ports 1-8)
Computer Science
(VLAN ports 9-15)
15
hellip
2
7 3
Ports 235 belong to EE VLAN
Ports 4678 belong to CS VLAN
5
4 6 8 16
1
Link Layer 5-75
type
2-byte Tag Protocol Identifier
(value 81-00)
Tag Control Information (12 bit VLAN ID field
3 bit priority field like IP TOS)
Recomputed CRC
8021Q VLAN frame format
8021 frame
8021Q frame
dest address
source address
data (payload) CRC preamble
dest address
source address
preamble data (payload) CRC
type
Link Layer 5-76
Link layer LANs outline
51 introduction services
52 error detection correction
53 multiple access protocols
54 LANs addressing ARP
Ethernet
switches
VLANS
55 link virtualization MPLS
56 data center networking
57 a day in the life of a web request
Link Layer 5-77
Multiprotocol label switching (MPLS)
initial goal high-speed IP forwarding using fixed length label (instead of IP address) fast lookup using fixed length identifier (rather than
shortest prefix matching)
borrowing ideas from Virtual Circuit (VC) approach
but IP datagram still keeps IP address
PPP or Ethernet
header IP header remainder of link-layer frame MPLS header
label Exp S TTL
20 3 1 5
Link Layer 5-78
MPLS capable routers
aka label-switched router
forward packets to outgoing interface based only on label value (donrsquot inspect IP address) MPLS forwarding table distinct from IP forwarding tables
flexibility MPLS forwarding decisions can differ from those of IP use destination and source addresses to route flows to
same destination differently (traffic engineering)
re-route flows quickly if link fails pre-computed backup paths (useful for VoIP)
Link Layer 5-79
R2
D
R3 R5
A
R6
MPLS versus IP paths
IP router IP routing path to destination determined
by destination address alone
R4
Link Layer 5-80
R2
D
R3 R4
R5
A
R6
MPLS versus IP paths
IP-only router
IP routing path to destination determined by destination address alone
MPLS and IP router
MPLS routing path to destination can be based on source and dest address fast reroute precompute backup routes in
case of link failure
entry router (R4) can use different MPLS
routes to A based eg on source address
Link Layer 5-81
MPLS signaling
modify OSPF IS-IS link-state flooding protocols to carry info used by MPLS routing eg link bandwidth amount of ldquoreservedrdquo link bandwidth
D
R4
R5
A
R6
entry MPLS router uses RSVP-TE signaling protocol to set up MPLS forwarding at downstream routers
modified link state flooding
RSVP-TE
Link Layer 5-82
R1 R2
D
R3 R4 R5
0
1
0 0
A
R6
in out out
label label dest interface
6 - A 0
in out out
label label dest interface
10 6 A 1
12 9 D 0
in out out
label label dest interface
10 A 0
12 D 0
1
in out out
label label dest interface
8 6 A 0
0
8 A 1
MPLS forwarding tables
Link Layer 5-83
Link layer LANs outline
51 introduction services
52 error detection correction
53 multiple access protocols
54 LANs addressing ARP
Ethernet
switches
VLANS
55 link virtualization MPLS
56 data center networking
57 a day in the life of a web request
Link Layer 5-84
Data center networks
10rsquos to 100rsquos of thousands of hosts often closely coupled in close proximity e-business (eg Amazon)
content-servers (eg YouTube Akamai Apple Microsoft)
search engines data mining (eg Google)
challenges
multiple applications each serving massive numbers of clients
managingbalancing load avoiding processing networking data bottlenecks
Inside a 40-ft Microsoft container
Chicago data center
Link Layer 5-85
Server racks
TOR switches
Tier-1 switches
Tier-2 switches
Load balancer
Load balancer
B
1 2 3 4 5 6 7 8
A C
Border router
Access router
Internet
Data center networks
load balancer application-layer routing receives external client requests
directs workload within data center
returns results to external client (hiding data
center internals from client)
Server racks
TOR switches
Tier-1 switches
Tier-2 switches
1 2 3 4 5 6 7 8
Data center networks
rich interconnection among switches racks
increased throughput between racks (multiple routing
paths possible)
increased reliability via redundancy
Link Layer 5-87
Link layer LANs outline
51 introduction services
52 error detection correction
53 multiple access protocols
54 LANs addressing ARP
Ethernet
switches
VLANS
55 link virtualization MPLS
56 data center networking
57 a day in the life of a web request
Link Layer 5-88
Synthesis a day in the life of a web request
journey down protocol stack complete application transport network link
putting-it-all-together synthesis goal identify review understand protocols (at all
layers) involved in seemingly simple scenario requesting www page
scenario student attaches laptop to campus network requestsreceives wwwgooglecom
Link Layer 5-89
A day in the life scenario
Comcast network
68800013
Googlersquos network
64233160019 64233169105
web server
DNS server
school network
68802024
web page
browser
router
(runs DHCP)
Link Layer 5-90
A day in the lifehellip connecting to the Internet
connecting laptop needs to get its own IP address addr of first-hop router addr of DNS server use DHCP
DHCP
UDP
IP
Eth
Phy
DHCP
DHCP
DHCP
DHCP
DHCP
DHCP
UDP
IP
Eth
Phy
DHCP
DHCP
DHCP
DHCP DHCP
DHCP request encapsulated in UDP encapsulated in IP encapsulated in 8023 Ethernet
Ethernet frame broadcast
(dest FFFFFFFFFFFF) on LAN received at router running DHCP server
Ethernet demuxed to IP demuxed UDP demuxed to DHCP
router
(runs DHCP)
Link Layer 5-91
DHCP server formulates DHCP ACK containing clientrsquos IP address IP address of first-hop router for client name amp IP address of DNS server
DHCP
UDP
IP
Eth
Phy
DHCP
DHCP
DHCP
DHCP
DHCP
UDP
IP
Eth
Phy
DHCP
DHCP
DHCP
DHCP
DHCP
encapsulation at DHCP server frame forwarded (switch learning) through LAN demultiplexing at client
Client now has IP address knows name amp addr of DNS
server IP address of its first-hop router
DHCP client receives DHCP ACK reply
A day in the lifehellip connecting to the Internet
router
(runs DHCP)
Link Layer 5-92
A day in the lifehellip ARP (before DNS before HTTP)
before sending HTTP request need IP address of wwwgooglecom DNS
DNS
UDP
IP
Eth
Phy
DNS
DNS
DNS
DNS query created encapsulated in UDP encapsulated in IP encapsulated in Eth To send frame to router need MAC address of router interface ARP
ARP query broadcast received by
router which replies with ARP reply giving MAC address of router interface
client now knows MAC address of first hop router so can now send frame containing DNS query
ARP query
Eth
Phy
ARP
ARP
ARP reply
router
(runs DHCP)
Link Layer 5-93
DNS
UDP
IP
Eth
Phy
DNS
DNS
DNS
DNS
DNS
IP datagram containing DNS query forwarded via LAN switch from client to 1st hop router
IP datagram forwarded from campus network into comcast network routed (tables created by RIP OSPF IS-IS andor BGP routing protocols) to DNS server
demuxrsquoed to DNS server
DNS server replies to client with IP address of wwwgooglecom
Comcast network
68800013
DNS server
DNS
UDP
IP
Eth
Phy
DNS
DNS
DNS
DNS
A day in the lifehellip using DNS
router
(runs DHCP)
Link Layer 5-94
A day in the lifehellipTCP connection carrying HTTP
HTTP
TCP
IP
Eth
Phy
HTTP
to send HTTP request client first opens TCP socket to web server
TCP SYN segment (step 1 in 3-way handshake) inter-domain routed to web server
TCP connection established 64233169105
web server
SYN
SYN
SYN
SYN
TCP
IP
Eth
Phy
SYN
SYN
SYN
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
web server responds with TCP SYNACK (step 2 in 3-way handshake)
router
(runs DHCP)
Link Layer 5-95
A day in the lifehellip HTTP requestreply
HTTP
TCP
IP
Eth
Phy
HTTP
HTTP request sent into TCP socket
IP datagram containing HTTP request routed to wwwgooglecom
IP datagram containing HTTP reply routed back to client
64233169105
web server
HTTP
TCP
IP
Eth
Phy web server responds with
HTTP reply (containing web page)
HTTP
HTTP
HTTP HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
web page finally () displayed
Link Layer 5-96
Chapter 5 Summary
principles behind data link layer services error detection correction
sharing a broadcast channel multiple access
link layer addressing
instantiation and implementation of various link layer technologies Ethernet
switched LANS VLANs
virtualized networks as a link layer MPLS
synthesis a day in the life of a web request
Link Layer 5-97
Chapter 5 letrsquos take a breath
journey down protocol stack complete (except PHY)
solid understanding of networking principles practice
hellip could stop here hellip but lots of interesting topics wireless
multimedia
security
network management
Link Layer 5-9
Adaptors communicating
sending side
encapsulates datagram in frame
adds error checking bits rdt flow control etc
receiving side
looks for errors rdt flow control etc
extracts datagram passes to upper layer at receiving side
controller controller
sending host receiving host
datagram datagram
datagram
frame
Link Layer 5-10
Link layer LANs outline
51 introduction services
52 error detection correction
53 multiple access protocols
54 LANs addressing ARP
Ethernet
switches
VLANS
55 link virtualization MPLS
56 data center networking
57 a day in the life of a web request
Link Layer 5-11
Error detection
EDC= Error Detection and Correction bits (redundancy)
D = Data protected by error checking may include header fields
bull Error detection not 100 reliable
bull protocol may miss some errors but rarely
bull larger EDC field yields better detection and correction
otherwise
Link Layer 5-12
Parity checking
single bit parity detect single bit
errors
two-dimensional bit parity detect and correct single bit errors
0 0
Link Layer 5-13
Internet checksum (review)
sender treat segment contents
as sequence of 16-bit integers
checksum addition (1rsquos complement sum) of segment contents
sender puts checksum value into UDP checksum field
receiver compute checksum of
received segment check if computed
checksum equals checksum field value NO - error detected YES - no error detected
But maybe errors nonetheless
goal detect ldquoerrorsrdquo (eg flipped bits) in transmitted packet (note used at transport layer only)
Link Layer 5-14
Cyclic redundancy check
more powerful error-detection coding
view data bits D as a binary number
choose r+1 bit pattern (generator) G
goal choose r CRC bits R such that ltDRgt exactly divisible by G (modulo 2)
receiver knows G divides ltDRgt by G If non-zero remainder error detected
can detect all burst errors less than r+1 bits
widely used in practice (Ethernet 80211 WiFi ATM)
Link Layer 5-15
CRC example
want D2r XOR R = nG
equivalently D2r = nG XOR R
equivalently if we divide D2r by
G want remainder R to satisfy
R = remainder[ ] D2r
G
1001 101110000 1001
1
101
01000
000 1010 1001
010 000
100 000 1000 0000 1000
D G
R
r = 3
Link Layer 5-16
Link layer LANs outline
51 introduction services
52 error detection correction
53 multiple access protocols
54 LANs addressing ARP
Ethernet
switches
VLANS
55 link virtualization MPLS
56 data center networking
57 a day in the life of a web request
Link Layer 5-17
Multiple access links protocols
two types of ldquolinksrdquo
point-to-point PPP for dial-up access
point-to-point link between Ethernet switch host
broadcast (shared wire or medium) old-fashioned Ethernet
upstream HFC
80211 wireless LAN
shared wire (eg cabled Ethernet)
shared RF (eg 80211 WiFi)
shared RF (satellite)
humans at a cocktail party
(shared air acoustical)
Link Layer 5-18
Multiple access protocols
single shared broadcast channel
two or more simultaneous transmissions by nodes interference
collision if node receives two or more signals at the same time
multiple access protocol distributed algorithm that determines how nodes share
channel ie determine when node can transmit
communication about channel sharing must use channel itself no out-of-band channel for coordination
Link Layer 5-19
An ideal multiple access protocol
given broadcast channel of rate R bps
desiderata 1 when one node wants to transmit it can send at rate R
2 when M nodes want to transmit each can send at average rate RM
3 fully decentralized
bull no special node to coordinate transmissions
bull no synchronization of clocks slots
4 simple
Link Layer 5-20
MAC protocols taxonomy
three broad classes
channel partitioning divide channel into smaller ldquopiecesrdquo (time slots frequency code)
allocate piece to node for exclusive use
random access channel not divided allow collisions
ldquorecoverrdquo from collisions
ldquotaking turnsrdquo nodes take turns but nodes with more to send can take longer
turns
Link Layer 5-21
Channel partitioning MAC protocols TDMA
TDMA time division multiple access access to channel in rounds each station gets fixed length slot (length = pkt
trans time) in each round unused slots go idle example 6-station LAN 134 have pkt slots
256 idle
1 3 4 1 3 4
6-slot
frame
6-slot
frame
Link Layer 5-22
FDMA frequency division multiple access channel spectrum divided into frequency bands
each station assigned fixed frequency band
unused transmission time in frequency bands go idle
example 6-station LAN 134 have pkt frequency bands 256 idle
frequency b
ands
FDM cable
Channel partitioning MAC protocols FDMA
Link Layer 5-23
Random access protocols
when node has packet to send transmit at full channel data rate R no a priori coordination among nodes
two or more transmitting nodes ldquocollisionrdquo random access MAC protocol specifies
how to detect collisions how to recover from collisions (eg via delayed
retransmissions)
examples of random access MAC protocols slotted ALOHA ALOHA CSMA CSMACD CSMACA
Link Layer 5-24
Slotted ALOHA
assumptions all frames same size
time divided into equal size slots (time to transmit 1 frame)
nodes start to transmit only slot beginning
nodes are synchronized
if 2 or more nodes transmit in slot all nodes detect collision
operation
when node obtains fresh frame transmits in next slot
if no collision node can send new frame in next slot
if collision node retransmits frame in each subsequent slot with prob p until success
Link Layer 5-25
Pros single active node can
continuously transmit at full rate of channel
highly decentralized only slots in nodes need to be in sync
simple
Cons
collisions wasting slots
idle slots
nodes may be able to detect collision in less than time to transmit packet
clock synchronization
Slotted ALOHA
1 1 1 1
2
3
2 2
3 3
node 1
node 2
node 3
C C C S S S E E E
Link Layer 5-26
suppose N nodes with many frames to send each transmits in slot with probability p
prob that given node has success in a slot = p(1-p)N-1
prob that any node has a success = Np(1-p)N-1
max efficiency find p that maximizes Np(1-p)N-1
for many nodes take limit of Np(1-p)N-1 as N goes to infinity gives
max efficiency = 1e = 37
efficiency long-run fraction of successful slots (many nodes all with many frames to send)
at best channel used for useful transmissions 37 of time
Slotted ALOHA efficiency
Link Layer 5-27
Pure (unslotted) ALOHA
unslotted Aloha simpler no synchronization
when frame first arrives
transmit immediately
collision probability increases
frame sent at t0 collides with other frames sent in [t0-1t0+1]
Link Layer 5-28
Pure ALOHA efficiency
P(success by given node) = P(node transmits)
P(no other node transmits in [t0-1t0]
P(no other node transmits in [t0-1t0]
= p (1-p)N-1 (1-p)N-1
= p (1-p)2(N-1)
hellip choosing optimum p and then letting n
= 1(2e) = 18
even worse than slotted Aloha
Link Layer 5-29
CSMA (carrier sense multiple access)
CSMA listen before transmit if channel sensed idle transmit entire frame
if channel sensed busy defer transmission
human analogy donrsquot interrupt others
Link Layer 5-30
CSMA collisions
collisions can still occur propagation delay means two nodes may not hear each otherrsquos transmission
collision entire packet transmission time wasted distance amp propagation
delay play role in in determining collision probability
spatial layout of nodes
Link Layer 5-31
CSMACD (collision detection)
CSMACD carrier sensing deferral as in CSMA collisions detected within short time
colliding transmissions aborted reducing channel wastage
collision detection easy in wired LANs measure signal strengths compare
transmitted received signals
difficult in wireless LANs received signal strength overwhelmed by local transmission strength
human analogy the polite conversationalist
Link Layer 5-32
CSMACD (collision detection)
spatial layout of nodes
Link Layer 5-33
Ethernet CSMACD algorithm
1 NIC receives datagram from network layer creates frame
2 If NIC senses channel idle starts frame transmission If NIC senses channel busy waits until channel idle then transmits
3 If NIC transmits entire frame without detecting another transmission NIC is done with frame
4 If NIC detects another transmission while transmitting aborts and sends jam signal
5 After aborting NIC enters binary (exponential) backoff
after mth collision NIC chooses K at random from 012 hellip 2m-1 NIC waits K512 bit times returns to Step 2
longer backoff interval with more collisions
Link Layer 5-34
CSMACD efficiency
Tprop = max prop delay between 2 nodes in LAN
ttrans = time to transmit max-size frame
efficiency goes to 1
as tprop goes to 0
as ttrans goes to infinity
better performance than ALOHA and simple cheap decentralized
transpropttefficiency
51
1
Link Layer 5-35
ldquoTaking turnsrdquo MAC protocols
channel partitioning MAC protocols share channel efficiently and fairly at high load
inefficient at low load delay in channel access 1N bandwidth allocated even if only 1 active node
random access MAC protocols efficient at low load single node can fully utilize
channel
high load collision overhead
ldquotaking turnsrdquo protocols look for best of both worlds
Link Layer 5-36
polling master node ldquoinvitesrdquo
slave nodes to transmit in turn
typically used with ldquodumbrdquo slave devices
concerns
polling overhead
latency
single point of failure (master)
master
slaves
poll
data
data
ldquoTaking turnsrdquo MAC protocols
Link Layer 5-37
token passing control token passed
from one node to next sequentially
token message
concerns
token overhead
latency
single point of failure (token)
T
data
(nothing
to send)
T
ldquoTaking turnsrdquo MAC protocols
cable headend
CMTS
ISP
cable modem termination system
multiple 40Mbps downstream (broadcast) channels
single CMTS transmits into channels
multiple 30 Mbps upstream channels
multiple access all users contend for certain upstream channel time slots (others assigned)
Cable access network
cable modem
splitter
hellip
hellip
Internet framesTV channels control transmitted downstream at different frequencies
upstream Internet frames TV control transmitted upstream at different frequencies in time slots
Link Layer 5-39
DOCSIS data over cable service interface spec FDM over upstream downstream frequency channels
TDM upstream some slots assigned some have contention
downstream MAP frame assigns upstream slots
request for upstream slots (and data) transmitted random access (binary backoff) in selected slots
MAP frame for
Interval [t1 t2]
Residences with cable modems
Downstream channel i
Upstream channel j
t1 t2
Assigned minislots containing cable modem
upstream data frames
Minislots containing
minislots request frames
cable headend
CMTS
Cable access network
Link Layer 5-40
Summary of MAC protocols
channel partitioning by time frequency or code Time Division Frequency Division
random access (dynamic)
ALOHA S-ALOHA CSMA CSMACD
carrier sensing easy in some technologies (wire) hard in others (wireless)
CSMACD used in Ethernet
CSMACA used in 80211
taking turns
polling from central site token passing
bluetooth FDDI token ring
Link Layer 5-41
Link layer LANs outline
51 introduction services
52 error detection correction
53 multiple access protocols
54 LANs addressing ARP
Ethernet
switches
VLANS
55 link virtualization MPLS
56 data center networking
57 a day in the life of a web request
Link Layer 5-42
MAC addresses and ARP
32-bit IP address network-layer address for interface
used for layer 3 (network layer) forwarding
MAC (or LAN or physical or Ethernet) address function used lsquolocallyrdquo to get frame from one interface to
another physically-connected interface (same network in IP-addressing sense)
48 bit MAC address (for most LANs) burned in NIC ROM also sometimes software settable
eg 1A-2F-BB-76-09-AD
hexadecimal (base 16) notation
(each ldquonumberrdquo represents 4 bits)
Link Layer 5-43
LAN addresses and ARP
each adapter on LAN has unique LAN address
adapter
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN
(wired or
wireless)
Link Layer 5-44
LAN addresses (more)
MAC address allocation administered by IEEE
manufacturer buys portion of MAC address space (to assure uniqueness)
analogy MAC address like Social Security Number
IP address like postal address
MAC flat address portability can move LAN card from one LAN to another
IP hierarchical address not portable address depends on IP subnet to which node is
attached
Link Layer 5-45
ARP address resolution protocol
ARP table each IP node (host router) on LAN has table
IPMAC address mappings for some LAN nodes
lt IP address MAC address TTLgt
TTL (Time To Live) time after which address mapping will be forgotten (typically 20 min)
Question how to determine
interfacersquos MAC address
knowing its IP address
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN
137196723
137196778
137196714
137196788
Link Layer 5-46
ARP protocol same LAN
A wants to send datagram to B Brsquos MAC address not in
Arsquos ARP table
A broadcasts ARP query packet containing Bs IP address dest MAC address = FF-FF-
FF-FF-FF-FF
all nodes on LAN receive ARP query
B receives ARP packet replies to A with its (Bs) MAC address frame sent to Arsquos MAC
address (unicast)
A caches (saves) IP-to-MAC address pair in its ARP table until information becomes old (times out) soft state information that
times out (goes away) unless refreshed
ARP is ldquoplug-and-playrdquo nodes create their ARP
tables without intervention from net administrator
Link Layer 5-47
walkthrough send datagram from A to B via R
focus on addressing ndash at IP (datagram) and MAC layer (frame)
assume A knows Brsquos IP address
assume A knows IP address of first hop router R (how)
assume A knows Rrsquos MAC address (how)
Addressing routing to another LAN
R
1A-23-F9-CD-06-9B 222222222220
111111111110 E6-E9-00-17-BB-4B CC-49-DE-D0-AB-7D
111111111112
111111111111
74-29-9C-E8-FF-55
A
222222222222
49-BD-D2-C7-56-2A
222222222221 88-B2-2F-54-1A-0F
B
R
1A-23-F9-CD-06-9B 222222222220
111111111110 E6-E9-00-17-BB-4B CC-49-DE-D0-AB-7D
111111111112
111111111111
74-29-9C-E8-FF-55
A
222222222222
49-BD-D2-C7-56-2A
222222222221 88-B2-2F-54-1A-0F
B
Link Layer 5-48
Addressing routing to another LAN
IP
Eth
Phy
IP src 111111111111
IP dest 222222222222
A creates IP datagram with IP source A destination B
A creates link-layer frame with Rs MAC address as dest frame contains A-to-B IP datagram
MAC src 74-29-9C-E8-FF-55
MAC dest E6-E9-00-17-BB-4B
R
1A-23-F9-CD-06-9B 222222222220
111111111110 E6-E9-00-17-BB-4B CC-49-DE-D0-AB-7D
111111111112
111111111111
74-29-9C-E8-FF-55
A
222222222222
49-BD-D2-C7-56-2A
222222222221 88-B2-2F-54-1A-0F
B
Link Layer 5-49
Addressing routing to another LAN
IP
Eth
Phy
frame sent from A to R
IP
Eth
Phy
frame received at R datagram removed passed up to IP
MAC src 74-29-9C-E8-FF-55
MAC dest E6-E9-00-17-BB-4B
IP src 111111111111
IP dest 222222222222
IP src 111111111111
IP dest 222222222222
R
1A-23-F9-CD-06-9B 222222222220
111111111110 E6-E9-00-17-BB-4B CC-49-DE-D0-AB-7D
111111111112
111111111111
74-29-9C-E8-FF-55
A
222222222222
49-BD-D2-C7-56-2A
222222222221 88-B2-2F-54-1A-0F
B
Link Layer 5-50
Addressing routing to another LAN
IP src 111111111111
IP dest 222222222222
R forwards datagram with IP source A destination B
R creates link-layer frame with Bs MAC address as dest frame contains A-to-B IP datagram
MAC src 1A-23-F9-CD-06-9B
MAC dest 49-BD-D2-C7-56-2A
IP
Eth
Phy
IP
Eth
Phy
R
1A-23-F9-CD-06-9B 222222222220
111111111110 E6-E9-00-17-BB-4B CC-49-DE-D0-AB-7D
111111111112
111111111111
74-29-9C-E8-FF-55
A
222222222222
49-BD-D2-C7-56-2A
222222222221 88-B2-2F-54-1A-0F
B
Link Layer 5-51
Addressing routing to another LAN R forwards datagram with IP source A destination B
R creates link-layer frame with Bs MAC address as dest frame contains A-to-B IP datagram
IP src 111111111111
IP dest 222222222222
MAC src 1A-23-F9-CD-06-9B
MAC dest 49-BD-D2-C7-56-2A
IP
Eth
Phy
IP
Eth
Phy
R
1A-23-F9-CD-06-9B 222222222220
111111111110 E6-E9-00-17-BB-4B CC-49-DE-D0-AB-7D
111111111112
111111111111
74-29-9C-E8-FF-55
A
222222222222
49-BD-D2-C7-56-2A
222222222221 88-B2-2F-54-1A-0F
B
Link Layer 5-52
Addressing routing to another LAN R forwards datagram with IP source A destination B
R creates link-layer frame with Bs MAC address as dest frame contains A-to-B IP datagram
IP src 111111111111
IP dest 222222222222
MAC src 1A-23-F9-CD-06-9B
MAC dest 49-BD-D2-C7-56-2A
IP
Eth
Phy
Link Layer 5-53
Link layer LANs outline
51 introduction services
52 error detection correction
53 multiple access protocols
54 LANs addressing ARP
Ethernet
switches
VLANS
55 link virtualization MPLS
56 data center networking
57 a day in the life of a web request
Link Layer 5-54
Ethernet
ldquodominantrdquo wired LAN technology
cheap $20 for NIC
first widely used LAN technology
simpler cheaper than token LANs and ATM
kept up with speed race 10 Mbps ndash 10 Gbps
Metcalfersquos Ethernet sketch
Link Layer 5-55
Ethernet physical topology
bus popular through mid 90s all nodes in same collision domain (can collide with each
other)
star prevails today active switch in center
each ldquospokerdquo runs a (separate) Ethernet protocol (nodes
do not collide with each other)
switch
bus coaxial cable star
Link Layer 5-56
Ethernet frame structure
sending adapter encapsulates IP datagram (or other network layer protocol packet) in Ethernet frame
preamble
7 bytes with pattern 10101010 followed by one byte with pattern 10101011
used to synchronize receiver sender clock rates
dest address
source address
data (payload) CRC preamble
type
Link Layer 5-57
Ethernet frame structure (more)
addresses 6 byte source destination MAC addresses if adapter receives frame with matching destination
address or with broadcast address (eg ARP packet) it passes data in frame to network layer protocol
otherwise adapter discards frame
type indicates higher layer protocol (mostly IP but others possible eg Novell IPX AppleTalk)
CRC cyclic redundancy check at receiver error detected frame is dropped
dest address
source address
data (payload) CRC preamble
type
Link Layer 5-58
Ethernet unreliable connectionless
connectionless no handshaking between sending and receiving NICs
unreliable receiving NIC doesnt send acks or nacks to sending NIC
data in dropped frames recovered only if initial sender uses higher layer rdt (eg TCP) otherwise dropped data lost
Ethernetrsquos MAC protocol unslotted CSMACD wth binary backoff
Link Layer 5-59
8023 Ethernet standards link amp physical layers
many different Ethernet standards
common MAC protocol and frame format
different speeds 2 Mbps 10 Mbps 100 Mbps 1Gbps 10G bps
different physical layer media fiber cable
application
transport
network
link
physical
MAC protocol
and frame format
100BASE-TX
100BASE-T4
100BASE-FX 100BASE-T2
100BASE-SX 100BASE-BX
fiber physical layer copper (twister
pair) physical layer
Link Layer 5-60
Link layer LANs outline
51 introduction services
52 error detection correction
53 multiple access protocols
54 LANs addressing ARP
Ethernet
switches
VLANS
55 link virtualization MPLS
56 data center networking
57 a day in the life of a web request
Link Layer 5-61
Ethernet switch link-layer device takes an active role
store forward Ethernet frames
examine incoming framersquos MAC address selectively forward frame to one-or-more outgoing links when frame is to be forwarded on segment uses CSMACD to access segment
transparent
hosts are unaware of presence of switches
plug-and-play self-learning
switches do not need to be configured
Link Layer 5-62
Switch multiple simultaneous transmissions
hosts have dedicated direct connection to switch
switches buffer packets
Ethernet protocol used on each incoming link but no collisions full duplex
each link is its own collision domain
switching A-to-Arsquo and B-to-Brsquo can transmit simultaneously without collisions switch with six interfaces
(123456)
A
Arsquo
B
Brsquo C
Crsquo
1 2
3 4 5
6
Link Layer 5-63
Switch forwarding table
Q how does switch know Arsquo reachable via interface 4 Brsquo reachable via interface 5
switch with six interfaces
(123456)
A
Arsquo
B
Brsquo C
Crsquo
1 2
3 4 5
6 A each switch has a switch table each entry
(MAC address of host interface to
reach host time stamp)
looks like a routing table
Q how are entries created maintained in switch table
something like a routing protocol
A
Arsquo
B
Brsquo C
Crsquo
1 2
3 4 5
6
Link Layer 5-64
Switch self-learning
switch learns which hosts can be reached through which interfaces
when frame received switch ldquolearnsrdquo location of sender incoming LAN segment
records senderlocation pair in switch table
A Arsquo
Source A
Dest Arsquo
MAC addr interface TTL
Switch table
(initially empty) A 1 60
Link Layer 5-65
Switch frame filteringforwarding
when frame received at switch 1 record incoming link MAC address of sending host
2 index switch table using MAC destination address
3 if entry found for destination then
if destination on segment from which frame arrived then drop frame
else forward frame on interface indicated by entry
else flood forward on all interfaces except arriving
interface
A
Arsquo
B
Brsquo C
Crsquo
1 2
3 4 5
6
Link Layer 5-66
Self-learning forwarding example
A Arsquo
Source A
Dest Arsquo
MAC addr interface TTL
switch table
(initially empty) A 1 60
A Arsquo A Arsquo A Arsquo A Arsquo A Arsquo
frame destination Arsquo locaton unknown flood
Arsquo A
destination A location
known
Arsquo 4 60
selectively send
on just one link
Link Layer 5-67
Interconnecting switches
switches can be connected together
Q sending from A to G - how does S1 know to forward frame destined to F via S4 and S3
A self learning (works exactly the same as in single-switch case)
A
B
S1
C D
E
F
S2
S4
S3
H
I
G
Link Layer 5-68
Self-learning multi-switch example
Suppose C sends frame to I I responds to C
Q show switch tables and packet forwarding in S1 S2 S3 S4
A
B
S1
C D
E
F
S2
S4
S3
H
I
G
Link Layer 5-69
Institutional network
to external
network
router
IP subnet
mail server
web server
Link Layer 5-70
Switches vs routers
both are store-and-forward
routers network-layer devices (examine network-layer headers)
switches link-layer devices (examine link-layer headers)
both have forwarding tables
routers compute tables using routing algorithms IP addresses
switches learn forwarding table using flooding learning MAC addresses
application
transport
network
link
physical
network
link
physical
link
physical
switch
datagram
application
transport
network
link
physical
frame
frame
frame
datagram
Link Layer 5-71
VLANs motivation
consider CS user moves office to
EE but wants connect to CS switch
single broadcast domain
all layer-2 broadcast traffic (ARP DHCP unknown location of destination MAC address) must cross entire LAN
securityprivacy efficiency issues
Computer
Science Electrical
Engineering
Computer
Engineering
Link Layer 5-72
VLANs port-based VLAN switch ports
grouped (by switch management software) so that single physical switch helliphellip
switch(es) supporting
VLAN capabilities can
be configured to
define multiple virtual
LANS over single
physical LAN
infrastructure
Virtual Local
Area Network 1
8
9
16 10 2
7
hellip
Electrical Engineering
(VLAN ports 1-8)
Computer Science
(VLAN ports 9-15)
15
hellip
Electrical Engineering
(VLAN ports 1-8)
hellip
1
8 2
7 9
16 10
15
hellip
Computer Science
(VLAN ports 9-16)
hellip operates as multiple virtual switches
Link Layer 5-73
Port-based VLAN
1
8
9
16 10 2
7
hellip
Electrical Engineering
(VLAN ports 1-8)
Computer Science
(VLAN ports 9-15)
15
hellip
traffic isolation frames tofrom ports 1-8 can only reach ports 1-8 can also define VLAN based on
MAC addresses of endpoints rather than switch port
dynamic membership ports can be dynamically assigned among VLANs
router
forwarding between VLANS done via routing (just as with separate switches) in practice vendors sell combined
switches plus routers
Link Layer 5-74
VLANS spanning multiple switches
trunk port carries frames between VLANS defined over multiple physical switches frames forwarded within VLAN between switches canrsquot be vanilla
8021 frames (must carry VLAN ID info)
8021q protocol addsremoved additional header fields for frames forwarded between trunk ports
1
8
9
10 2
7
hellip
Electrical Engineering
(VLAN ports 1-8)
Computer Science
(VLAN ports 9-15)
15
hellip
2
7 3
Ports 235 belong to EE VLAN
Ports 4678 belong to CS VLAN
5
4 6 8 16
1
Link Layer 5-75
type
2-byte Tag Protocol Identifier
(value 81-00)
Tag Control Information (12 bit VLAN ID field
3 bit priority field like IP TOS)
Recomputed CRC
8021Q VLAN frame format
8021 frame
8021Q frame
dest address
source address
data (payload) CRC preamble
dest address
source address
preamble data (payload) CRC
type
Link Layer 5-76
Link layer LANs outline
51 introduction services
52 error detection correction
53 multiple access protocols
54 LANs addressing ARP
Ethernet
switches
VLANS
55 link virtualization MPLS
56 data center networking
57 a day in the life of a web request
Link Layer 5-77
Multiprotocol label switching (MPLS)
initial goal high-speed IP forwarding using fixed length label (instead of IP address) fast lookup using fixed length identifier (rather than
shortest prefix matching)
borrowing ideas from Virtual Circuit (VC) approach
but IP datagram still keeps IP address
PPP or Ethernet
header IP header remainder of link-layer frame MPLS header
label Exp S TTL
20 3 1 5
Link Layer 5-78
MPLS capable routers
aka label-switched router
forward packets to outgoing interface based only on label value (donrsquot inspect IP address) MPLS forwarding table distinct from IP forwarding tables
flexibility MPLS forwarding decisions can differ from those of IP use destination and source addresses to route flows to
same destination differently (traffic engineering)
re-route flows quickly if link fails pre-computed backup paths (useful for VoIP)
Link Layer 5-79
R2
D
R3 R5
A
R6
MPLS versus IP paths
IP router IP routing path to destination determined
by destination address alone
R4
Link Layer 5-80
R2
D
R3 R4
R5
A
R6
MPLS versus IP paths
IP-only router
IP routing path to destination determined by destination address alone
MPLS and IP router
MPLS routing path to destination can be based on source and dest address fast reroute precompute backup routes in
case of link failure
entry router (R4) can use different MPLS
routes to A based eg on source address
Link Layer 5-81
MPLS signaling
modify OSPF IS-IS link-state flooding protocols to carry info used by MPLS routing eg link bandwidth amount of ldquoreservedrdquo link bandwidth
D
R4
R5
A
R6
entry MPLS router uses RSVP-TE signaling protocol to set up MPLS forwarding at downstream routers
modified link state flooding
RSVP-TE
Link Layer 5-82
R1 R2
D
R3 R4 R5
0
1
0 0
A
R6
in out out
label label dest interface
6 - A 0
in out out
label label dest interface
10 6 A 1
12 9 D 0
in out out
label label dest interface
10 A 0
12 D 0
1
in out out
label label dest interface
8 6 A 0
0
8 A 1
MPLS forwarding tables
Link Layer 5-83
Link layer LANs outline
51 introduction services
52 error detection correction
53 multiple access protocols
54 LANs addressing ARP
Ethernet
switches
VLANS
55 link virtualization MPLS
56 data center networking
57 a day in the life of a web request
Link Layer 5-84
Data center networks
10rsquos to 100rsquos of thousands of hosts often closely coupled in close proximity e-business (eg Amazon)
content-servers (eg YouTube Akamai Apple Microsoft)
search engines data mining (eg Google)
challenges
multiple applications each serving massive numbers of clients
managingbalancing load avoiding processing networking data bottlenecks
Inside a 40-ft Microsoft container
Chicago data center
Link Layer 5-85
Server racks
TOR switches
Tier-1 switches
Tier-2 switches
Load balancer
Load balancer
B
1 2 3 4 5 6 7 8
A C
Border router
Access router
Internet
Data center networks
load balancer application-layer routing receives external client requests
directs workload within data center
returns results to external client (hiding data
center internals from client)
Server racks
TOR switches
Tier-1 switches
Tier-2 switches
1 2 3 4 5 6 7 8
Data center networks
rich interconnection among switches racks
increased throughput between racks (multiple routing
paths possible)
increased reliability via redundancy
Link Layer 5-87
Link layer LANs outline
51 introduction services
52 error detection correction
53 multiple access protocols
54 LANs addressing ARP
Ethernet
switches
VLANS
55 link virtualization MPLS
56 data center networking
57 a day in the life of a web request
Link Layer 5-88
Synthesis a day in the life of a web request
journey down protocol stack complete application transport network link
putting-it-all-together synthesis goal identify review understand protocols (at all
layers) involved in seemingly simple scenario requesting www page
scenario student attaches laptop to campus network requestsreceives wwwgooglecom
Link Layer 5-89
A day in the life scenario
Comcast network
68800013
Googlersquos network
64233160019 64233169105
web server
DNS server
school network
68802024
web page
browser
router
(runs DHCP)
Link Layer 5-90
A day in the lifehellip connecting to the Internet
connecting laptop needs to get its own IP address addr of first-hop router addr of DNS server use DHCP
DHCP
UDP
IP
Eth
Phy
DHCP
DHCP
DHCP
DHCP
DHCP
DHCP
UDP
IP
Eth
Phy
DHCP
DHCP
DHCP
DHCP DHCP
DHCP request encapsulated in UDP encapsulated in IP encapsulated in 8023 Ethernet
Ethernet frame broadcast
(dest FFFFFFFFFFFF) on LAN received at router running DHCP server
Ethernet demuxed to IP demuxed UDP demuxed to DHCP
router
(runs DHCP)
Link Layer 5-91
DHCP server formulates DHCP ACK containing clientrsquos IP address IP address of first-hop router for client name amp IP address of DNS server
DHCP
UDP
IP
Eth
Phy
DHCP
DHCP
DHCP
DHCP
DHCP
UDP
IP
Eth
Phy
DHCP
DHCP
DHCP
DHCP
DHCP
encapsulation at DHCP server frame forwarded (switch learning) through LAN demultiplexing at client
Client now has IP address knows name amp addr of DNS
server IP address of its first-hop router
DHCP client receives DHCP ACK reply
A day in the lifehellip connecting to the Internet
router
(runs DHCP)
Link Layer 5-92
A day in the lifehellip ARP (before DNS before HTTP)
before sending HTTP request need IP address of wwwgooglecom DNS
DNS
UDP
IP
Eth
Phy
DNS
DNS
DNS
DNS query created encapsulated in UDP encapsulated in IP encapsulated in Eth To send frame to router need MAC address of router interface ARP
ARP query broadcast received by
router which replies with ARP reply giving MAC address of router interface
client now knows MAC address of first hop router so can now send frame containing DNS query
ARP query
Eth
Phy
ARP
ARP
ARP reply
router
(runs DHCP)
Link Layer 5-93
DNS
UDP
IP
Eth
Phy
DNS
DNS
DNS
DNS
DNS
IP datagram containing DNS query forwarded via LAN switch from client to 1st hop router
IP datagram forwarded from campus network into comcast network routed (tables created by RIP OSPF IS-IS andor BGP routing protocols) to DNS server
demuxrsquoed to DNS server
DNS server replies to client with IP address of wwwgooglecom
Comcast network
68800013
DNS server
DNS
UDP
IP
Eth
Phy
DNS
DNS
DNS
DNS
A day in the lifehellip using DNS
router
(runs DHCP)
Link Layer 5-94
A day in the lifehellipTCP connection carrying HTTP
HTTP
TCP
IP
Eth
Phy
HTTP
to send HTTP request client first opens TCP socket to web server
TCP SYN segment (step 1 in 3-way handshake) inter-domain routed to web server
TCP connection established 64233169105
web server
SYN
SYN
SYN
SYN
TCP
IP
Eth
Phy
SYN
SYN
SYN
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
web server responds with TCP SYNACK (step 2 in 3-way handshake)
router
(runs DHCP)
Link Layer 5-95
A day in the lifehellip HTTP requestreply
HTTP
TCP
IP
Eth
Phy
HTTP
HTTP request sent into TCP socket
IP datagram containing HTTP request routed to wwwgooglecom
IP datagram containing HTTP reply routed back to client
64233169105
web server
HTTP
TCP
IP
Eth
Phy web server responds with
HTTP reply (containing web page)
HTTP
HTTP
HTTP HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
web page finally () displayed
Link Layer 5-96
Chapter 5 Summary
principles behind data link layer services error detection correction
sharing a broadcast channel multiple access
link layer addressing
instantiation and implementation of various link layer technologies Ethernet
switched LANS VLANs
virtualized networks as a link layer MPLS
synthesis a day in the life of a web request
Link Layer 5-97
Chapter 5 letrsquos take a breath
journey down protocol stack complete (except PHY)
solid understanding of networking principles practice
hellip could stop here hellip but lots of interesting topics wireless
multimedia
security
network management
Link Layer 5-10
Link layer LANs outline
51 introduction services
52 error detection correction
53 multiple access protocols
54 LANs addressing ARP
Ethernet
switches
VLANS
55 link virtualization MPLS
56 data center networking
57 a day in the life of a web request
Link Layer 5-11
Error detection
EDC= Error Detection and Correction bits (redundancy)
D = Data protected by error checking may include header fields
bull Error detection not 100 reliable
bull protocol may miss some errors but rarely
bull larger EDC field yields better detection and correction
otherwise
Link Layer 5-12
Parity checking
single bit parity detect single bit
errors
two-dimensional bit parity detect and correct single bit errors
0 0
Link Layer 5-13
Internet checksum (review)
sender treat segment contents
as sequence of 16-bit integers
checksum addition (1rsquos complement sum) of segment contents
sender puts checksum value into UDP checksum field
receiver compute checksum of
received segment check if computed
checksum equals checksum field value NO - error detected YES - no error detected
But maybe errors nonetheless
goal detect ldquoerrorsrdquo (eg flipped bits) in transmitted packet (note used at transport layer only)
Link Layer 5-14
Cyclic redundancy check
more powerful error-detection coding
view data bits D as a binary number
choose r+1 bit pattern (generator) G
goal choose r CRC bits R such that ltDRgt exactly divisible by G (modulo 2)
receiver knows G divides ltDRgt by G If non-zero remainder error detected
can detect all burst errors less than r+1 bits
widely used in practice (Ethernet 80211 WiFi ATM)
Link Layer 5-15
CRC example
want D2r XOR R = nG
equivalently D2r = nG XOR R
equivalently if we divide D2r by
G want remainder R to satisfy
R = remainder[ ] D2r
G
1001 101110000 1001
1
101
01000
000 1010 1001
010 000
100 000 1000 0000 1000
D G
R
r = 3
Link Layer 5-16
Link layer LANs outline
51 introduction services
52 error detection correction
53 multiple access protocols
54 LANs addressing ARP
Ethernet
switches
VLANS
55 link virtualization MPLS
56 data center networking
57 a day in the life of a web request
Link Layer 5-17
Multiple access links protocols
two types of ldquolinksrdquo
point-to-point PPP for dial-up access
point-to-point link between Ethernet switch host
broadcast (shared wire or medium) old-fashioned Ethernet
upstream HFC
80211 wireless LAN
shared wire (eg cabled Ethernet)
shared RF (eg 80211 WiFi)
shared RF (satellite)
humans at a cocktail party
(shared air acoustical)
Link Layer 5-18
Multiple access protocols
single shared broadcast channel
two or more simultaneous transmissions by nodes interference
collision if node receives two or more signals at the same time
multiple access protocol distributed algorithm that determines how nodes share
channel ie determine when node can transmit
communication about channel sharing must use channel itself no out-of-band channel for coordination
Link Layer 5-19
An ideal multiple access protocol
given broadcast channel of rate R bps
desiderata 1 when one node wants to transmit it can send at rate R
2 when M nodes want to transmit each can send at average rate RM
3 fully decentralized
bull no special node to coordinate transmissions
bull no synchronization of clocks slots
4 simple
Link Layer 5-20
MAC protocols taxonomy
three broad classes
channel partitioning divide channel into smaller ldquopiecesrdquo (time slots frequency code)
allocate piece to node for exclusive use
random access channel not divided allow collisions
ldquorecoverrdquo from collisions
ldquotaking turnsrdquo nodes take turns but nodes with more to send can take longer
turns
Link Layer 5-21
Channel partitioning MAC protocols TDMA
TDMA time division multiple access access to channel in rounds each station gets fixed length slot (length = pkt
trans time) in each round unused slots go idle example 6-station LAN 134 have pkt slots
256 idle
1 3 4 1 3 4
6-slot
frame
6-slot
frame
Link Layer 5-22
FDMA frequency division multiple access channel spectrum divided into frequency bands
each station assigned fixed frequency band
unused transmission time in frequency bands go idle
example 6-station LAN 134 have pkt frequency bands 256 idle
frequency b
ands
FDM cable
Channel partitioning MAC protocols FDMA
Link Layer 5-23
Random access protocols
when node has packet to send transmit at full channel data rate R no a priori coordination among nodes
two or more transmitting nodes ldquocollisionrdquo random access MAC protocol specifies
how to detect collisions how to recover from collisions (eg via delayed
retransmissions)
examples of random access MAC protocols slotted ALOHA ALOHA CSMA CSMACD CSMACA
Link Layer 5-24
Slotted ALOHA
assumptions all frames same size
time divided into equal size slots (time to transmit 1 frame)
nodes start to transmit only slot beginning
nodes are synchronized
if 2 or more nodes transmit in slot all nodes detect collision
operation
when node obtains fresh frame transmits in next slot
if no collision node can send new frame in next slot
if collision node retransmits frame in each subsequent slot with prob p until success
Link Layer 5-25
Pros single active node can
continuously transmit at full rate of channel
highly decentralized only slots in nodes need to be in sync
simple
Cons
collisions wasting slots
idle slots
nodes may be able to detect collision in less than time to transmit packet
clock synchronization
Slotted ALOHA
1 1 1 1
2
3
2 2
3 3
node 1
node 2
node 3
C C C S S S E E E
Link Layer 5-26
suppose N nodes with many frames to send each transmits in slot with probability p
prob that given node has success in a slot = p(1-p)N-1
prob that any node has a success = Np(1-p)N-1
max efficiency find p that maximizes Np(1-p)N-1
for many nodes take limit of Np(1-p)N-1 as N goes to infinity gives
max efficiency = 1e = 37
efficiency long-run fraction of successful slots (many nodes all with many frames to send)
at best channel used for useful transmissions 37 of time
Slotted ALOHA efficiency
Link Layer 5-27
Pure (unslotted) ALOHA
unslotted Aloha simpler no synchronization
when frame first arrives
transmit immediately
collision probability increases
frame sent at t0 collides with other frames sent in [t0-1t0+1]
Link Layer 5-28
Pure ALOHA efficiency
P(success by given node) = P(node transmits)
P(no other node transmits in [t0-1t0]
P(no other node transmits in [t0-1t0]
= p (1-p)N-1 (1-p)N-1
= p (1-p)2(N-1)
hellip choosing optimum p and then letting n
= 1(2e) = 18
even worse than slotted Aloha
Link Layer 5-29
CSMA (carrier sense multiple access)
CSMA listen before transmit if channel sensed idle transmit entire frame
if channel sensed busy defer transmission
human analogy donrsquot interrupt others
Link Layer 5-30
CSMA collisions
collisions can still occur propagation delay means two nodes may not hear each otherrsquos transmission
collision entire packet transmission time wasted distance amp propagation
delay play role in in determining collision probability
spatial layout of nodes
Link Layer 5-31
CSMACD (collision detection)
CSMACD carrier sensing deferral as in CSMA collisions detected within short time
colliding transmissions aborted reducing channel wastage
collision detection easy in wired LANs measure signal strengths compare
transmitted received signals
difficult in wireless LANs received signal strength overwhelmed by local transmission strength
human analogy the polite conversationalist
Link Layer 5-32
CSMACD (collision detection)
spatial layout of nodes
Link Layer 5-33
Ethernet CSMACD algorithm
1 NIC receives datagram from network layer creates frame
2 If NIC senses channel idle starts frame transmission If NIC senses channel busy waits until channel idle then transmits
3 If NIC transmits entire frame without detecting another transmission NIC is done with frame
4 If NIC detects another transmission while transmitting aborts and sends jam signal
5 After aborting NIC enters binary (exponential) backoff
after mth collision NIC chooses K at random from 012 hellip 2m-1 NIC waits K512 bit times returns to Step 2
longer backoff interval with more collisions
Link Layer 5-34
CSMACD efficiency
Tprop = max prop delay between 2 nodes in LAN
ttrans = time to transmit max-size frame
efficiency goes to 1
as tprop goes to 0
as ttrans goes to infinity
better performance than ALOHA and simple cheap decentralized
transpropttefficiency
51
1
Link Layer 5-35
ldquoTaking turnsrdquo MAC protocols
channel partitioning MAC protocols share channel efficiently and fairly at high load
inefficient at low load delay in channel access 1N bandwidth allocated even if only 1 active node
random access MAC protocols efficient at low load single node can fully utilize
channel
high load collision overhead
ldquotaking turnsrdquo protocols look for best of both worlds
Link Layer 5-36
polling master node ldquoinvitesrdquo
slave nodes to transmit in turn
typically used with ldquodumbrdquo slave devices
concerns
polling overhead
latency
single point of failure (master)
master
slaves
poll
data
data
ldquoTaking turnsrdquo MAC protocols
Link Layer 5-37
token passing control token passed
from one node to next sequentially
token message
concerns
token overhead
latency
single point of failure (token)
T
data
(nothing
to send)
T
ldquoTaking turnsrdquo MAC protocols
cable headend
CMTS
ISP
cable modem termination system
multiple 40Mbps downstream (broadcast) channels
single CMTS transmits into channels
multiple 30 Mbps upstream channels
multiple access all users contend for certain upstream channel time slots (others assigned)
Cable access network
cable modem
splitter
hellip
hellip
Internet framesTV channels control transmitted downstream at different frequencies
upstream Internet frames TV control transmitted upstream at different frequencies in time slots
Link Layer 5-39
DOCSIS data over cable service interface spec FDM over upstream downstream frequency channels
TDM upstream some slots assigned some have contention
downstream MAP frame assigns upstream slots
request for upstream slots (and data) transmitted random access (binary backoff) in selected slots
MAP frame for
Interval [t1 t2]
Residences with cable modems
Downstream channel i
Upstream channel j
t1 t2
Assigned minislots containing cable modem
upstream data frames
Minislots containing
minislots request frames
cable headend
CMTS
Cable access network
Link Layer 5-40
Summary of MAC protocols
channel partitioning by time frequency or code Time Division Frequency Division
random access (dynamic)
ALOHA S-ALOHA CSMA CSMACD
carrier sensing easy in some technologies (wire) hard in others (wireless)
CSMACD used in Ethernet
CSMACA used in 80211
taking turns
polling from central site token passing
bluetooth FDDI token ring
Link Layer 5-41
Link layer LANs outline
51 introduction services
52 error detection correction
53 multiple access protocols
54 LANs addressing ARP
Ethernet
switches
VLANS
55 link virtualization MPLS
56 data center networking
57 a day in the life of a web request
Link Layer 5-42
MAC addresses and ARP
32-bit IP address network-layer address for interface
used for layer 3 (network layer) forwarding
MAC (or LAN or physical or Ethernet) address function used lsquolocallyrdquo to get frame from one interface to
another physically-connected interface (same network in IP-addressing sense)
48 bit MAC address (for most LANs) burned in NIC ROM also sometimes software settable
eg 1A-2F-BB-76-09-AD
hexadecimal (base 16) notation
(each ldquonumberrdquo represents 4 bits)
Link Layer 5-43
LAN addresses and ARP
each adapter on LAN has unique LAN address
adapter
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN
(wired or
wireless)
Link Layer 5-44
LAN addresses (more)
MAC address allocation administered by IEEE
manufacturer buys portion of MAC address space (to assure uniqueness)
analogy MAC address like Social Security Number
IP address like postal address
MAC flat address portability can move LAN card from one LAN to another
IP hierarchical address not portable address depends on IP subnet to which node is
attached
Link Layer 5-45
ARP address resolution protocol
ARP table each IP node (host router) on LAN has table
IPMAC address mappings for some LAN nodes
lt IP address MAC address TTLgt
TTL (Time To Live) time after which address mapping will be forgotten (typically 20 min)
Question how to determine
interfacersquos MAC address
knowing its IP address
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN
137196723
137196778
137196714
137196788
Link Layer 5-46
ARP protocol same LAN
A wants to send datagram to B Brsquos MAC address not in
Arsquos ARP table
A broadcasts ARP query packet containing Bs IP address dest MAC address = FF-FF-
FF-FF-FF-FF
all nodes on LAN receive ARP query
B receives ARP packet replies to A with its (Bs) MAC address frame sent to Arsquos MAC
address (unicast)
A caches (saves) IP-to-MAC address pair in its ARP table until information becomes old (times out) soft state information that
times out (goes away) unless refreshed
ARP is ldquoplug-and-playrdquo nodes create their ARP
tables without intervention from net administrator
Link Layer 5-47
walkthrough send datagram from A to B via R
focus on addressing ndash at IP (datagram) and MAC layer (frame)
assume A knows Brsquos IP address
assume A knows IP address of first hop router R (how)
assume A knows Rrsquos MAC address (how)
Addressing routing to another LAN
R
1A-23-F9-CD-06-9B 222222222220
111111111110 E6-E9-00-17-BB-4B CC-49-DE-D0-AB-7D
111111111112
111111111111
74-29-9C-E8-FF-55
A
222222222222
49-BD-D2-C7-56-2A
222222222221 88-B2-2F-54-1A-0F
B
R
1A-23-F9-CD-06-9B 222222222220
111111111110 E6-E9-00-17-BB-4B CC-49-DE-D0-AB-7D
111111111112
111111111111
74-29-9C-E8-FF-55
A
222222222222
49-BD-D2-C7-56-2A
222222222221 88-B2-2F-54-1A-0F
B
Link Layer 5-48
Addressing routing to another LAN
IP
Eth
Phy
IP src 111111111111
IP dest 222222222222
A creates IP datagram with IP source A destination B
A creates link-layer frame with Rs MAC address as dest frame contains A-to-B IP datagram
MAC src 74-29-9C-E8-FF-55
MAC dest E6-E9-00-17-BB-4B
R
1A-23-F9-CD-06-9B 222222222220
111111111110 E6-E9-00-17-BB-4B CC-49-DE-D0-AB-7D
111111111112
111111111111
74-29-9C-E8-FF-55
A
222222222222
49-BD-D2-C7-56-2A
222222222221 88-B2-2F-54-1A-0F
B
Link Layer 5-49
Addressing routing to another LAN
IP
Eth
Phy
frame sent from A to R
IP
Eth
Phy
frame received at R datagram removed passed up to IP
MAC src 74-29-9C-E8-FF-55
MAC dest E6-E9-00-17-BB-4B
IP src 111111111111
IP dest 222222222222
IP src 111111111111
IP dest 222222222222
R
1A-23-F9-CD-06-9B 222222222220
111111111110 E6-E9-00-17-BB-4B CC-49-DE-D0-AB-7D
111111111112
111111111111
74-29-9C-E8-FF-55
A
222222222222
49-BD-D2-C7-56-2A
222222222221 88-B2-2F-54-1A-0F
B
Link Layer 5-50
Addressing routing to another LAN
IP src 111111111111
IP dest 222222222222
R forwards datagram with IP source A destination B
R creates link-layer frame with Bs MAC address as dest frame contains A-to-B IP datagram
MAC src 1A-23-F9-CD-06-9B
MAC dest 49-BD-D2-C7-56-2A
IP
Eth
Phy
IP
Eth
Phy
R
1A-23-F9-CD-06-9B 222222222220
111111111110 E6-E9-00-17-BB-4B CC-49-DE-D0-AB-7D
111111111112
111111111111
74-29-9C-E8-FF-55
A
222222222222
49-BD-D2-C7-56-2A
222222222221 88-B2-2F-54-1A-0F
B
Link Layer 5-51
Addressing routing to another LAN R forwards datagram with IP source A destination B
R creates link-layer frame with Bs MAC address as dest frame contains A-to-B IP datagram
IP src 111111111111
IP dest 222222222222
MAC src 1A-23-F9-CD-06-9B
MAC dest 49-BD-D2-C7-56-2A
IP
Eth
Phy
IP
Eth
Phy
R
1A-23-F9-CD-06-9B 222222222220
111111111110 E6-E9-00-17-BB-4B CC-49-DE-D0-AB-7D
111111111112
111111111111
74-29-9C-E8-FF-55
A
222222222222
49-BD-D2-C7-56-2A
222222222221 88-B2-2F-54-1A-0F
B
Link Layer 5-52
Addressing routing to another LAN R forwards datagram with IP source A destination B
R creates link-layer frame with Bs MAC address as dest frame contains A-to-B IP datagram
IP src 111111111111
IP dest 222222222222
MAC src 1A-23-F9-CD-06-9B
MAC dest 49-BD-D2-C7-56-2A
IP
Eth
Phy
Link Layer 5-53
Link layer LANs outline
51 introduction services
52 error detection correction
53 multiple access protocols
54 LANs addressing ARP
Ethernet
switches
VLANS
55 link virtualization MPLS
56 data center networking
57 a day in the life of a web request
Link Layer 5-54
Ethernet
ldquodominantrdquo wired LAN technology
cheap $20 for NIC
first widely used LAN technology
simpler cheaper than token LANs and ATM
kept up with speed race 10 Mbps ndash 10 Gbps
Metcalfersquos Ethernet sketch
Link Layer 5-55
Ethernet physical topology
bus popular through mid 90s all nodes in same collision domain (can collide with each
other)
star prevails today active switch in center
each ldquospokerdquo runs a (separate) Ethernet protocol (nodes
do not collide with each other)
switch
bus coaxial cable star
Link Layer 5-56
Ethernet frame structure
sending adapter encapsulates IP datagram (or other network layer protocol packet) in Ethernet frame
preamble
7 bytes with pattern 10101010 followed by one byte with pattern 10101011
used to synchronize receiver sender clock rates
dest address
source address
data (payload) CRC preamble
type
Link Layer 5-57
Ethernet frame structure (more)
addresses 6 byte source destination MAC addresses if adapter receives frame with matching destination
address or with broadcast address (eg ARP packet) it passes data in frame to network layer protocol
otherwise adapter discards frame
type indicates higher layer protocol (mostly IP but others possible eg Novell IPX AppleTalk)
CRC cyclic redundancy check at receiver error detected frame is dropped
dest address
source address
data (payload) CRC preamble
type
Link Layer 5-58
Ethernet unreliable connectionless
connectionless no handshaking between sending and receiving NICs
unreliable receiving NIC doesnt send acks or nacks to sending NIC
data in dropped frames recovered only if initial sender uses higher layer rdt (eg TCP) otherwise dropped data lost
Ethernetrsquos MAC protocol unslotted CSMACD wth binary backoff
Link Layer 5-59
8023 Ethernet standards link amp physical layers
many different Ethernet standards
common MAC protocol and frame format
different speeds 2 Mbps 10 Mbps 100 Mbps 1Gbps 10G bps
different physical layer media fiber cable
application
transport
network
link
physical
MAC protocol
and frame format
100BASE-TX
100BASE-T4
100BASE-FX 100BASE-T2
100BASE-SX 100BASE-BX
fiber physical layer copper (twister
pair) physical layer
Link Layer 5-60
Link layer LANs outline
51 introduction services
52 error detection correction
53 multiple access protocols
54 LANs addressing ARP
Ethernet
switches
VLANS
55 link virtualization MPLS
56 data center networking
57 a day in the life of a web request
Link Layer 5-61
Ethernet switch link-layer device takes an active role
store forward Ethernet frames
examine incoming framersquos MAC address selectively forward frame to one-or-more outgoing links when frame is to be forwarded on segment uses CSMACD to access segment
transparent
hosts are unaware of presence of switches
plug-and-play self-learning
switches do not need to be configured
Link Layer 5-62
Switch multiple simultaneous transmissions
hosts have dedicated direct connection to switch
switches buffer packets
Ethernet protocol used on each incoming link but no collisions full duplex
each link is its own collision domain
switching A-to-Arsquo and B-to-Brsquo can transmit simultaneously without collisions switch with six interfaces
(123456)
A
Arsquo
B
Brsquo C
Crsquo
1 2
3 4 5
6
Link Layer 5-63
Switch forwarding table
Q how does switch know Arsquo reachable via interface 4 Brsquo reachable via interface 5
switch with six interfaces
(123456)
A
Arsquo
B
Brsquo C
Crsquo
1 2
3 4 5
6 A each switch has a switch table each entry
(MAC address of host interface to
reach host time stamp)
looks like a routing table
Q how are entries created maintained in switch table
something like a routing protocol
A
Arsquo
B
Brsquo C
Crsquo
1 2
3 4 5
6
Link Layer 5-64
Switch self-learning
switch learns which hosts can be reached through which interfaces
when frame received switch ldquolearnsrdquo location of sender incoming LAN segment
records senderlocation pair in switch table
A Arsquo
Source A
Dest Arsquo
MAC addr interface TTL
Switch table
(initially empty) A 1 60
Link Layer 5-65
Switch frame filteringforwarding
when frame received at switch 1 record incoming link MAC address of sending host
2 index switch table using MAC destination address
3 if entry found for destination then
if destination on segment from which frame arrived then drop frame
else forward frame on interface indicated by entry
else flood forward on all interfaces except arriving
interface
A
Arsquo
B
Brsquo C
Crsquo
1 2
3 4 5
6
Link Layer 5-66
Self-learning forwarding example
A Arsquo
Source A
Dest Arsquo
MAC addr interface TTL
switch table
(initially empty) A 1 60
A Arsquo A Arsquo A Arsquo A Arsquo A Arsquo
frame destination Arsquo locaton unknown flood
Arsquo A
destination A location
known
Arsquo 4 60
selectively send
on just one link
Link Layer 5-67
Interconnecting switches
switches can be connected together
Q sending from A to G - how does S1 know to forward frame destined to F via S4 and S3
A self learning (works exactly the same as in single-switch case)
A
B
S1
C D
E
F
S2
S4
S3
H
I
G
Link Layer 5-68
Self-learning multi-switch example
Suppose C sends frame to I I responds to C
Q show switch tables and packet forwarding in S1 S2 S3 S4
A
B
S1
C D
E
F
S2
S4
S3
H
I
G
Link Layer 5-69
Institutional network
to external
network
router
IP subnet
mail server
web server
Link Layer 5-70
Switches vs routers
both are store-and-forward
routers network-layer devices (examine network-layer headers)
switches link-layer devices (examine link-layer headers)
both have forwarding tables
routers compute tables using routing algorithms IP addresses
switches learn forwarding table using flooding learning MAC addresses
application
transport
network
link
physical
network
link
physical
link
physical
switch
datagram
application
transport
network
link
physical
frame
frame
frame
datagram
Link Layer 5-71
VLANs motivation
consider CS user moves office to
EE but wants connect to CS switch
single broadcast domain
all layer-2 broadcast traffic (ARP DHCP unknown location of destination MAC address) must cross entire LAN
securityprivacy efficiency issues
Computer
Science Electrical
Engineering
Computer
Engineering
Link Layer 5-72
VLANs port-based VLAN switch ports
grouped (by switch management software) so that single physical switch helliphellip
switch(es) supporting
VLAN capabilities can
be configured to
define multiple virtual
LANS over single
physical LAN
infrastructure
Virtual Local
Area Network 1
8
9
16 10 2
7
hellip
Electrical Engineering
(VLAN ports 1-8)
Computer Science
(VLAN ports 9-15)
15
hellip
Electrical Engineering
(VLAN ports 1-8)
hellip
1
8 2
7 9
16 10
15
hellip
Computer Science
(VLAN ports 9-16)
hellip operates as multiple virtual switches
Link Layer 5-73
Port-based VLAN
1
8
9
16 10 2
7
hellip
Electrical Engineering
(VLAN ports 1-8)
Computer Science
(VLAN ports 9-15)
15
hellip
traffic isolation frames tofrom ports 1-8 can only reach ports 1-8 can also define VLAN based on
MAC addresses of endpoints rather than switch port
dynamic membership ports can be dynamically assigned among VLANs
router
forwarding between VLANS done via routing (just as with separate switches) in practice vendors sell combined
switches plus routers
Link Layer 5-74
VLANS spanning multiple switches
trunk port carries frames between VLANS defined over multiple physical switches frames forwarded within VLAN between switches canrsquot be vanilla
8021 frames (must carry VLAN ID info)
8021q protocol addsremoved additional header fields for frames forwarded between trunk ports
1
8
9
10 2
7
hellip
Electrical Engineering
(VLAN ports 1-8)
Computer Science
(VLAN ports 9-15)
15
hellip
2
7 3
Ports 235 belong to EE VLAN
Ports 4678 belong to CS VLAN
5
4 6 8 16
1
Link Layer 5-75
type
2-byte Tag Protocol Identifier
(value 81-00)
Tag Control Information (12 bit VLAN ID field
3 bit priority field like IP TOS)
Recomputed CRC
8021Q VLAN frame format
8021 frame
8021Q frame
dest address
source address
data (payload) CRC preamble
dest address
source address
preamble data (payload) CRC
type
Link Layer 5-76
Link layer LANs outline
51 introduction services
52 error detection correction
53 multiple access protocols
54 LANs addressing ARP
Ethernet
switches
VLANS
55 link virtualization MPLS
56 data center networking
57 a day in the life of a web request
Link Layer 5-77
Multiprotocol label switching (MPLS)
initial goal high-speed IP forwarding using fixed length label (instead of IP address) fast lookup using fixed length identifier (rather than
shortest prefix matching)
borrowing ideas from Virtual Circuit (VC) approach
but IP datagram still keeps IP address
PPP or Ethernet
header IP header remainder of link-layer frame MPLS header
label Exp S TTL
20 3 1 5
Link Layer 5-78
MPLS capable routers
aka label-switched router
forward packets to outgoing interface based only on label value (donrsquot inspect IP address) MPLS forwarding table distinct from IP forwarding tables
flexibility MPLS forwarding decisions can differ from those of IP use destination and source addresses to route flows to
same destination differently (traffic engineering)
re-route flows quickly if link fails pre-computed backup paths (useful for VoIP)
Link Layer 5-79
R2
D
R3 R5
A
R6
MPLS versus IP paths
IP router IP routing path to destination determined
by destination address alone
R4
Link Layer 5-80
R2
D
R3 R4
R5
A
R6
MPLS versus IP paths
IP-only router
IP routing path to destination determined by destination address alone
MPLS and IP router
MPLS routing path to destination can be based on source and dest address fast reroute precompute backup routes in
case of link failure
entry router (R4) can use different MPLS
routes to A based eg on source address
Link Layer 5-81
MPLS signaling
modify OSPF IS-IS link-state flooding protocols to carry info used by MPLS routing eg link bandwidth amount of ldquoreservedrdquo link bandwidth
D
R4
R5
A
R6
entry MPLS router uses RSVP-TE signaling protocol to set up MPLS forwarding at downstream routers
modified link state flooding
RSVP-TE
Link Layer 5-82
R1 R2
D
R3 R4 R5
0
1
0 0
A
R6
in out out
label label dest interface
6 - A 0
in out out
label label dest interface
10 6 A 1
12 9 D 0
in out out
label label dest interface
10 A 0
12 D 0
1
in out out
label label dest interface
8 6 A 0
0
8 A 1
MPLS forwarding tables
Link Layer 5-83
Link layer LANs outline
51 introduction services
52 error detection correction
53 multiple access protocols
54 LANs addressing ARP
Ethernet
switches
VLANS
55 link virtualization MPLS
56 data center networking
57 a day in the life of a web request
Link Layer 5-84
Data center networks
10rsquos to 100rsquos of thousands of hosts often closely coupled in close proximity e-business (eg Amazon)
content-servers (eg YouTube Akamai Apple Microsoft)
search engines data mining (eg Google)
challenges
multiple applications each serving massive numbers of clients
managingbalancing load avoiding processing networking data bottlenecks
Inside a 40-ft Microsoft container
Chicago data center
Link Layer 5-85
Server racks
TOR switches
Tier-1 switches
Tier-2 switches
Load balancer
Load balancer
B
1 2 3 4 5 6 7 8
A C
Border router
Access router
Internet
Data center networks
load balancer application-layer routing receives external client requests
directs workload within data center
returns results to external client (hiding data
center internals from client)
Server racks
TOR switches
Tier-1 switches
Tier-2 switches
1 2 3 4 5 6 7 8
Data center networks
rich interconnection among switches racks
increased throughput between racks (multiple routing
paths possible)
increased reliability via redundancy
Link Layer 5-87
Link layer LANs outline
51 introduction services
52 error detection correction
53 multiple access protocols
54 LANs addressing ARP
Ethernet
switches
VLANS
55 link virtualization MPLS
56 data center networking
57 a day in the life of a web request
Link Layer 5-88
Synthesis a day in the life of a web request
journey down protocol stack complete application transport network link
putting-it-all-together synthesis goal identify review understand protocols (at all
layers) involved in seemingly simple scenario requesting www page
scenario student attaches laptop to campus network requestsreceives wwwgooglecom
Link Layer 5-89
A day in the life scenario
Comcast network
68800013
Googlersquos network
64233160019 64233169105
web server
DNS server
school network
68802024
web page
browser
router
(runs DHCP)
Link Layer 5-90
A day in the lifehellip connecting to the Internet
connecting laptop needs to get its own IP address addr of first-hop router addr of DNS server use DHCP
DHCP
UDP
IP
Eth
Phy
DHCP
DHCP
DHCP
DHCP
DHCP
DHCP
UDP
IP
Eth
Phy
DHCP
DHCP
DHCP
DHCP DHCP
DHCP request encapsulated in UDP encapsulated in IP encapsulated in 8023 Ethernet
Ethernet frame broadcast
(dest FFFFFFFFFFFF) on LAN received at router running DHCP server
Ethernet demuxed to IP demuxed UDP demuxed to DHCP
router
(runs DHCP)
Link Layer 5-91
DHCP server formulates DHCP ACK containing clientrsquos IP address IP address of first-hop router for client name amp IP address of DNS server
DHCP
UDP
IP
Eth
Phy
DHCP
DHCP
DHCP
DHCP
DHCP
UDP
IP
Eth
Phy
DHCP
DHCP
DHCP
DHCP
DHCP
encapsulation at DHCP server frame forwarded (switch learning) through LAN demultiplexing at client
Client now has IP address knows name amp addr of DNS
server IP address of its first-hop router
DHCP client receives DHCP ACK reply
A day in the lifehellip connecting to the Internet
router
(runs DHCP)
Link Layer 5-92
A day in the lifehellip ARP (before DNS before HTTP)
before sending HTTP request need IP address of wwwgooglecom DNS
DNS
UDP
IP
Eth
Phy
DNS
DNS
DNS
DNS query created encapsulated in UDP encapsulated in IP encapsulated in Eth To send frame to router need MAC address of router interface ARP
ARP query broadcast received by
router which replies with ARP reply giving MAC address of router interface
client now knows MAC address of first hop router so can now send frame containing DNS query
ARP query
Eth
Phy
ARP
ARP
ARP reply
router
(runs DHCP)
Link Layer 5-93
DNS
UDP
IP
Eth
Phy
DNS
DNS
DNS
DNS
DNS
IP datagram containing DNS query forwarded via LAN switch from client to 1st hop router
IP datagram forwarded from campus network into comcast network routed (tables created by RIP OSPF IS-IS andor BGP routing protocols) to DNS server
demuxrsquoed to DNS server
DNS server replies to client with IP address of wwwgooglecom
Comcast network
68800013
DNS server
DNS
UDP
IP
Eth
Phy
DNS
DNS
DNS
DNS
A day in the lifehellip using DNS
router
(runs DHCP)
Link Layer 5-94
A day in the lifehellipTCP connection carrying HTTP
HTTP
TCP
IP
Eth
Phy
HTTP
to send HTTP request client first opens TCP socket to web server
TCP SYN segment (step 1 in 3-way handshake) inter-domain routed to web server
TCP connection established 64233169105
web server
SYN
SYN
SYN
SYN
TCP
IP
Eth
Phy
SYN
SYN
SYN
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
web server responds with TCP SYNACK (step 2 in 3-way handshake)
router
(runs DHCP)
Link Layer 5-95
A day in the lifehellip HTTP requestreply
HTTP
TCP
IP
Eth
Phy
HTTP
HTTP request sent into TCP socket
IP datagram containing HTTP request routed to wwwgooglecom
IP datagram containing HTTP reply routed back to client
64233169105
web server
HTTP
TCP
IP
Eth
Phy web server responds with
HTTP reply (containing web page)
HTTP
HTTP
HTTP HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
web page finally () displayed
Link Layer 5-96
Chapter 5 Summary
principles behind data link layer services error detection correction
sharing a broadcast channel multiple access
link layer addressing
instantiation and implementation of various link layer technologies Ethernet
switched LANS VLANs
virtualized networks as a link layer MPLS
synthesis a day in the life of a web request
Link Layer 5-97
Chapter 5 letrsquos take a breath
journey down protocol stack complete (except PHY)
solid understanding of networking principles practice
hellip could stop here hellip but lots of interesting topics wireless
multimedia
security
network management
Link Layer 5-11
Error detection
EDC= Error Detection and Correction bits (redundancy)
D = Data protected by error checking may include header fields
bull Error detection not 100 reliable
bull protocol may miss some errors but rarely
bull larger EDC field yields better detection and correction
otherwise
Link Layer 5-12
Parity checking
single bit parity detect single bit
errors
two-dimensional bit parity detect and correct single bit errors
0 0
Link Layer 5-13
Internet checksum (review)
sender treat segment contents
as sequence of 16-bit integers
checksum addition (1rsquos complement sum) of segment contents
sender puts checksum value into UDP checksum field
receiver compute checksum of
received segment check if computed
checksum equals checksum field value NO - error detected YES - no error detected
But maybe errors nonetheless
goal detect ldquoerrorsrdquo (eg flipped bits) in transmitted packet (note used at transport layer only)
Link Layer 5-14
Cyclic redundancy check
more powerful error-detection coding
view data bits D as a binary number
choose r+1 bit pattern (generator) G
goal choose r CRC bits R such that ltDRgt exactly divisible by G (modulo 2)
receiver knows G divides ltDRgt by G If non-zero remainder error detected
can detect all burst errors less than r+1 bits
widely used in practice (Ethernet 80211 WiFi ATM)
Link Layer 5-15
CRC example
want D2r XOR R = nG
equivalently D2r = nG XOR R
equivalently if we divide D2r by
G want remainder R to satisfy
R = remainder[ ] D2r
G
1001 101110000 1001
1
101
01000
000 1010 1001
010 000
100 000 1000 0000 1000
D G
R
r = 3
Link Layer 5-16
Link layer LANs outline
51 introduction services
52 error detection correction
53 multiple access protocols
54 LANs addressing ARP
Ethernet
switches
VLANS
55 link virtualization MPLS
56 data center networking
57 a day in the life of a web request
Link Layer 5-17
Multiple access links protocols
two types of ldquolinksrdquo
point-to-point PPP for dial-up access
point-to-point link between Ethernet switch host
broadcast (shared wire or medium) old-fashioned Ethernet
upstream HFC
80211 wireless LAN
shared wire (eg cabled Ethernet)
shared RF (eg 80211 WiFi)
shared RF (satellite)
humans at a cocktail party
(shared air acoustical)
Link Layer 5-18
Multiple access protocols
single shared broadcast channel
two or more simultaneous transmissions by nodes interference
collision if node receives two or more signals at the same time
multiple access protocol distributed algorithm that determines how nodes share
channel ie determine when node can transmit
communication about channel sharing must use channel itself no out-of-band channel for coordination
Link Layer 5-19
An ideal multiple access protocol
given broadcast channel of rate R bps
desiderata 1 when one node wants to transmit it can send at rate R
2 when M nodes want to transmit each can send at average rate RM
3 fully decentralized
bull no special node to coordinate transmissions
bull no synchronization of clocks slots
4 simple
Link Layer 5-20
MAC protocols taxonomy
three broad classes
channel partitioning divide channel into smaller ldquopiecesrdquo (time slots frequency code)
allocate piece to node for exclusive use
random access channel not divided allow collisions
ldquorecoverrdquo from collisions
ldquotaking turnsrdquo nodes take turns but nodes with more to send can take longer
turns
Link Layer 5-21
Channel partitioning MAC protocols TDMA
TDMA time division multiple access access to channel in rounds each station gets fixed length slot (length = pkt
trans time) in each round unused slots go idle example 6-station LAN 134 have pkt slots
256 idle
1 3 4 1 3 4
6-slot
frame
6-slot
frame
Link Layer 5-22
FDMA frequency division multiple access channel spectrum divided into frequency bands
each station assigned fixed frequency band
unused transmission time in frequency bands go idle
example 6-station LAN 134 have pkt frequency bands 256 idle
frequency b
ands
FDM cable
Channel partitioning MAC protocols FDMA
Link Layer 5-23
Random access protocols
when node has packet to send transmit at full channel data rate R no a priori coordination among nodes
two or more transmitting nodes ldquocollisionrdquo random access MAC protocol specifies
how to detect collisions how to recover from collisions (eg via delayed
retransmissions)
examples of random access MAC protocols slotted ALOHA ALOHA CSMA CSMACD CSMACA
Link Layer 5-24
Slotted ALOHA
assumptions all frames same size
time divided into equal size slots (time to transmit 1 frame)
nodes start to transmit only slot beginning
nodes are synchronized
if 2 or more nodes transmit in slot all nodes detect collision
operation
when node obtains fresh frame transmits in next slot
if no collision node can send new frame in next slot
if collision node retransmits frame in each subsequent slot with prob p until success
Link Layer 5-25
Pros single active node can
continuously transmit at full rate of channel
highly decentralized only slots in nodes need to be in sync
simple
Cons
collisions wasting slots
idle slots
nodes may be able to detect collision in less than time to transmit packet
clock synchronization
Slotted ALOHA
1 1 1 1
2
3
2 2
3 3
node 1
node 2
node 3
C C C S S S E E E
Link Layer 5-26
suppose N nodes with many frames to send each transmits in slot with probability p
prob that given node has success in a slot = p(1-p)N-1
prob that any node has a success = Np(1-p)N-1
max efficiency find p that maximizes Np(1-p)N-1
for many nodes take limit of Np(1-p)N-1 as N goes to infinity gives
max efficiency = 1e = 37
efficiency long-run fraction of successful slots (many nodes all with many frames to send)
at best channel used for useful transmissions 37 of time
Slotted ALOHA efficiency
Link Layer 5-27
Pure (unslotted) ALOHA
unslotted Aloha simpler no synchronization
when frame first arrives
transmit immediately
collision probability increases
frame sent at t0 collides with other frames sent in [t0-1t0+1]
Link Layer 5-28
Pure ALOHA efficiency
P(success by given node) = P(node transmits)
P(no other node transmits in [t0-1t0]
P(no other node transmits in [t0-1t0]
= p (1-p)N-1 (1-p)N-1
= p (1-p)2(N-1)
hellip choosing optimum p and then letting n
= 1(2e) = 18
even worse than slotted Aloha
Link Layer 5-29
CSMA (carrier sense multiple access)
CSMA listen before transmit if channel sensed idle transmit entire frame
if channel sensed busy defer transmission
human analogy donrsquot interrupt others
Link Layer 5-30
CSMA collisions
collisions can still occur propagation delay means two nodes may not hear each otherrsquos transmission
collision entire packet transmission time wasted distance amp propagation
delay play role in in determining collision probability
spatial layout of nodes
Link Layer 5-31
CSMACD (collision detection)
CSMACD carrier sensing deferral as in CSMA collisions detected within short time
colliding transmissions aborted reducing channel wastage
collision detection easy in wired LANs measure signal strengths compare
transmitted received signals
difficult in wireless LANs received signal strength overwhelmed by local transmission strength
human analogy the polite conversationalist
Link Layer 5-32
CSMACD (collision detection)
spatial layout of nodes
Link Layer 5-33
Ethernet CSMACD algorithm
1 NIC receives datagram from network layer creates frame
2 If NIC senses channel idle starts frame transmission If NIC senses channel busy waits until channel idle then transmits
3 If NIC transmits entire frame without detecting another transmission NIC is done with frame
4 If NIC detects another transmission while transmitting aborts and sends jam signal
5 After aborting NIC enters binary (exponential) backoff
after mth collision NIC chooses K at random from 012 hellip 2m-1 NIC waits K512 bit times returns to Step 2
longer backoff interval with more collisions
Link Layer 5-34
CSMACD efficiency
Tprop = max prop delay between 2 nodes in LAN
ttrans = time to transmit max-size frame
efficiency goes to 1
as tprop goes to 0
as ttrans goes to infinity
better performance than ALOHA and simple cheap decentralized
transpropttefficiency
51
1
Link Layer 5-35
ldquoTaking turnsrdquo MAC protocols
channel partitioning MAC protocols share channel efficiently and fairly at high load
inefficient at low load delay in channel access 1N bandwidth allocated even if only 1 active node
random access MAC protocols efficient at low load single node can fully utilize
channel
high load collision overhead
ldquotaking turnsrdquo protocols look for best of both worlds
Link Layer 5-36
polling master node ldquoinvitesrdquo
slave nodes to transmit in turn
typically used with ldquodumbrdquo slave devices
concerns
polling overhead
latency
single point of failure (master)
master
slaves
poll
data
data
ldquoTaking turnsrdquo MAC protocols
Link Layer 5-37
token passing control token passed
from one node to next sequentially
token message
concerns
token overhead
latency
single point of failure (token)
T
data
(nothing
to send)
T
ldquoTaking turnsrdquo MAC protocols
cable headend
CMTS
ISP
cable modem termination system
multiple 40Mbps downstream (broadcast) channels
single CMTS transmits into channels
multiple 30 Mbps upstream channels
multiple access all users contend for certain upstream channel time slots (others assigned)
Cable access network
cable modem
splitter
hellip
hellip
Internet framesTV channels control transmitted downstream at different frequencies
upstream Internet frames TV control transmitted upstream at different frequencies in time slots
Link Layer 5-39
DOCSIS data over cable service interface spec FDM over upstream downstream frequency channels
TDM upstream some slots assigned some have contention
downstream MAP frame assigns upstream slots
request for upstream slots (and data) transmitted random access (binary backoff) in selected slots
MAP frame for
Interval [t1 t2]
Residences with cable modems
Downstream channel i
Upstream channel j
t1 t2
Assigned minislots containing cable modem
upstream data frames
Minislots containing
minislots request frames
cable headend
CMTS
Cable access network
Link Layer 5-40
Summary of MAC protocols
channel partitioning by time frequency or code Time Division Frequency Division
random access (dynamic)
ALOHA S-ALOHA CSMA CSMACD
carrier sensing easy in some technologies (wire) hard in others (wireless)
CSMACD used in Ethernet
CSMACA used in 80211
taking turns
polling from central site token passing
bluetooth FDDI token ring
Link Layer 5-41
Link layer LANs outline
51 introduction services
52 error detection correction
53 multiple access protocols
54 LANs addressing ARP
Ethernet
switches
VLANS
55 link virtualization MPLS
56 data center networking
57 a day in the life of a web request
Link Layer 5-42
MAC addresses and ARP
32-bit IP address network-layer address for interface
used for layer 3 (network layer) forwarding
MAC (or LAN or physical or Ethernet) address function used lsquolocallyrdquo to get frame from one interface to
another physically-connected interface (same network in IP-addressing sense)
48 bit MAC address (for most LANs) burned in NIC ROM also sometimes software settable
eg 1A-2F-BB-76-09-AD
hexadecimal (base 16) notation
(each ldquonumberrdquo represents 4 bits)
Link Layer 5-43
LAN addresses and ARP
each adapter on LAN has unique LAN address
adapter
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN
(wired or
wireless)
Link Layer 5-44
LAN addresses (more)
MAC address allocation administered by IEEE
manufacturer buys portion of MAC address space (to assure uniqueness)
analogy MAC address like Social Security Number
IP address like postal address
MAC flat address portability can move LAN card from one LAN to another
IP hierarchical address not portable address depends on IP subnet to which node is
attached
Link Layer 5-45
ARP address resolution protocol
ARP table each IP node (host router) on LAN has table
IPMAC address mappings for some LAN nodes
lt IP address MAC address TTLgt
TTL (Time To Live) time after which address mapping will be forgotten (typically 20 min)
Question how to determine
interfacersquos MAC address
knowing its IP address
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN
137196723
137196778
137196714
137196788
Link Layer 5-46
ARP protocol same LAN
A wants to send datagram to B Brsquos MAC address not in
Arsquos ARP table
A broadcasts ARP query packet containing Bs IP address dest MAC address = FF-FF-
FF-FF-FF-FF
all nodes on LAN receive ARP query
B receives ARP packet replies to A with its (Bs) MAC address frame sent to Arsquos MAC
address (unicast)
A caches (saves) IP-to-MAC address pair in its ARP table until information becomes old (times out) soft state information that
times out (goes away) unless refreshed
ARP is ldquoplug-and-playrdquo nodes create their ARP
tables without intervention from net administrator
Link Layer 5-47
walkthrough send datagram from A to B via R
focus on addressing ndash at IP (datagram) and MAC layer (frame)
assume A knows Brsquos IP address
assume A knows IP address of first hop router R (how)
assume A knows Rrsquos MAC address (how)
Addressing routing to another LAN
R
1A-23-F9-CD-06-9B 222222222220
111111111110 E6-E9-00-17-BB-4B CC-49-DE-D0-AB-7D
111111111112
111111111111
74-29-9C-E8-FF-55
A
222222222222
49-BD-D2-C7-56-2A
222222222221 88-B2-2F-54-1A-0F
B
R
1A-23-F9-CD-06-9B 222222222220
111111111110 E6-E9-00-17-BB-4B CC-49-DE-D0-AB-7D
111111111112
111111111111
74-29-9C-E8-FF-55
A
222222222222
49-BD-D2-C7-56-2A
222222222221 88-B2-2F-54-1A-0F
B
Link Layer 5-48
Addressing routing to another LAN
IP
Eth
Phy
IP src 111111111111
IP dest 222222222222
A creates IP datagram with IP source A destination B
A creates link-layer frame with Rs MAC address as dest frame contains A-to-B IP datagram
MAC src 74-29-9C-E8-FF-55
MAC dest E6-E9-00-17-BB-4B
R
1A-23-F9-CD-06-9B 222222222220
111111111110 E6-E9-00-17-BB-4B CC-49-DE-D0-AB-7D
111111111112
111111111111
74-29-9C-E8-FF-55
A
222222222222
49-BD-D2-C7-56-2A
222222222221 88-B2-2F-54-1A-0F
B
Link Layer 5-49
Addressing routing to another LAN
IP
Eth
Phy
frame sent from A to R
IP
Eth
Phy
frame received at R datagram removed passed up to IP
MAC src 74-29-9C-E8-FF-55
MAC dest E6-E9-00-17-BB-4B
IP src 111111111111
IP dest 222222222222
IP src 111111111111
IP dest 222222222222
R
1A-23-F9-CD-06-9B 222222222220
111111111110 E6-E9-00-17-BB-4B CC-49-DE-D0-AB-7D
111111111112
111111111111
74-29-9C-E8-FF-55
A
222222222222
49-BD-D2-C7-56-2A
222222222221 88-B2-2F-54-1A-0F
B
Link Layer 5-50
Addressing routing to another LAN
IP src 111111111111
IP dest 222222222222
R forwards datagram with IP source A destination B
R creates link-layer frame with Bs MAC address as dest frame contains A-to-B IP datagram
MAC src 1A-23-F9-CD-06-9B
MAC dest 49-BD-D2-C7-56-2A
IP
Eth
Phy
IP
Eth
Phy
R
1A-23-F9-CD-06-9B 222222222220
111111111110 E6-E9-00-17-BB-4B CC-49-DE-D0-AB-7D
111111111112
111111111111
74-29-9C-E8-FF-55
A
222222222222
49-BD-D2-C7-56-2A
222222222221 88-B2-2F-54-1A-0F
B
Link Layer 5-51
Addressing routing to another LAN R forwards datagram with IP source A destination B
R creates link-layer frame with Bs MAC address as dest frame contains A-to-B IP datagram
IP src 111111111111
IP dest 222222222222
MAC src 1A-23-F9-CD-06-9B
MAC dest 49-BD-D2-C7-56-2A
IP
Eth
Phy
IP
Eth
Phy
R
1A-23-F9-CD-06-9B 222222222220
111111111110 E6-E9-00-17-BB-4B CC-49-DE-D0-AB-7D
111111111112
111111111111
74-29-9C-E8-FF-55
A
222222222222
49-BD-D2-C7-56-2A
222222222221 88-B2-2F-54-1A-0F
B
Link Layer 5-52
Addressing routing to another LAN R forwards datagram with IP source A destination B
R creates link-layer frame with Bs MAC address as dest frame contains A-to-B IP datagram
IP src 111111111111
IP dest 222222222222
MAC src 1A-23-F9-CD-06-9B
MAC dest 49-BD-D2-C7-56-2A
IP
Eth
Phy
Link Layer 5-53
Link layer LANs outline
51 introduction services
52 error detection correction
53 multiple access protocols
54 LANs addressing ARP
Ethernet
switches
VLANS
55 link virtualization MPLS
56 data center networking
57 a day in the life of a web request
Link Layer 5-54
Ethernet
ldquodominantrdquo wired LAN technology
cheap $20 for NIC
first widely used LAN technology
simpler cheaper than token LANs and ATM
kept up with speed race 10 Mbps ndash 10 Gbps
Metcalfersquos Ethernet sketch
Link Layer 5-55
Ethernet physical topology
bus popular through mid 90s all nodes in same collision domain (can collide with each
other)
star prevails today active switch in center
each ldquospokerdquo runs a (separate) Ethernet protocol (nodes
do not collide with each other)
switch
bus coaxial cable star
Link Layer 5-56
Ethernet frame structure
sending adapter encapsulates IP datagram (or other network layer protocol packet) in Ethernet frame
preamble
7 bytes with pattern 10101010 followed by one byte with pattern 10101011
used to synchronize receiver sender clock rates
dest address
source address
data (payload) CRC preamble
type
Link Layer 5-57
Ethernet frame structure (more)
addresses 6 byte source destination MAC addresses if adapter receives frame with matching destination
address or with broadcast address (eg ARP packet) it passes data in frame to network layer protocol
otherwise adapter discards frame
type indicates higher layer protocol (mostly IP but others possible eg Novell IPX AppleTalk)
CRC cyclic redundancy check at receiver error detected frame is dropped
dest address
source address
data (payload) CRC preamble
type
Link Layer 5-58
Ethernet unreliable connectionless
connectionless no handshaking between sending and receiving NICs
unreliable receiving NIC doesnt send acks or nacks to sending NIC
data in dropped frames recovered only if initial sender uses higher layer rdt (eg TCP) otherwise dropped data lost
Ethernetrsquos MAC protocol unslotted CSMACD wth binary backoff
Link Layer 5-59
8023 Ethernet standards link amp physical layers
many different Ethernet standards
common MAC protocol and frame format
different speeds 2 Mbps 10 Mbps 100 Mbps 1Gbps 10G bps
different physical layer media fiber cable
application
transport
network
link
physical
MAC protocol
and frame format
100BASE-TX
100BASE-T4
100BASE-FX 100BASE-T2
100BASE-SX 100BASE-BX
fiber physical layer copper (twister
pair) physical layer
Link Layer 5-60
Link layer LANs outline
51 introduction services
52 error detection correction
53 multiple access protocols
54 LANs addressing ARP
Ethernet
switches
VLANS
55 link virtualization MPLS
56 data center networking
57 a day in the life of a web request
Link Layer 5-61
Ethernet switch link-layer device takes an active role
store forward Ethernet frames
examine incoming framersquos MAC address selectively forward frame to one-or-more outgoing links when frame is to be forwarded on segment uses CSMACD to access segment
transparent
hosts are unaware of presence of switches
plug-and-play self-learning
switches do not need to be configured
Link Layer 5-62
Switch multiple simultaneous transmissions
hosts have dedicated direct connection to switch
switches buffer packets
Ethernet protocol used on each incoming link but no collisions full duplex
each link is its own collision domain
switching A-to-Arsquo and B-to-Brsquo can transmit simultaneously without collisions switch with six interfaces
(123456)
A
Arsquo
B
Brsquo C
Crsquo
1 2
3 4 5
6
Link Layer 5-63
Switch forwarding table
Q how does switch know Arsquo reachable via interface 4 Brsquo reachable via interface 5
switch with six interfaces
(123456)
A
Arsquo
B
Brsquo C
Crsquo
1 2
3 4 5
6 A each switch has a switch table each entry
(MAC address of host interface to
reach host time stamp)
looks like a routing table
Q how are entries created maintained in switch table
something like a routing protocol
A
Arsquo
B
Brsquo C
Crsquo
1 2
3 4 5
6
Link Layer 5-64
Switch self-learning
switch learns which hosts can be reached through which interfaces
when frame received switch ldquolearnsrdquo location of sender incoming LAN segment
records senderlocation pair in switch table
A Arsquo
Source A
Dest Arsquo
MAC addr interface TTL
Switch table
(initially empty) A 1 60
Link Layer 5-65
Switch frame filteringforwarding
when frame received at switch 1 record incoming link MAC address of sending host
2 index switch table using MAC destination address
3 if entry found for destination then
if destination on segment from which frame arrived then drop frame
else forward frame on interface indicated by entry
else flood forward on all interfaces except arriving
interface
A
Arsquo
B
Brsquo C
Crsquo
1 2
3 4 5
6
Link Layer 5-66
Self-learning forwarding example
A Arsquo
Source A
Dest Arsquo
MAC addr interface TTL
switch table
(initially empty) A 1 60
A Arsquo A Arsquo A Arsquo A Arsquo A Arsquo
frame destination Arsquo locaton unknown flood
Arsquo A
destination A location
known
Arsquo 4 60
selectively send
on just one link
Link Layer 5-67
Interconnecting switches
switches can be connected together
Q sending from A to G - how does S1 know to forward frame destined to F via S4 and S3
A self learning (works exactly the same as in single-switch case)
A
B
S1
C D
E
F
S2
S4
S3
H
I
G
Link Layer 5-68
Self-learning multi-switch example
Suppose C sends frame to I I responds to C
Q show switch tables and packet forwarding in S1 S2 S3 S4
A
B
S1
C D
E
F
S2
S4
S3
H
I
G
Link Layer 5-69
Institutional network
to external
network
router
IP subnet
mail server
web server
Link Layer 5-70
Switches vs routers
both are store-and-forward
routers network-layer devices (examine network-layer headers)
switches link-layer devices (examine link-layer headers)
both have forwarding tables
routers compute tables using routing algorithms IP addresses
switches learn forwarding table using flooding learning MAC addresses
application
transport
network
link
physical
network
link
physical
link
physical
switch
datagram
application
transport
network
link
physical
frame
frame
frame
datagram
Link Layer 5-71
VLANs motivation
consider CS user moves office to
EE but wants connect to CS switch
single broadcast domain
all layer-2 broadcast traffic (ARP DHCP unknown location of destination MAC address) must cross entire LAN
securityprivacy efficiency issues
Computer
Science Electrical
Engineering
Computer
Engineering
Link Layer 5-72
VLANs port-based VLAN switch ports
grouped (by switch management software) so that single physical switch helliphellip
switch(es) supporting
VLAN capabilities can
be configured to
define multiple virtual
LANS over single
physical LAN
infrastructure
Virtual Local
Area Network 1
8
9
16 10 2
7
hellip
Electrical Engineering
(VLAN ports 1-8)
Computer Science
(VLAN ports 9-15)
15
hellip
Electrical Engineering
(VLAN ports 1-8)
hellip
1
8 2
7 9
16 10
15
hellip
Computer Science
(VLAN ports 9-16)
hellip operates as multiple virtual switches
Link Layer 5-73
Port-based VLAN
1
8
9
16 10 2
7
hellip
Electrical Engineering
(VLAN ports 1-8)
Computer Science
(VLAN ports 9-15)
15
hellip
traffic isolation frames tofrom ports 1-8 can only reach ports 1-8 can also define VLAN based on
MAC addresses of endpoints rather than switch port
dynamic membership ports can be dynamically assigned among VLANs
router
forwarding between VLANS done via routing (just as with separate switches) in practice vendors sell combined
switches plus routers
Link Layer 5-74
VLANS spanning multiple switches
trunk port carries frames between VLANS defined over multiple physical switches frames forwarded within VLAN between switches canrsquot be vanilla
8021 frames (must carry VLAN ID info)
8021q protocol addsremoved additional header fields for frames forwarded between trunk ports
1
8
9
10 2
7
hellip
Electrical Engineering
(VLAN ports 1-8)
Computer Science
(VLAN ports 9-15)
15
hellip
2
7 3
Ports 235 belong to EE VLAN
Ports 4678 belong to CS VLAN
5
4 6 8 16
1
Link Layer 5-75
type
2-byte Tag Protocol Identifier
(value 81-00)
Tag Control Information (12 bit VLAN ID field
3 bit priority field like IP TOS)
Recomputed CRC
8021Q VLAN frame format
8021 frame
8021Q frame
dest address
source address
data (payload) CRC preamble
dest address
source address
preamble data (payload) CRC
type
Link Layer 5-76
Link layer LANs outline
51 introduction services
52 error detection correction
53 multiple access protocols
54 LANs addressing ARP
Ethernet
switches
VLANS
55 link virtualization MPLS
56 data center networking
57 a day in the life of a web request
Link Layer 5-77
Multiprotocol label switching (MPLS)
initial goal high-speed IP forwarding using fixed length label (instead of IP address) fast lookup using fixed length identifier (rather than
shortest prefix matching)
borrowing ideas from Virtual Circuit (VC) approach
but IP datagram still keeps IP address
PPP or Ethernet
header IP header remainder of link-layer frame MPLS header
label Exp S TTL
20 3 1 5
Link Layer 5-78
MPLS capable routers
aka label-switched router
forward packets to outgoing interface based only on label value (donrsquot inspect IP address) MPLS forwarding table distinct from IP forwarding tables
flexibility MPLS forwarding decisions can differ from those of IP use destination and source addresses to route flows to
same destination differently (traffic engineering)
re-route flows quickly if link fails pre-computed backup paths (useful for VoIP)
Link Layer 5-79
R2
D
R3 R5
A
R6
MPLS versus IP paths
IP router IP routing path to destination determined
by destination address alone
R4
Link Layer 5-80
R2
D
R3 R4
R5
A
R6
MPLS versus IP paths
IP-only router
IP routing path to destination determined by destination address alone
MPLS and IP router
MPLS routing path to destination can be based on source and dest address fast reroute precompute backup routes in
case of link failure
entry router (R4) can use different MPLS
routes to A based eg on source address
Link Layer 5-81
MPLS signaling
modify OSPF IS-IS link-state flooding protocols to carry info used by MPLS routing eg link bandwidth amount of ldquoreservedrdquo link bandwidth
D
R4
R5
A
R6
entry MPLS router uses RSVP-TE signaling protocol to set up MPLS forwarding at downstream routers
modified link state flooding
RSVP-TE
Link Layer 5-82
R1 R2
D
R3 R4 R5
0
1
0 0
A
R6
in out out
label label dest interface
6 - A 0
in out out
label label dest interface
10 6 A 1
12 9 D 0
in out out
label label dest interface
10 A 0
12 D 0
1
in out out
label label dest interface
8 6 A 0
0
8 A 1
MPLS forwarding tables
Link Layer 5-83
Link layer LANs outline
51 introduction services
52 error detection correction
53 multiple access protocols
54 LANs addressing ARP
Ethernet
switches
VLANS
55 link virtualization MPLS
56 data center networking
57 a day in the life of a web request
Link Layer 5-84
Data center networks
10rsquos to 100rsquos of thousands of hosts often closely coupled in close proximity e-business (eg Amazon)
content-servers (eg YouTube Akamai Apple Microsoft)
search engines data mining (eg Google)
challenges
multiple applications each serving massive numbers of clients
managingbalancing load avoiding processing networking data bottlenecks
Inside a 40-ft Microsoft container
Chicago data center
Link Layer 5-85
Server racks
TOR switches
Tier-1 switches
Tier-2 switches
Load balancer
Load balancer
B
1 2 3 4 5 6 7 8
A C
Border router
Access router
Internet
Data center networks
load balancer application-layer routing receives external client requests
directs workload within data center
returns results to external client (hiding data
center internals from client)
Server racks
TOR switches
Tier-1 switches
Tier-2 switches
1 2 3 4 5 6 7 8
Data center networks
rich interconnection among switches racks
increased throughput between racks (multiple routing
paths possible)
increased reliability via redundancy
Link Layer 5-87
Link layer LANs outline
51 introduction services
52 error detection correction
53 multiple access protocols
54 LANs addressing ARP
Ethernet
switches
VLANS
55 link virtualization MPLS
56 data center networking
57 a day in the life of a web request
Link Layer 5-88
Synthesis a day in the life of a web request
journey down protocol stack complete application transport network link
putting-it-all-together synthesis goal identify review understand protocols (at all
layers) involved in seemingly simple scenario requesting www page
scenario student attaches laptop to campus network requestsreceives wwwgooglecom
Link Layer 5-89
A day in the life scenario
Comcast network
68800013
Googlersquos network
64233160019 64233169105
web server
DNS server
school network
68802024
web page
browser
router
(runs DHCP)
Link Layer 5-90
A day in the lifehellip connecting to the Internet
connecting laptop needs to get its own IP address addr of first-hop router addr of DNS server use DHCP
DHCP
UDP
IP
Eth
Phy
DHCP
DHCP
DHCP
DHCP
DHCP
DHCP
UDP
IP
Eth
Phy
DHCP
DHCP
DHCP
DHCP DHCP
DHCP request encapsulated in UDP encapsulated in IP encapsulated in 8023 Ethernet
Ethernet frame broadcast
(dest FFFFFFFFFFFF) on LAN received at router running DHCP server
Ethernet demuxed to IP demuxed UDP demuxed to DHCP
router
(runs DHCP)
Link Layer 5-91
DHCP server formulates DHCP ACK containing clientrsquos IP address IP address of first-hop router for client name amp IP address of DNS server
DHCP
UDP
IP
Eth
Phy
DHCP
DHCP
DHCP
DHCP
DHCP
UDP
IP
Eth
Phy
DHCP
DHCP
DHCP
DHCP
DHCP
encapsulation at DHCP server frame forwarded (switch learning) through LAN demultiplexing at client
Client now has IP address knows name amp addr of DNS
server IP address of its first-hop router
DHCP client receives DHCP ACK reply
A day in the lifehellip connecting to the Internet
router
(runs DHCP)
Link Layer 5-92
A day in the lifehellip ARP (before DNS before HTTP)
before sending HTTP request need IP address of wwwgooglecom DNS
DNS
UDP
IP
Eth
Phy
DNS
DNS
DNS
DNS query created encapsulated in UDP encapsulated in IP encapsulated in Eth To send frame to router need MAC address of router interface ARP
ARP query broadcast received by
router which replies with ARP reply giving MAC address of router interface
client now knows MAC address of first hop router so can now send frame containing DNS query
ARP query
Eth
Phy
ARP
ARP
ARP reply
router
(runs DHCP)
Link Layer 5-93
DNS
UDP
IP
Eth
Phy
DNS
DNS
DNS
DNS
DNS
IP datagram containing DNS query forwarded via LAN switch from client to 1st hop router
IP datagram forwarded from campus network into comcast network routed (tables created by RIP OSPF IS-IS andor BGP routing protocols) to DNS server
demuxrsquoed to DNS server
DNS server replies to client with IP address of wwwgooglecom
Comcast network
68800013
DNS server
DNS
UDP
IP
Eth
Phy
DNS
DNS
DNS
DNS
A day in the lifehellip using DNS
router
(runs DHCP)
Link Layer 5-94
A day in the lifehellipTCP connection carrying HTTP
HTTP
TCP
IP
Eth
Phy
HTTP
to send HTTP request client first opens TCP socket to web server
TCP SYN segment (step 1 in 3-way handshake) inter-domain routed to web server
TCP connection established 64233169105
web server
SYN
SYN
SYN
SYN
TCP
IP
Eth
Phy
SYN
SYN
SYN
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
web server responds with TCP SYNACK (step 2 in 3-way handshake)
router
(runs DHCP)
Link Layer 5-95
A day in the lifehellip HTTP requestreply
HTTP
TCP
IP
Eth
Phy
HTTP
HTTP request sent into TCP socket
IP datagram containing HTTP request routed to wwwgooglecom
IP datagram containing HTTP reply routed back to client
64233169105
web server
HTTP
TCP
IP
Eth
Phy web server responds with
HTTP reply (containing web page)
HTTP
HTTP
HTTP HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
web page finally () displayed
Link Layer 5-96
Chapter 5 Summary
principles behind data link layer services error detection correction
sharing a broadcast channel multiple access
link layer addressing
instantiation and implementation of various link layer technologies Ethernet
switched LANS VLANs
virtualized networks as a link layer MPLS
synthesis a day in the life of a web request
Link Layer 5-97
Chapter 5 letrsquos take a breath
journey down protocol stack complete (except PHY)
solid understanding of networking principles practice
hellip could stop here hellip but lots of interesting topics wireless
multimedia
security
network management
Link Layer 5-12
Parity checking
single bit parity detect single bit
errors
two-dimensional bit parity detect and correct single bit errors
0 0
Link Layer 5-13
Internet checksum (review)
sender treat segment contents
as sequence of 16-bit integers
checksum addition (1rsquos complement sum) of segment contents
sender puts checksum value into UDP checksum field
receiver compute checksum of
received segment check if computed
checksum equals checksum field value NO - error detected YES - no error detected
But maybe errors nonetheless
goal detect ldquoerrorsrdquo (eg flipped bits) in transmitted packet (note used at transport layer only)
Link Layer 5-14
Cyclic redundancy check
more powerful error-detection coding
view data bits D as a binary number
choose r+1 bit pattern (generator) G
goal choose r CRC bits R such that ltDRgt exactly divisible by G (modulo 2)
receiver knows G divides ltDRgt by G If non-zero remainder error detected
can detect all burst errors less than r+1 bits
widely used in practice (Ethernet 80211 WiFi ATM)
Link Layer 5-15
CRC example
want D2r XOR R = nG
equivalently D2r = nG XOR R
equivalently if we divide D2r by
G want remainder R to satisfy
R = remainder[ ] D2r
G
1001 101110000 1001
1
101
01000
000 1010 1001
010 000
100 000 1000 0000 1000
D G
R
r = 3
Link Layer 5-16
Link layer LANs outline
51 introduction services
52 error detection correction
53 multiple access protocols
54 LANs addressing ARP
Ethernet
switches
VLANS
55 link virtualization MPLS
56 data center networking
57 a day in the life of a web request
Link Layer 5-17
Multiple access links protocols
two types of ldquolinksrdquo
point-to-point PPP for dial-up access
point-to-point link between Ethernet switch host
broadcast (shared wire or medium) old-fashioned Ethernet
upstream HFC
80211 wireless LAN
shared wire (eg cabled Ethernet)
shared RF (eg 80211 WiFi)
shared RF (satellite)
humans at a cocktail party
(shared air acoustical)
Link Layer 5-18
Multiple access protocols
single shared broadcast channel
two or more simultaneous transmissions by nodes interference
collision if node receives two or more signals at the same time
multiple access protocol distributed algorithm that determines how nodes share
channel ie determine when node can transmit
communication about channel sharing must use channel itself no out-of-band channel for coordination
Link Layer 5-19
An ideal multiple access protocol
given broadcast channel of rate R bps
desiderata 1 when one node wants to transmit it can send at rate R
2 when M nodes want to transmit each can send at average rate RM
3 fully decentralized
bull no special node to coordinate transmissions
bull no synchronization of clocks slots
4 simple
Link Layer 5-20
MAC protocols taxonomy
three broad classes
channel partitioning divide channel into smaller ldquopiecesrdquo (time slots frequency code)
allocate piece to node for exclusive use
random access channel not divided allow collisions
ldquorecoverrdquo from collisions
ldquotaking turnsrdquo nodes take turns but nodes with more to send can take longer
turns
Link Layer 5-21
Channel partitioning MAC protocols TDMA
TDMA time division multiple access access to channel in rounds each station gets fixed length slot (length = pkt
trans time) in each round unused slots go idle example 6-station LAN 134 have pkt slots
256 idle
1 3 4 1 3 4
6-slot
frame
6-slot
frame
Link Layer 5-22
FDMA frequency division multiple access channel spectrum divided into frequency bands
each station assigned fixed frequency band
unused transmission time in frequency bands go idle
example 6-station LAN 134 have pkt frequency bands 256 idle
frequency b
ands
FDM cable
Channel partitioning MAC protocols FDMA
Link Layer 5-23
Random access protocols
when node has packet to send transmit at full channel data rate R no a priori coordination among nodes
two or more transmitting nodes ldquocollisionrdquo random access MAC protocol specifies
how to detect collisions how to recover from collisions (eg via delayed
retransmissions)
examples of random access MAC protocols slotted ALOHA ALOHA CSMA CSMACD CSMACA
Link Layer 5-24
Slotted ALOHA
assumptions all frames same size
time divided into equal size slots (time to transmit 1 frame)
nodes start to transmit only slot beginning
nodes are synchronized
if 2 or more nodes transmit in slot all nodes detect collision
operation
when node obtains fresh frame transmits in next slot
if no collision node can send new frame in next slot
if collision node retransmits frame in each subsequent slot with prob p until success
Link Layer 5-25
Pros single active node can
continuously transmit at full rate of channel
highly decentralized only slots in nodes need to be in sync
simple
Cons
collisions wasting slots
idle slots
nodes may be able to detect collision in less than time to transmit packet
clock synchronization
Slotted ALOHA
1 1 1 1
2
3
2 2
3 3
node 1
node 2
node 3
C C C S S S E E E
Link Layer 5-26
suppose N nodes with many frames to send each transmits in slot with probability p
prob that given node has success in a slot = p(1-p)N-1
prob that any node has a success = Np(1-p)N-1
max efficiency find p that maximizes Np(1-p)N-1
for many nodes take limit of Np(1-p)N-1 as N goes to infinity gives
max efficiency = 1e = 37
efficiency long-run fraction of successful slots (many nodes all with many frames to send)
at best channel used for useful transmissions 37 of time
Slotted ALOHA efficiency
Link Layer 5-27
Pure (unslotted) ALOHA
unslotted Aloha simpler no synchronization
when frame first arrives
transmit immediately
collision probability increases
frame sent at t0 collides with other frames sent in [t0-1t0+1]
Link Layer 5-28
Pure ALOHA efficiency
P(success by given node) = P(node transmits)
P(no other node transmits in [t0-1t0]
P(no other node transmits in [t0-1t0]
= p (1-p)N-1 (1-p)N-1
= p (1-p)2(N-1)
hellip choosing optimum p and then letting n
= 1(2e) = 18
even worse than slotted Aloha
Link Layer 5-29
CSMA (carrier sense multiple access)
CSMA listen before transmit if channel sensed idle transmit entire frame
if channel sensed busy defer transmission
human analogy donrsquot interrupt others
Link Layer 5-30
CSMA collisions
collisions can still occur propagation delay means two nodes may not hear each otherrsquos transmission
collision entire packet transmission time wasted distance amp propagation
delay play role in in determining collision probability
spatial layout of nodes
Link Layer 5-31
CSMACD (collision detection)
CSMACD carrier sensing deferral as in CSMA collisions detected within short time
colliding transmissions aborted reducing channel wastage
collision detection easy in wired LANs measure signal strengths compare
transmitted received signals
difficult in wireless LANs received signal strength overwhelmed by local transmission strength
human analogy the polite conversationalist
Link Layer 5-32
CSMACD (collision detection)
spatial layout of nodes
Link Layer 5-33
Ethernet CSMACD algorithm
1 NIC receives datagram from network layer creates frame
2 If NIC senses channel idle starts frame transmission If NIC senses channel busy waits until channel idle then transmits
3 If NIC transmits entire frame without detecting another transmission NIC is done with frame
4 If NIC detects another transmission while transmitting aborts and sends jam signal
5 After aborting NIC enters binary (exponential) backoff
after mth collision NIC chooses K at random from 012 hellip 2m-1 NIC waits K512 bit times returns to Step 2
longer backoff interval with more collisions
Link Layer 5-34
CSMACD efficiency
Tprop = max prop delay between 2 nodes in LAN
ttrans = time to transmit max-size frame
efficiency goes to 1
as tprop goes to 0
as ttrans goes to infinity
better performance than ALOHA and simple cheap decentralized
transpropttefficiency
51
1
Link Layer 5-35
ldquoTaking turnsrdquo MAC protocols
channel partitioning MAC protocols share channel efficiently and fairly at high load
inefficient at low load delay in channel access 1N bandwidth allocated even if only 1 active node
random access MAC protocols efficient at low load single node can fully utilize
channel
high load collision overhead
ldquotaking turnsrdquo protocols look for best of both worlds
Link Layer 5-36
polling master node ldquoinvitesrdquo
slave nodes to transmit in turn
typically used with ldquodumbrdquo slave devices
concerns
polling overhead
latency
single point of failure (master)
master
slaves
poll
data
data
ldquoTaking turnsrdquo MAC protocols
Link Layer 5-37
token passing control token passed
from one node to next sequentially
token message
concerns
token overhead
latency
single point of failure (token)
T
data
(nothing
to send)
T
ldquoTaking turnsrdquo MAC protocols
cable headend
CMTS
ISP
cable modem termination system
multiple 40Mbps downstream (broadcast) channels
single CMTS transmits into channels
multiple 30 Mbps upstream channels
multiple access all users contend for certain upstream channel time slots (others assigned)
Cable access network
cable modem
splitter
hellip
hellip
Internet framesTV channels control transmitted downstream at different frequencies
upstream Internet frames TV control transmitted upstream at different frequencies in time slots
Link Layer 5-39
DOCSIS data over cable service interface spec FDM over upstream downstream frequency channels
TDM upstream some slots assigned some have contention
downstream MAP frame assigns upstream slots
request for upstream slots (and data) transmitted random access (binary backoff) in selected slots
MAP frame for
Interval [t1 t2]
Residences with cable modems
Downstream channel i
Upstream channel j
t1 t2
Assigned minislots containing cable modem
upstream data frames
Minislots containing
minislots request frames
cable headend
CMTS
Cable access network
Link Layer 5-40
Summary of MAC protocols
channel partitioning by time frequency or code Time Division Frequency Division
random access (dynamic)
ALOHA S-ALOHA CSMA CSMACD
carrier sensing easy in some technologies (wire) hard in others (wireless)
CSMACD used in Ethernet
CSMACA used in 80211
taking turns
polling from central site token passing
bluetooth FDDI token ring
Link Layer 5-41
Link layer LANs outline
51 introduction services
52 error detection correction
53 multiple access protocols
54 LANs addressing ARP
Ethernet
switches
VLANS
55 link virtualization MPLS
56 data center networking
57 a day in the life of a web request
Link Layer 5-42
MAC addresses and ARP
32-bit IP address network-layer address for interface
used for layer 3 (network layer) forwarding
MAC (or LAN or physical or Ethernet) address function used lsquolocallyrdquo to get frame from one interface to
another physically-connected interface (same network in IP-addressing sense)
48 bit MAC address (for most LANs) burned in NIC ROM also sometimes software settable
eg 1A-2F-BB-76-09-AD
hexadecimal (base 16) notation
(each ldquonumberrdquo represents 4 bits)
Link Layer 5-43
LAN addresses and ARP
each adapter on LAN has unique LAN address
adapter
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN
(wired or
wireless)
Link Layer 5-44
LAN addresses (more)
MAC address allocation administered by IEEE
manufacturer buys portion of MAC address space (to assure uniqueness)
analogy MAC address like Social Security Number
IP address like postal address
MAC flat address portability can move LAN card from one LAN to another
IP hierarchical address not portable address depends on IP subnet to which node is
attached
Link Layer 5-45
ARP address resolution protocol
ARP table each IP node (host router) on LAN has table
IPMAC address mappings for some LAN nodes
lt IP address MAC address TTLgt
TTL (Time To Live) time after which address mapping will be forgotten (typically 20 min)
Question how to determine
interfacersquos MAC address
knowing its IP address
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN
137196723
137196778
137196714
137196788
Link Layer 5-46
ARP protocol same LAN
A wants to send datagram to B Brsquos MAC address not in
Arsquos ARP table
A broadcasts ARP query packet containing Bs IP address dest MAC address = FF-FF-
FF-FF-FF-FF
all nodes on LAN receive ARP query
B receives ARP packet replies to A with its (Bs) MAC address frame sent to Arsquos MAC
address (unicast)
A caches (saves) IP-to-MAC address pair in its ARP table until information becomes old (times out) soft state information that
times out (goes away) unless refreshed
ARP is ldquoplug-and-playrdquo nodes create their ARP
tables without intervention from net administrator
Link Layer 5-47
walkthrough send datagram from A to B via R
focus on addressing ndash at IP (datagram) and MAC layer (frame)
assume A knows Brsquos IP address
assume A knows IP address of first hop router R (how)
assume A knows Rrsquos MAC address (how)
Addressing routing to another LAN
R
1A-23-F9-CD-06-9B 222222222220
111111111110 E6-E9-00-17-BB-4B CC-49-DE-D0-AB-7D
111111111112
111111111111
74-29-9C-E8-FF-55
A
222222222222
49-BD-D2-C7-56-2A
222222222221 88-B2-2F-54-1A-0F
B
R
1A-23-F9-CD-06-9B 222222222220
111111111110 E6-E9-00-17-BB-4B CC-49-DE-D0-AB-7D
111111111112
111111111111
74-29-9C-E8-FF-55
A
222222222222
49-BD-D2-C7-56-2A
222222222221 88-B2-2F-54-1A-0F
B
Link Layer 5-48
Addressing routing to another LAN
IP
Eth
Phy
IP src 111111111111
IP dest 222222222222
A creates IP datagram with IP source A destination B
A creates link-layer frame with Rs MAC address as dest frame contains A-to-B IP datagram
MAC src 74-29-9C-E8-FF-55
MAC dest E6-E9-00-17-BB-4B
R
1A-23-F9-CD-06-9B 222222222220
111111111110 E6-E9-00-17-BB-4B CC-49-DE-D0-AB-7D
111111111112
111111111111
74-29-9C-E8-FF-55
A
222222222222
49-BD-D2-C7-56-2A
222222222221 88-B2-2F-54-1A-0F
B
Link Layer 5-49
Addressing routing to another LAN
IP
Eth
Phy
frame sent from A to R
IP
Eth
Phy
frame received at R datagram removed passed up to IP
MAC src 74-29-9C-E8-FF-55
MAC dest E6-E9-00-17-BB-4B
IP src 111111111111
IP dest 222222222222
IP src 111111111111
IP dest 222222222222
R
1A-23-F9-CD-06-9B 222222222220
111111111110 E6-E9-00-17-BB-4B CC-49-DE-D0-AB-7D
111111111112
111111111111
74-29-9C-E8-FF-55
A
222222222222
49-BD-D2-C7-56-2A
222222222221 88-B2-2F-54-1A-0F
B
Link Layer 5-50
Addressing routing to another LAN
IP src 111111111111
IP dest 222222222222
R forwards datagram with IP source A destination B
R creates link-layer frame with Bs MAC address as dest frame contains A-to-B IP datagram
MAC src 1A-23-F9-CD-06-9B
MAC dest 49-BD-D2-C7-56-2A
IP
Eth
Phy
IP
Eth
Phy
R
1A-23-F9-CD-06-9B 222222222220
111111111110 E6-E9-00-17-BB-4B CC-49-DE-D0-AB-7D
111111111112
111111111111
74-29-9C-E8-FF-55
A
222222222222
49-BD-D2-C7-56-2A
222222222221 88-B2-2F-54-1A-0F
B
Link Layer 5-51
Addressing routing to another LAN R forwards datagram with IP source A destination B
R creates link-layer frame with Bs MAC address as dest frame contains A-to-B IP datagram
IP src 111111111111
IP dest 222222222222
MAC src 1A-23-F9-CD-06-9B
MAC dest 49-BD-D2-C7-56-2A
IP
Eth
Phy
IP
Eth
Phy
R
1A-23-F9-CD-06-9B 222222222220
111111111110 E6-E9-00-17-BB-4B CC-49-DE-D0-AB-7D
111111111112
111111111111
74-29-9C-E8-FF-55
A
222222222222
49-BD-D2-C7-56-2A
222222222221 88-B2-2F-54-1A-0F
B
Link Layer 5-52
Addressing routing to another LAN R forwards datagram with IP source A destination B
R creates link-layer frame with Bs MAC address as dest frame contains A-to-B IP datagram
IP src 111111111111
IP dest 222222222222
MAC src 1A-23-F9-CD-06-9B
MAC dest 49-BD-D2-C7-56-2A
IP
Eth
Phy
Link Layer 5-53
Link layer LANs outline
51 introduction services
52 error detection correction
53 multiple access protocols
54 LANs addressing ARP
Ethernet
switches
VLANS
55 link virtualization MPLS
56 data center networking
57 a day in the life of a web request
Link Layer 5-54
Ethernet
ldquodominantrdquo wired LAN technology
cheap $20 for NIC
first widely used LAN technology
simpler cheaper than token LANs and ATM
kept up with speed race 10 Mbps ndash 10 Gbps
Metcalfersquos Ethernet sketch
Link Layer 5-55
Ethernet physical topology
bus popular through mid 90s all nodes in same collision domain (can collide with each
other)
star prevails today active switch in center
each ldquospokerdquo runs a (separate) Ethernet protocol (nodes
do not collide with each other)
switch
bus coaxial cable star
Link Layer 5-56
Ethernet frame structure
sending adapter encapsulates IP datagram (or other network layer protocol packet) in Ethernet frame
preamble
7 bytes with pattern 10101010 followed by one byte with pattern 10101011
used to synchronize receiver sender clock rates
dest address
source address
data (payload) CRC preamble
type
Link Layer 5-57
Ethernet frame structure (more)
addresses 6 byte source destination MAC addresses if adapter receives frame with matching destination
address or with broadcast address (eg ARP packet) it passes data in frame to network layer protocol
otherwise adapter discards frame
type indicates higher layer protocol (mostly IP but others possible eg Novell IPX AppleTalk)
CRC cyclic redundancy check at receiver error detected frame is dropped
dest address
source address
data (payload) CRC preamble
type
Link Layer 5-58
Ethernet unreliable connectionless
connectionless no handshaking between sending and receiving NICs
unreliable receiving NIC doesnt send acks or nacks to sending NIC
data in dropped frames recovered only if initial sender uses higher layer rdt (eg TCP) otherwise dropped data lost
Ethernetrsquos MAC protocol unslotted CSMACD wth binary backoff
Link Layer 5-59
8023 Ethernet standards link amp physical layers
many different Ethernet standards
common MAC protocol and frame format
different speeds 2 Mbps 10 Mbps 100 Mbps 1Gbps 10G bps
different physical layer media fiber cable
application
transport
network
link
physical
MAC protocol
and frame format
100BASE-TX
100BASE-T4
100BASE-FX 100BASE-T2
100BASE-SX 100BASE-BX
fiber physical layer copper (twister
pair) physical layer
Link Layer 5-60
Link layer LANs outline
51 introduction services
52 error detection correction
53 multiple access protocols
54 LANs addressing ARP
Ethernet
switches
VLANS
55 link virtualization MPLS
56 data center networking
57 a day in the life of a web request
Link Layer 5-61
Ethernet switch link-layer device takes an active role
store forward Ethernet frames
examine incoming framersquos MAC address selectively forward frame to one-or-more outgoing links when frame is to be forwarded on segment uses CSMACD to access segment
transparent
hosts are unaware of presence of switches
plug-and-play self-learning
switches do not need to be configured
Link Layer 5-62
Switch multiple simultaneous transmissions
hosts have dedicated direct connection to switch
switches buffer packets
Ethernet protocol used on each incoming link but no collisions full duplex
each link is its own collision domain
switching A-to-Arsquo and B-to-Brsquo can transmit simultaneously without collisions switch with six interfaces
(123456)
A
Arsquo
B
Brsquo C
Crsquo
1 2
3 4 5
6
Link Layer 5-63
Switch forwarding table
Q how does switch know Arsquo reachable via interface 4 Brsquo reachable via interface 5
switch with six interfaces
(123456)
A
Arsquo
B
Brsquo C
Crsquo
1 2
3 4 5
6 A each switch has a switch table each entry
(MAC address of host interface to
reach host time stamp)
looks like a routing table
Q how are entries created maintained in switch table
something like a routing protocol
A
Arsquo
B
Brsquo C
Crsquo
1 2
3 4 5
6
Link Layer 5-64
Switch self-learning
switch learns which hosts can be reached through which interfaces
when frame received switch ldquolearnsrdquo location of sender incoming LAN segment
records senderlocation pair in switch table
A Arsquo
Source A
Dest Arsquo
MAC addr interface TTL
Switch table
(initially empty) A 1 60
Link Layer 5-65
Switch frame filteringforwarding
when frame received at switch 1 record incoming link MAC address of sending host
2 index switch table using MAC destination address
3 if entry found for destination then
if destination on segment from which frame arrived then drop frame
else forward frame on interface indicated by entry
else flood forward on all interfaces except arriving
interface
A
Arsquo
B
Brsquo C
Crsquo
1 2
3 4 5
6
Link Layer 5-66
Self-learning forwarding example
A Arsquo
Source A
Dest Arsquo
MAC addr interface TTL
switch table
(initially empty) A 1 60
A Arsquo A Arsquo A Arsquo A Arsquo A Arsquo
frame destination Arsquo locaton unknown flood
Arsquo A
destination A location
known
Arsquo 4 60
selectively send
on just one link
Link Layer 5-67
Interconnecting switches
switches can be connected together
Q sending from A to G - how does S1 know to forward frame destined to F via S4 and S3
A self learning (works exactly the same as in single-switch case)
A
B
S1
C D
E
F
S2
S4
S3
H
I
G
Link Layer 5-68
Self-learning multi-switch example
Suppose C sends frame to I I responds to C
Q show switch tables and packet forwarding in S1 S2 S3 S4
A
B
S1
C D
E
F
S2
S4
S3
H
I
G
Link Layer 5-69
Institutional network
to external
network
router
IP subnet
mail server
web server
Link Layer 5-70
Switches vs routers
both are store-and-forward
routers network-layer devices (examine network-layer headers)
switches link-layer devices (examine link-layer headers)
both have forwarding tables
routers compute tables using routing algorithms IP addresses
switches learn forwarding table using flooding learning MAC addresses
application
transport
network
link
physical
network
link
physical
link
physical
switch
datagram
application
transport
network
link
physical
frame
frame
frame
datagram
Link Layer 5-71
VLANs motivation
consider CS user moves office to
EE but wants connect to CS switch
single broadcast domain
all layer-2 broadcast traffic (ARP DHCP unknown location of destination MAC address) must cross entire LAN
securityprivacy efficiency issues
Computer
Science Electrical
Engineering
Computer
Engineering
Link Layer 5-72
VLANs port-based VLAN switch ports
grouped (by switch management software) so that single physical switch helliphellip
switch(es) supporting
VLAN capabilities can
be configured to
define multiple virtual
LANS over single
physical LAN
infrastructure
Virtual Local
Area Network 1
8
9
16 10 2
7
hellip
Electrical Engineering
(VLAN ports 1-8)
Computer Science
(VLAN ports 9-15)
15
hellip
Electrical Engineering
(VLAN ports 1-8)
hellip
1
8 2
7 9
16 10
15
hellip
Computer Science
(VLAN ports 9-16)
hellip operates as multiple virtual switches
Link Layer 5-73
Port-based VLAN
1
8
9
16 10 2
7
hellip
Electrical Engineering
(VLAN ports 1-8)
Computer Science
(VLAN ports 9-15)
15
hellip
traffic isolation frames tofrom ports 1-8 can only reach ports 1-8 can also define VLAN based on
MAC addresses of endpoints rather than switch port
dynamic membership ports can be dynamically assigned among VLANs
router
forwarding between VLANS done via routing (just as with separate switches) in practice vendors sell combined
switches plus routers
Link Layer 5-74
VLANS spanning multiple switches
trunk port carries frames between VLANS defined over multiple physical switches frames forwarded within VLAN between switches canrsquot be vanilla
8021 frames (must carry VLAN ID info)
8021q protocol addsremoved additional header fields for frames forwarded between trunk ports
1
8
9
10 2
7
hellip
Electrical Engineering
(VLAN ports 1-8)
Computer Science
(VLAN ports 9-15)
15
hellip
2
7 3
Ports 235 belong to EE VLAN
Ports 4678 belong to CS VLAN
5
4 6 8 16
1
Link Layer 5-75
type
2-byte Tag Protocol Identifier
(value 81-00)
Tag Control Information (12 bit VLAN ID field
3 bit priority field like IP TOS)
Recomputed CRC
8021Q VLAN frame format
8021 frame
8021Q frame
dest address
source address
data (payload) CRC preamble
dest address
source address
preamble data (payload) CRC
type
Link Layer 5-76
Link layer LANs outline
51 introduction services
52 error detection correction
53 multiple access protocols
54 LANs addressing ARP
Ethernet
switches
VLANS
55 link virtualization MPLS
56 data center networking
57 a day in the life of a web request
Link Layer 5-77
Multiprotocol label switching (MPLS)
initial goal high-speed IP forwarding using fixed length label (instead of IP address) fast lookup using fixed length identifier (rather than
shortest prefix matching)
borrowing ideas from Virtual Circuit (VC) approach
but IP datagram still keeps IP address
PPP or Ethernet
header IP header remainder of link-layer frame MPLS header
label Exp S TTL
20 3 1 5
Link Layer 5-78
MPLS capable routers
aka label-switched router
forward packets to outgoing interface based only on label value (donrsquot inspect IP address) MPLS forwarding table distinct from IP forwarding tables
flexibility MPLS forwarding decisions can differ from those of IP use destination and source addresses to route flows to
same destination differently (traffic engineering)
re-route flows quickly if link fails pre-computed backup paths (useful for VoIP)
Link Layer 5-79
R2
D
R3 R5
A
R6
MPLS versus IP paths
IP router IP routing path to destination determined
by destination address alone
R4
Link Layer 5-80
R2
D
R3 R4
R5
A
R6
MPLS versus IP paths
IP-only router
IP routing path to destination determined by destination address alone
MPLS and IP router
MPLS routing path to destination can be based on source and dest address fast reroute precompute backup routes in
case of link failure
entry router (R4) can use different MPLS
routes to A based eg on source address
Link Layer 5-81
MPLS signaling
modify OSPF IS-IS link-state flooding protocols to carry info used by MPLS routing eg link bandwidth amount of ldquoreservedrdquo link bandwidth
D
R4
R5
A
R6
entry MPLS router uses RSVP-TE signaling protocol to set up MPLS forwarding at downstream routers
modified link state flooding
RSVP-TE
Link Layer 5-82
R1 R2
D
R3 R4 R5
0
1
0 0
A
R6
in out out
label label dest interface
6 - A 0
in out out
label label dest interface
10 6 A 1
12 9 D 0
in out out
label label dest interface
10 A 0
12 D 0
1
in out out
label label dest interface
8 6 A 0
0
8 A 1
MPLS forwarding tables
Link Layer 5-83
Link layer LANs outline
51 introduction services
52 error detection correction
53 multiple access protocols
54 LANs addressing ARP
Ethernet
switches
VLANS
55 link virtualization MPLS
56 data center networking
57 a day in the life of a web request
Link Layer 5-84
Data center networks
10rsquos to 100rsquos of thousands of hosts often closely coupled in close proximity e-business (eg Amazon)
content-servers (eg YouTube Akamai Apple Microsoft)
search engines data mining (eg Google)
challenges
multiple applications each serving massive numbers of clients
managingbalancing load avoiding processing networking data bottlenecks
Inside a 40-ft Microsoft container
Chicago data center
Link Layer 5-85
Server racks
TOR switches
Tier-1 switches
Tier-2 switches
Load balancer
Load balancer
B
1 2 3 4 5 6 7 8
A C
Border router
Access router
Internet
Data center networks
load balancer application-layer routing receives external client requests
directs workload within data center
returns results to external client (hiding data
center internals from client)
Server racks
TOR switches
Tier-1 switches
Tier-2 switches
1 2 3 4 5 6 7 8
Data center networks
rich interconnection among switches racks
increased throughput between racks (multiple routing
paths possible)
increased reliability via redundancy
Link Layer 5-87
Link layer LANs outline
51 introduction services
52 error detection correction
53 multiple access protocols
54 LANs addressing ARP
Ethernet
switches
VLANS
55 link virtualization MPLS
56 data center networking
57 a day in the life of a web request
Link Layer 5-88
Synthesis a day in the life of a web request
journey down protocol stack complete application transport network link
putting-it-all-together synthesis goal identify review understand protocols (at all
layers) involved in seemingly simple scenario requesting www page
scenario student attaches laptop to campus network requestsreceives wwwgooglecom
Link Layer 5-89
A day in the life scenario
Comcast network
68800013
Googlersquos network
64233160019 64233169105
web server
DNS server
school network
68802024
web page
browser
router
(runs DHCP)
Link Layer 5-90
A day in the lifehellip connecting to the Internet
connecting laptop needs to get its own IP address addr of first-hop router addr of DNS server use DHCP
DHCP
UDP
IP
Eth
Phy
DHCP
DHCP
DHCP
DHCP
DHCP
DHCP
UDP
IP
Eth
Phy
DHCP
DHCP
DHCP
DHCP DHCP
DHCP request encapsulated in UDP encapsulated in IP encapsulated in 8023 Ethernet
Ethernet frame broadcast
(dest FFFFFFFFFFFF) on LAN received at router running DHCP server
Ethernet demuxed to IP demuxed UDP demuxed to DHCP
router
(runs DHCP)
Link Layer 5-91
DHCP server formulates DHCP ACK containing clientrsquos IP address IP address of first-hop router for client name amp IP address of DNS server
DHCP
UDP
IP
Eth
Phy
DHCP
DHCP
DHCP
DHCP
DHCP
UDP
IP
Eth
Phy
DHCP
DHCP
DHCP
DHCP
DHCP
encapsulation at DHCP server frame forwarded (switch learning) through LAN demultiplexing at client
Client now has IP address knows name amp addr of DNS
server IP address of its first-hop router
DHCP client receives DHCP ACK reply
A day in the lifehellip connecting to the Internet
router
(runs DHCP)
Link Layer 5-92
A day in the lifehellip ARP (before DNS before HTTP)
before sending HTTP request need IP address of wwwgooglecom DNS
DNS
UDP
IP
Eth
Phy
DNS
DNS
DNS
DNS query created encapsulated in UDP encapsulated in IP encapsulated in Eth To send frame to router need MAC address of router interface ARP
ARP query broadcast received by
router which replies with ARP reply giving MAC address of router interface
client now knows MAC address of first hop router so can now send frame containing DNS query
ARP query
Eth
Phy
ARP
ARP
ARP reply
router
(runs DHCP)
Link Layer 5-93
DNS
UDP
IP
Eth
Phy
DNS
DNS
DNS
DNS
DNS
IP datagram containing DNS query forwarded via LAN switch from client to 1st hop router
IP datagram forwarded from campus network into comcast network routed (tables created by RIP OSPF IS-IS andor BGP routing protocols) to DNS server
demuxrsquoed to DNS server
DNS server replies to client with IP address of wwwgooglecom
Comcast network
68800013
DNS server
DNS
UDP
IP
Eth
Phy
DNS
DNS
DNS
DNS
A day in the lifehellip using DNS
router
(runs DHCP)
Link Layer 5-94
A day in the lifehellipTCP connection carrying HTTP
HTTP
TCP
IP
Eth
Phy
HTTP
to send HTTP request client first opens TCP socket to web server
TCP SYN segment (step 1 in 3-way handshake) inter-domain routed to web server
TCP connection established 64233169105
web server
SYN
SYN
SYN
SYN
TCP
IP
Eth
Phy
SYN
SYN
SYN
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
web server responds with TCP SYNACK (step 2 in 3-way handshake)
router
(runs DHCP)
Link Layer 5-95
A day in the lifehellip HTTP requestreply
HTTP
TCP
IP
Eth
Phy
HTTP
HTTP request sent into TCP socket
IP datagram containing HTTP request routed to wwwgooglecom
IP datagram containing HTTP reply routed back to client
64233169105
web server
HTTP
TCP
IP
Eth
Phy web server responds with
HTTP reply (containing web page)
HTTP
HTTP
HTTP HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
web page finally () displayed
Link Layer 5-96
Chapter 5 Summary
principles behind data link layer services error detection correction
sharing a broadcast channel multiple access
link layer addressing
instantiation and implementation of various link layer technologies Ethernet
switched LANS VLANs
virtualized networks as a link layer MPLS
synthesis a day in the life of a web request
Link Layer 5-97
Chapter 5 letrsquos take a breath
journey down protocol stack complete (except PHY)
solid understanding of networking principles practice
hellip could stop here hellip but lots of interesting topics wireless
multimedia
security
network management
Link Layer 5-13
Internet checksum (review)
sender treat segment contents
as sequence of 16-bit integers
checksum addition (1rsquos complement sum) of segment contents
sender puts checksum value into UDP checksum field
receiver compute checksum of
received segment check if computed
checksum equals checksum field value NO - error detected YES - no error detected
But maybe errors nonetheless
goal detect ldquoerrorsrdquo (eg flipped bits) in transmitted packet (note used at transport layer only)
Link Layer 5-14
Cyclic redundancy check
more powerful error-detection coding
view data bits D as a binary number
choose r+1 bit pattern (generator) G
goal choose r CRC bits R such that ltDRgt exactly divisible by G (modulo 2)
receiver knows G divides ltDRgt by G If non-zero remainder error detected
can detect all burst errors less than r+1 bits
widely used in practice (Ethernet 80211 WiFi ATM)
Link Layer 5-15
CRC example
want D2r XOR R = nG
equivalently D2r = nG XOR R
equivalently if we divide D2r by
G want remainder R to satisfy
R = remainder[ ] D2r
G
1001 101110000 1001
1
101
01000
000 1010 1001
010 000
100 000 1000 0000 1000
D G
R
r = 3
Link Layer 5-16
Link layer LANs outline
51 introduction services
52 error detection correction
53 multiple access protocols
54 LANs addressing ARP
Ethernet
switches
VLANS
55 link virtualization MPLS
56 data center networking
57 a day in the life of a web request
Link Layer 5-17
Multiple access links protocols
two types of ldquolinksrdquo
point-to-point PPP for dial-up access
point-to-point link between Ethernet switch host
broadcast (shared wire or medium) old-fashioned Ethernet
upstream HFC
80211 wireless LAN
shared wire (eg cabled Ethernet)
shared RF (eg 80211 WiFi)
shared RF (satellite)
humans at a cocktail party
(shared air acoustical)
Link Layer 5-18
Multiple access protocols
single shared broadcast channel
two or more simultaneous transmissions by nodes interference
collision if node receives two or more signals at the same time
multiple access protocol distributed algorithm that determines how nodes share
channel ie determine when node can transmit
communication about channel sharing must use channel itself no out-of-band channel for coordination
Link Layer 5-19
An ideal multiple access protocol
given broadcast channel of rate R bps
desiderata 1 when one node wants to transmit it can send at rate R
2 when M nodes want to transmit each can send at average rate RM
3 fully decentralized
bull no special node to coordinate transmissions
bull no synchronization of clocks slots
4 simple
Link Layer 5-20
MAC protocols taxonomy
three broad classes
channel partitioning divide channel into smaller ldquopiecesrdquo (time slots frequency code)
allocate piece to node for exclusive use
random access channel not divided allow collisions
ldquorecoverrdquo from collisions
ldquotaking turnsrdquo nodes take turns but nodes with more to send can take longer
turns
Link Layer 5-21
Channel partitioning MAC protocols TDMA
TDMA time division multiple access access to channel in rounds each station gets fixed length slot (length = pkt
trans time) in each round unused slots go idle example 6-station LAN 134 have pkt slots
256 idle
1 3 4 1 3 4
6-slot
frame
6-slot
frame
Link Layer 5-22
FDMA frequency division multiple access channel spectrum divided into frequency bands
each station assigned fixed frequency band
unused transmission time in frequency bands go idle
example 6-station LAN 134 have pkt frequency bands 256 idle
frequency b
ands
FDM cable
Channel partitioning MAC protocols FDMA
Link Layer 5-23
Random access protocols
when node has packet to send transmit at full channel data rate R no a priori coordination among nodes
two or more transmitting nodes ldquocollisionrdquo random access MAC protocol specifies
how to detect collisions how to recover from collisions (eg via delayed
retransmissions)
examples of random access MAC protocols slotted ALOHA ALOHA CSMA CSMACD CSMACA
Link Layer 5-24
Slotted ALOHA
assumptions all frames same size
time divided into equal size slots (time to transmit 1 frame)
nodes start to transmit only slot beginning
nodes are synchronized
if 2 or more nodes transmit in slot all nodes detect collision
operation
when node obtains fresh frame transmits in next slot
if no collision node can send new frame in next slot
if collision node retransmits frame in each subsequent slot with prob p until success
Link Layer 5-25
Pros single active node can
continuously transmit at full rate of channel
highly decentralized only slots in nodes need to be in sync
simple
Cons
collisions wasting slots
idle slots
nodes may be able to detect collision in less than time to transmit packet
clock synchronization
Slotted ALOHA
1 1 1 1
2
3
2 2
3 3
node 1
node 2
node 3
C C C S S S E E E
Link Layer 5-26
suppose N nodes with many frames to send each transmits in slot with probability p
prob that given node has success in a slot = p(1-p)N-1
prob that any node has a success = Np(1-p)N-1
max efficiency find p that maximizes Np(1-p)N-1
for many nodes take limit of Np(1-p)N-1 as N goes to infinity gives
max efficiency = 1e = 37
efficiency long-run fraction of successful slots (many nodes all with many frames to send)
at best channel used for useful transmissions 37 of time
Slotted ALOHA efficiency
Link Layer 5-27
Pure (unslotted) ALOHA
unslotted Aloha simpler no synchronization
when frame first arrives
transmit immediately
collision probability increases
frame sent at t0 collides with other frames sent in [t0-1t0+1]
Link Layer 5-28
Pure ALOHA efficiency
P(success by given node) = P(node transmits)
P(no other node transmits in [t0-1t0]
P(no other node transmits in [t0-1t0]
= p (1-p)N-1 (1-p)N-1
= p (1-p)2(N-1)
hellip choosing optimum p and then letting n
= 1(2e) = 18
even worse than slotted Aloha
Link Layer 5-29
CSMA (carrier sense multiple access)
CSMA listen before transmit if channel sensed idle transmit entire frame
if channel sensed busy defer transmission
human analogy donrsquot interrupt others
Link Layer 5-30
CSMA collisions
collisions can still occur propagation delay means two nodes may not hear each otherrsquos transmission
collision entire packet transmission time wasted distance amp propagation
delay play role in in determining collision probability
spatial layout of nodes
Link Layer 5-31
CSMACD (collision detection)
CSMACD carrier sensing deferral as in CSMA collisions detected within short time
colliding transmissions aborted reducing channel wastage
collision detection easy in wired LANs measure signal strengths compare
transmitted received signals
difficult in wireless LANs received signal strength overwhelmed by local transmission strength
human analogy the polite conversationalist
Link Layer 5-32
CSMACD (collision detection)
spatial layout of nodes
Link Layer 5-33
Ethernet CSMACD algorithm
1 NIC receives datagram from network layer creates frame
2 If NIC senses channel idle starts frame transmission If NIC senses channel busy waits until channel idle then transmits
3 If NIC transmits entire frame without detecting another transmission NIC is done with frame
4 If NIC detects another transmission while transmitting aborts and sends jam signal
5 After aborting NIC enters binary (exponential) backoff
after mth collision NIC chooses K at random from 012 hellip 2m-1 NIC waits K512 bit times returns to Step 2
longer backoff interval with more collisions
Link Layer 5-34
CSMACD efficiency
Tprop = max prop delay between 2 nodes in LAN
ttrans = time to transmit max-size frame
efficiency goes to 1
as tprop goes to 0
as ttrans goes to infinity
better performance than ALOHA and simple cheap decentralized
transpropttefficiency
51
1
Link Layer 5-35
ldquoTaking turnsrdquo MAC protocols
channel partitioning MAC protocols share channel efficiently and fairly at high load
inefficient at low load delay in channel access 1N bandwidth allocated even if only 1 active node
random access MAC protocols efficient at low load single node can fully utilize
channel
high load collision overhead
ldquotaking turnsrdquo protocols look for best of both worlds
Link Layer 5-36
polling master node ldquoinvitesrdquo
slave nodes to transmit in turn
typically used with ldquodumbrdquo slave devices
concerns
polling overhead
latency
single point of failure (master)
master
slaves
poll
data
data
ldquoTaking turnsrdquo MAC protocols
Link Layer 5-37
token passing control token passed
from one node to next sequentially
token message
concerns
token overhead
latency
single point of failure (token)
T
data
(nothing
to send)
T
ldquoTaking turnsrdquo MAC protocols
cable headend
CMTS
ISP
cable modem termination system
multiple 40Mbps downstream (broadcast) channels
single CMTS transmits into channels
multiple 30 Mbps upstream channels
multiple access all users contend for certain upstream channel time slots (others assigned)
Cable access network
cable modem
splitter
hellip
hellip
Internet framesTV channels control transmitted downstream at different frequencies
upstream Internet frames TV control transmitted upstream at different frequencies in time slots
Link Layer 5-39
DOCSIS data over cable service interface spec FDM over upstream downstream frequency channels
TDM upstream some slots assigned some have contention
downstream MAP frame assigns upstream slots
request for upstream slots (and data) transmitted random access (binary backoff) in selected slots
MAP frame for
Interval [t1 t2]
Residences with cable modems
Downstream channel i
Upstream channel j
t1 t2
Assigned minislots containing cable modem
upstream data frames
Minislots containing
minislots request frames
cable headend
CMTS
Cable access network
Link Layer 5-40
Summary of MAC protocols
channel partitioning by time frequency or code Time Division Frequency Division
random access (dynamic)
ALOHA S-ALOHA CSMA CSMACD
carrier sensing easy in some technologies (wire) hard in others (wireless)
CSMACD used in Ethernet
CSMACA used in 80211
taking turns
polling from central site token passing
bluetooth FDDI token ring
Link Layer 5-41
Link layer LANs outline
51 introduction services
52 error detection correction
53 multiple access protocols
54 LANs addressing ARP
Ethernet
switches
VLANS
55 link virtualization MPLS
56 data center networking
57 a day in the life of a web request
Link Layer 5-42
MAC addresses and ARP
32-bit IP address network-layer address for interface
used for layer 3 (network layer) forwarding
MAC (or LAN or physical or Ethernet) address function used lsquolocallyrdquo to get frame from one interface to
another physically-connected interface (same network in IP-addressing sense)
48 bit MAC address (for most LANs) burned in NIC ROM also sometimes software settable
eg 1A-2F-BB-76-09-AD
hexadecimal (base 16) notation
(each ldquonumberrdquo represents 4 bits)
Link Layer 5-43
LAN addresses and ARP
each adapter on LAN has unique LAN address
adapter
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN
(wired or
wireless)
Link Layer 5-44
LAN addresses (more)
MAC address allocation administered by IEEE
manufacturer buys portion of MAC address space (to assure uniqueness)
analogy MAC address like Social Security Number
IP address like postal address
MAC flat address portability can move LAN card from one LAN to another
IP hierarchical address not portable address depends on IP subnet to which node is
attached
Link Layer 5-45
ARP address resolution protocol
ARP table each IP node (host router) on LAN has table
IPMAC address mappings for some LAN nodes
lt IP address MAC address TTLgt
TTL (Time To Live) time after which address mapping will be forgotten (typically 20 min)
Question how to determine
interfacersquos MAC address
knowing its IP address
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN
137196723
137196778
137196714
137196788
Link Layer 5-46
ARP protocol same LAN
A wants to send datagram to B Brsquos MAC address not in
Arsquos ARP table
A broadcasts ARP query packet containing Bs IP address dest MAC address = FF-FF-
FF-FF-FF-FF
all nodes on LAN receive ARP query
B receives ARP packet replies to A with its (Bs) MAC address frame sent to Arsquos MAC
address (unicast)
A caches (saves) IP-to-MAC address pair in its ARP table until information becomes old (times out) soft state information that
times out (goes away) unless refreshed
ARP is ldquoplug-and-playrdquo nodes create their ARP
tables without intervention from net administrator
Link Layer 5-47
walkthrough send datagram from A to B via R
focus on addressing ndash at IP (datagram) and MAC layer (frame)
assume A knows Brsquos IP address
assume A knows IP address of first hop router R (how)
assume A knows Rrsquos MAC address (how)
Addressing routing to another LAN
R
1A-23-F9-CD-06-9B 222222222220
111111111110 E6-E9-00-17-BB-4B CC-49-DE-D0-AB-7D
111111111112
111111111111
74-29-9C-E8-FF-55
A
222222222222
49-BD-D2-C7-56-2A
222222222221 88-B2-2F-54-1A-0F
B
R
1A-23-F9-CD-06-9B 222222222220
111111111110 E6-E9-00-17-BB-4B CC-49-DE-D0-AB-7D
111111111112
111111111111
74-29-9C-E8-FF-55
A
222222222222
49-BD-D2-C7-56-2A
222222222221 88-B2-2F-54-1A-0F
B
Link Layer 5-48
Addressing routing to another LAN
IP
Eth
Phy
IP src 111111111111
IP dest 222222222222
A creates IP datagram with IP source A destination B
A creates link-layer frame with Rs MAC address as dest frame contains A-to-B IP datagram
MAC src 74-29-9C-E8-FF-55
MAC dest E6-E9-00-17-BB-4B
R
1A-23-F9-CD-06-9B 222222222220
111111111110 E6-E9-00-17-BB-4B CC-49-DE-D0-AB-7D
111111111112
111111111111
74-29-9C-E8-FF-55
A
222222222222
49-BD-D2-C7-56-2A
222222222221 88-B2-2F-54-1A-0F
B
Link Layer 5-49
Addressing routing to another LAN
IP
Eth
Phy
frame sent from A to R
IP
Eth
Phy
frame received at R datagram removed passed up to IP
MAC src 74-29-9C-E8-FF-55
MAC dest E6-E9-00-17-BB-4B
IP src 111111111111
IP dest 222222222222
IP src 111111111111
IP dest 222222222222
R
1A-23-F9-CD-06-9B 222222222220
111111111110 E6-E9-00-17-BB-4B CC-49-DE-D0-AB-7D
111111111112
111111111111
74-29-9C-E8-FF-55
A
222222222222
49-BD-D2-C7-56-2A
222222222221 88-B2-2F-54-1A-0F
B
Link Layer 5-50
Addressing routing to another LAN
IP src 111111111111
IP dest 222222222222
R forwards datagram with IP source A destination B
R creates link-layer frame with Bs MAC address as dest frame contains A-to-B IP datagram
MAC src 1A-23-F9-CD-06-9B
MAC dest 49-BD-D2-C7-56-2A
IP
Eth
Phy
IP
Eth
Phy
R
1A-23-F9-CD-06-9B 222222222220
111111111110 E6-E9-00-17-BB-4B CC-49-DE-D0-AB-7D
111111111112
111111111111
74-29-9C-E8-FF-55
A
222222222222
49-BD-D2-C7-56-2A
222222222221 88-B2-2F-54-1A-0F
B
Link Layer 5-51
Addressing routing to another LAN R forwards datagram with IP source A destination B
R creates link-layer frame with Bs MAC address as dest frame contains A-to-B IP datagram
IP src 111111111111
IP dest 222222222222
MAC src 1A-23-F9-CD-06-9B
MAC dest 49-BD-D2-C7-56-2A
IP
Eth
Phy
IP
Eth
Phy
R
1A-23-F9-CD-06-9B 222222222220
111111111110 E6-E9-00-17-BB-4B CC-49-DE-D0-AB-7D
111111111112
111111111111
74-29-9C-E8-FF-55
A
222222222222
49-BD-D2-C7-56-2A
222222222221 88-B2-2F-54-1A-0F
B
Link Layer 5-52
Addressing routing to another LAN R forwards datagram with IP source A destination B
R creates link-layer frame with Bs MAC address as dest frame contains A-to-B IP datagram
IP src 111111111111
IP dest 222222222222
MAC src 1A-23-F9-CD-06-9B
MAC dest 49-BD-D2-C7-56-2A
IP
Eth
Phy
Link Layer 5-53
Link layer LANs outline
51 introduction services
52 error detection correction
53 multiple access protocols
54 LANs addressing ARP
Ethernet
switches
VLANS
55 link virtualization MPLS
56 data center networking
57 a day in the life of a web request
Link Layer 5-54
Ethernet
ldquodominantrdquo wired LAN technology
cheap $20 for NIC
first widely used LAN technology
simpler cheaper than token LANs and ATM
kept up with speed race 10 Mbps ndash 10 Gbps
Metcalfersquos Ethernet sketch
Link Layer 5-55
Ethernet physical topology
bus popular through mid 90s all nodes in same collision domain (can collide with each
other)
star prevails today active switch in center
each ldquospokerdquo runs a (separate) Ethernet protocol (nodes
do not collide with each other)
switch
bus coaxial cable star
Link Layer 5-56
Ethernet frame structure
sending adapter encapsulates IP datagram (or other network layer protocol packet) in Ethernet frame
preamble
7 bytes with pattern 10101010 followed by one byte with pattern 10101011
used to synchronize receiver sender clock rates
dest address
source address
data (payload) CRC preamble
type
Link Layer 5-57
Ethernet frame structure (more)
addresses 6 byte source destination MAC addresses if adapter receives frame with matching destination
address or with broadcast address (eg ARP packet) it passes data in frame to network layer protocol
otherwise adapter discards frame
type indicates higher layer protocol (mostly IP but others possible eg Novell IPX AppleTalk)
CRC cyclic redundancy check at receiver error detected frame is dropped
dest address
source address
data (payload) CRC preamble
type
Link Layer 5-58
Ethernet unreliable connectionless
connectionless no handshaking between sending and receiving NICs
unreliable receiving NIC doesnt send acks or nacks to sending NIC
data in dropped frames recovered only if initial sender uses higher layer rdt (eg TCP) otherwise dropped data lost
Ethernetrsquos MAC protocol unslotted CSMACD wth binary backoff
Link Layer 5-59
8023 Ethernet standards link amp physical layers
many different Ethernet standards
common MAC protocol and frame format
different speeds 2 Mbps 10 Mbps 100 Mbps 1Gbps 10G bps
different physical layer media fiber cable
application
transport
network
link
physical
MAC protocol
and frame format
100BASE-TX
100BASE-T4
100BASE-FX 100BASE-T2
100BASE-SX 100BASE-BX
fiber physical layer copper (twister
pair) physical layer
Link Layer 5-60
Link layer LANs outline
51 introduction services
52 error detection correction
53 multiple access protocols
54 LANs addressing ARP
Ethernet
switches
VLANS
55 link virtualization MPLS
56 data center networking
57 a day in the life of a web request
Link Layer 5-61
Ethernet switch link-layer device takes an active role
store forward Ethernet frames
examine incoming framersquos MAC address selectively forward frame to one-or-more outgoing links when frame is to be forwarded on segment uses CSMACD to access segment
transparent
hosts are unaware of presence of switches
plug-and-play self-learning
switches do not need to be configured
Link Layer 5-62
Switch multiple simultaneous transmissions
hosts have dedicated direct connection to switch
switches buffer packets
Ethernet protocol used on each incoming link but no collisions full duplex
each link is its own collision domain
switching A-to-Arsquo and B-to-Brsquo can transmit simultaneously without collisions switch with six interfaces
(123456)
A
Arsquo
B
Brsquo C
Crsquo
1 2
3 4 5
6
Link Layer 5-63
Switch forwarding table
Q how does switch know Arsquo reachable via interface 4 Brsquo reachable via interface 5
switch with six interfaces
(123456)
A
Arsquo
B
Brsquo C
Crsquo
1 2
3 4 5
6 A each switch has a switch table each entry
(MAC address of host interface to
reach host time stamp)
looks like a routing table
Q how are entries created maintained in switch table
something like a routing protocol
A
Arsquo
B
Brsquo C
Crsquo
1 2
3 4 5
6
Link Layer 5-64
Switch self-learning
switch learns which hosts can be reached through which interfaces
when frame received switch ldquolearnsrdquo location of sender incoming LAN segment
records senderlocation pair in switch table
A Arsquo
Source A
Dest Arsquo
MAC addr interface TTL
Switch table
(initially empty) A 1 60
Link Layer 5-65
Switch frame filteringforwarding
when frame received at switch 1 record incoming link MAC address of sending host
2 index switch table using MAC destination address
3 if entry found for destination then
if destination on segment from which frame arrived then drop frame
else forward frame on interface indicated by entry
else flood forward on all interfaces except arriving
interface
A
Arsquo
B
Brsquo C
Crsquo
1 2
3 4 5
6
Link Layer 5-66
Self-learning forwarding example
A Arsquo
Source A
Dest Arsquo
MAC addr interface TTL
switch table
(initially empty) A 1 60
A Arsquo A Arsquo A Arsquo A Arsquo A Arsquo
frame destination Arsquo locaton unknown flood
Arsquo A
destination A location
known
Arsquo 4 60
selectively send
on just one link
Link Layer 5-67
Interconnecting switches
switches can be connected together
Q sending from A to G - how does S1 know to forward frame destined to F via S4 and S3
A self learning (works exactly the same as in single-switch case)
A
B
S1
C D
E
F
S2
S4
S3
H
I
G
Link Layer 5-68
Self-learning multi-switch example
Suppose C sends frame to I I responds to C
Q show switch tables and packet forwarding in S1 S2 S3 S4
A
B
S1
C D
E
F
S2
S4
S3
H
I
G
Link Layer 5-69
Institutional network
to external
network
router
IP subnet
mail server
web server
Link Layer 5-70
Switches vs routers
both are store-and-forward
routers network-layer devices (examine network-layer headers)
switches link-layer devices (examine link-layer headers)
both have forwarding tables
routers compute tables using routing algorithms IP addresses
switches learn forwarding table using flooding learning MAC addresses
application
transport
network
link
physical
network
link
physical
link
physical
switch
datagram
application
transport
network
link
physical
frame
frame
frame
datagram
Link Layer 5-71
VLANs motivation
consider CS user moves office to
EE but wants connect to CS switch
single broadcast domain
all layer-2 broadcast traffic (ARP DHCP unknown location of destination MAC address) must cross entire LAN
securityprivacy efficiency issues
Computer
Science Electrical
Engineering
Computer
Engineering
Link Layer 5-72
VLANs port-based VLAN switch ports
grouped (by switch management software) so that single physical switch helliphellip
switch(es) supporting
VLAN capabilities can
be configured to
define multiple virtual
LANS over single
physical LAN
infrastructure
Virtual Local
Area Network 1
8
9
16 10 2
7
hellip
Electrical Engineering
(VLAN ports 1-8)
Computer Science
(VLAN ports 9-15)
15
hellip
Electrical Engineering
(VLAN ports 1-8)
hellip
1
8 2
7 9
16 10
15
hellip
Computer Science
(VLAN ports 9-16)
hellip operates as multiple virtual switches
Link Layer 5-73
Port-based VLAN
1
8
9
16 10 2
7
hellip
Electrical Engineering
(VLAN ports 1-8)
Computer Science
(VLAN ports 9-15)
15
hellip
traffic isolation frames tofrom ports 1-8 can only reach ports 1-8 can also define VLAN based on
MAC addresses of endpoints rather than switch port
dynamic membership ports can be dynamically assigned among VLANs
router
forwarding between VLANS done via routing (just as with separate switches) in practice vendors sell combined
switches plus routers
Link Layer 5-74
VLANS spanning multiple switches
trunk port carries frames between VLANS defined over multiple physical switches frames forwarded within VLAN between switches canrsquot be vanilla
8021 frames (must carry VLAN ID info)
8021q protocol addsremoved additional header fields for frames forwarded between trunk ports
1
8
9
10 2
7
hellip
Electrical Engineering
(VLAN ports 1-8)
Computer Science
(VLAN ports 9-15)
15
hellip
2
7 3
Ports 235 belong to EE VLAN
Ports 4678 belong to CS VLAN
5
4 6 8 16
1
Link Layer 5-75
type
2-byte Tag Protocol Identifier
(value 81-00)
Tag Control Information (12 bit VLAN ID field
3 bit priority field like IP TOS)
Recomputed CRC
8021Q VLAN frame format
8021 frame
8021Q frame
dest address
source address
data (payload) CRC preamble
dest address
source address
preamble data (payload) CRC
type
Link Layer 5-76
Link layer LANs outline
51 introduction services
52 error detection correction
53 multiple access protocols
54 LANs addressing ARP
Ethernet
switches
VLANS
55 link virtualization MPLS
56 data center networking
57 a day in the life of a web request
Link Layer 5-77
Multiprotocol label switching (MPLS)
initial goal high-speed IP forwarding using fixed length label (instead of IP address) fast lookup using fixed length identifier (rather than
shortest prefix matching)
borrowing ideas from Virtual Circuit (VC) approach
but IP datagram still keeps IP address
PPP or Ethernet
header IP header remainder of link-layer frame MPLS header
label Exp S TTL
20 3 1 5
Link Layer 5-78
MPLS capable routers
aka label-switched router
forward packets to outgoing interface based only on label value (donrsquot inspect IP address) MPLS forwarding table distinct from IP forwarding tables
flexibility MPLS forwarding decisions can differ from those of IP use destination and source addresses to route flows to
same destination differently (traffic engineering)
re-route flows quickly if link fails pre-computed backup paths (useful for VoIP)
Link Layer 5-79
R2
D
R3 R5
A
R6
MPLS versus IP paths
IP router IP routing path to destination determined
by destination address alone
R4
Link Layer 5-80
R2
D
R3 R4
R5
A
R6
MPLS versus IP paths
IP-only router
IP routing path to destination determined by destination address alone
MPLS and IP router
MPLS routing path to destination can be based on source and dest address fast reroute precompute backup routes in
case of link failure
entry router (R4) can use different MPLS
routes to A based eg on source address
Link Layer 5-81
MPLS signaling
modify OSPF IS-IS link-state flooding protocols to carry info used by MPLS routing eg link bandwidth amount of ldquoreservedrdquo link bandwidth
D
R4
R5
A
R6
entry MPLS router uses RSVP-TE signaling protocol to set up MPLS forwarding at downstream routers
modified link state flooding
RSVP-TE
Link Layer 5-82
R1 R2
D
R3 R4 R5
0
1
0 0
A
R6
in out out
label label dest interface
6 - A 0
in out out
label label dest interface
10 6 A 1
12 9 D 0
in out out
label label dest interface
10 A 0
12 D 0
1
in out out
label label dest interface
8 6 A 0
0
8 A 1
MPLS forwarding tables
Link Layer 5-83
Link layer LANs outline
51 introduction services
52 error detection correction
53 multiple access protocols
54 LANs addressing ARP
Ethernet
switches
VLANS
55 link virtualization MPLS
56 data center networking
57 a day in the life of a web request
Link Layer 5-84
Data center networks
10rsquos to 100rsquos of thousands of hosts often closely coupled in close proximity e-business (eg Amazon)
content-servers (eg YouTube Akamai Apple Microsoft)
search engines data mining (eg Google)
challenges
multiple applications each serving massive numbers of clients
managingbalancing load avoiding processing networking data bottlenecks
Inside a 40-ft Microsoft container
Chicago data center
Link Layer 5-85
Server racks
TOR switches
Tier-1 switches
Tier-2 switches
Load balancer
Load balancer
B
1 2 3 4 5 6 7 8
A C
Border router
Access router
Internet
Data center networks
load balancer application-layer routing receives external client requests
directs workload within data center
returns results to external client (hiding data
center internals from client)
Server racks
TOR switches
Tier-1 switches
Tier-2 switches
1 2 3 4 5 6 7 8
Data center networks
rich interconnection among switches racks
increased throughput between racks (multiple routing
paths possible)
increased reliability via redundancy
Link Layer 5-87
Link layer LANs outline
51 introduction services
52 error detection correction
53 multiple access protocols
54 LANs addressing ARP
Ethernet
switches
VLANS
55 link virtualization MPLS
56 data center networking
57 a day in the life of a web request
Link Layer 5-88
Synthesis a day in the life of a web request
journey down protocol stack complete application transport network link
putting-it-all-together synthesis goal identify review understand protocols (at all
layers) involved in seemingly simple scenario requesting www page
scenario student attaches laptop to campus network requestsreceives wwwgooglecom
Link Layer 5-89
A day in the life scenario
Comcast network
68800013
Googlersquos network
64233160019 64233169105
web server
DNS server
school network
68802024
web page
browser
router
(runs DHCP)
Link Layer 5-90
A day in the lifehellip connecting to the Internet
connecting laptop needs to get its own IP address addr of first-hop router addr of DNS server use DHCP
DHCP
UDP
IP
Eth
Phy
DHCP
DHCP
DHCP
DHCP
DHCP
DHCP
UDP
IP
Eth
Phy
DHCP
DHCP
DHCP
DHCP DHCP
DHCP request encapsulated in UDP encapsulated in IP encapsulated in 8023 Ethernet
Ethernet frame broadcast
(dest FFFFFFFFFFFF) on LAN received at router running DHCP server
Ethernet demuxed to IP demuxed UDP demuxed to DHCP
router
(runs DHCP)
Link Layer 5-91
DHCP server formulates DHCP ACK containing clientrsquos IP address IP address of first-hop router for client name amp IP address of DNS server
DHCP
UDP
IP
Eth
Phy
DHCP
DHCP
DHCP
DHCP
DHCP
UDP
IP
Eth
Phy
DHCP
DHCP
DHCP
DHCP
DHCP
encapsulation at DHCP server frame forwarded (switch learning) through LAN demultiplexing at client
Client now has IP address knows name amp addr of DNS
server IP address of its first-hop router
DHCP client receives DHCP ACK reply
A day in the lifehellip connecting to the Internet
router
(runs DHCP)
Link Layer 5-92
A day in the lifehellip ARP (before DNS before HTTP)
before sending HTTP request need IP address of wwwgooglecom DNS
DNS
UDP
IP
Eth
Phy
DNS
DNS
DNS
DNS query created encapsulated in UDP encapsulated in IP encapsulated in Eth To send frame to router need MAC address of router interface ARP
ARP query broadcast received by
router which replies with ARP reply giving MAC address of router interface
client now knows MAC address of first hop router so can now send frame containing DNS query
ARP query
Eth
Phy
ARP
ARP
ARP reply
router
(runs DHCP)
Link Layer 5-93
DNS
UDP
IP
Eth
Phy
DNS
DNS
DNS
DNS
DNS
IP datagram containing DNS query forwarded via LAN switch from client to 1st hop router
IP datagram forwarded from campus network into comcast network routed (tables created by RIP OSPF IS-IS andor BGP routing protocols) to DNS server
demuxrsquoed to DNS server
DNS server replies to client with IP address of wwwgooglecom
Comcast network
68800013
DNS server
DNS
UDP
IP
Eth
Phy
DNS
DNS
DNS
DNS
A day in the lifehellip using DNS
router
(runs DHCP)
Link Layer 5-94
A day in the lifehellipTCP connection carrying HTTP
HTTP
TCP
IP
Eth
Phy
HTTP
to send HTTP request client first opens TCP socket to web server
TCP SYN segment (step 1 in 3-way handshake) inter-domain routed to web server
TCP connection established 64233169105
web server
SYN
SYN
SYN
SYN
TCP
IP
Eth
Phy
SYN
SYN
SYN
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
web server responds with TCP SYNACK (step 2 in 3-way handshake)
router
(runs DHCP)
Link Layer 5-95
A day in the lifehellip HTTP requestreply
HTTP
TCP
IP
Eth
Phy
HTTP
HTTP request sent into TCP socket
IP datagram containing HTTP request routed to wwwgooglecom
IP datagram containing HTTP reply routed back to client
64233169105
web server
HTTP
TCP
IP
Eth
Phy web server responds with
HTTP reply (containing web page)
HTTP
HTTP
HTTP HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
web page finally () displayed
Link Layer 5-96
Chapter 5 Summary
principles behind data link layer services error detection correction
sharing a broadcast channel multiple access
link layer addressing
instantiation and implementation of various link layer technologies Ethernet
switched LANS VLANs
virtualized networks as a link layer MPLS
synthesis a day in the life of a web request
Link Layer 5-97
Chapter 5 letrsquos take a breath
journey down protocol stack complete (except PHY)
solid understanding of networking principles practice
hellip could stop here hellip but lots of interesting topics wireless
multimedia
security
network management
Link Layer 5-14
Cyclic redundancy check
more powerful error-detection coding
view data bits D as a binary number
choose r+1 bit pattern (generator) G
goal choose r CRC bits R such that ltDRgt exactly divisible by G (modulo 2)
receiver knows G divides ltDRgt by G If non-zero remainder error detected
can detect all burst errors less than r+1 bits
widely used in practice (Ethernet 80211 WiFi ATM)
Link Layer 5-15
CRC example
want D2r XOR R = nG
equivalently D2r = nG XOR R
equivalently if we divide D2r by
G want remainder R to satisfy
R = remainder[ ] D2r
G
1001 101110000 1001
1
101
01000
000 1010 1001
010 000
100 000 1000 0000 1000
D G
R
r = 3
Link Layer 5-16
Link layer LANs outline
51 introduction services
52 error detection correction
53 multiple access protocols
54 LANs addressing ARP
Ethernet
switches
VLANS
55 link virtualization MPLS
56 data center networking
57 a day in the life of a web request
Link Layer 5-17
Multiple access links protocols
two types of ldquolinksrdquo
point-to-point PPP for dial-up access
point-to-point link between Ethernet switch host
broadcast (shared wire or medium) old-fashioned Ethernet
upstream HFC
80211 wireless LAN
shared wire (eg cabled Ethernet)
shared RF (eg 80211 WiFi)
shared RF (satellite)
humans at a cocktail party
(shared air acoustical)
Link Layer 5-18
Multiple access protocols
single shared broadcast channel
two or more simultaneous transmissions by nodes interference
collision if node receives two or more signals at the same time
multiple access protocol distributed algorithm that determines how nodes share
channel ie determine when node can transmit
communication about channel sharing must use channel itself no out-of-band channel for coordination
Link Layer 5-19
An ideal multiple access protocol
given broadcast channel of rate R bps
desiderata 1 when one node wants to transmit it can send at rate R
2 when M nodes want to transmit each can send at average rate RM
3 fully decentralized
bull no special node to coordinate transmissions
bull no synchronization of clocks slots
4 simple
Link Layer 5-20
MAC protocols taxonomy
three broad classes
channel partitioning divide channel into smaller ldquopiecesrdquo (time slots frequency code)
allocate piece to node for exclusive use
random access channel not divided allow collisions
ldquorecoverrdquo from collisions
ldquotaking turnsrdquo nodes take turns but nodes with more to send can take longer
turns
Link Layer 5-21
Channel partitioning MAC protocols TDMA
TDMA time division multiple access access to channel in rounds each station gets fixed length slot (length = pkt
trans time) in each round unused slots go idle example 6-station LAN 134 have pkt slots
256 idle
1 3 4 1 3 4
6-slot
frame
6-slot
frame
Link Layer 5-22
FDMA frequency division multiple access channel spectrum divided into frequency bands
each station assigned fixed frequency band
unused transmission time in frequency bands go idle
example 6-station LAN 134 have pkt frequency bands 256 idle
frequency b
ands
FDM cable
Channel partitioning MAC protocols FDMA
Link Layer 5-23
Random access protocols
when node has packet to send transmit at full channel data rate R no a priori coordination among nodes
two or more transmitting nodes ldquocollisionrdquo random access MAC protocol specifies
how to detect collisions how to recover from collisions (eg via delayed
retransmissions)
examples of random access MAC protocols slotted ALOHA ALOHA CSMA CSMACD CSMACA
Link Layer 5-24
Slotted ALOHA
assumptions all frames same size
time divided into equal size slots (time to transmit 1 frame)
nodes start to transmit only slot beginning
nodes are synchronized
if 2 or more nodes transmit in slot all nodes detect collision
operation
when node obtains fresh frame transmits in next slot
if no collision node can send new frame in next slot
if collision node retransmits frame in each subsequent slot with prob p until success
Link Layer 5-25
Pros single active node can
continuously transmit at full rate of channel
highly decentralized only slots in nodes need to be in sync
simple
Cons
collisions wasting slots
idle slots
nodes may be able to detect collision in less than time to transmit packet
clock synchronization
Slotted ALOHA
1 1 1 1
2
3
2 2
3 3
node 1
node 2
node 3
C C C S S S E E E
Link Layer 5-26
suppose N nodes with many frames to send each transmits in slot with probability p
prob that given node has success in a slot = p(1-p)N-1
prob that any node has a success = Np(1-p)N-1
max efficiency find p that maximizes Np(1-p)N-1
for many nodes take limit of Np(1-p)N-1 as N goes to infinity gives
max efficiency = 1e = 37
efficiency long-run fraction of successful slots (many nodes all with many frames to send)
at best channel used for useful transmissions 37 of time
Slotted ALOHA efficiency
Link Layer 5-27
Pure (unslotted) ALOHA
unslotted Aloha simpler no synchronization
when frame first arrives
transmit immediately
collision probability increases
frame sent at t0 collides with other frames sent in [t0-1t0+1]
Link Layer 5-28
Pure ALOHA efficiency
P(success by given node) = P(node transmits)
P(no other node transmits in [t0-1t0]
P(no other node transmits in [t0-1t0]
= p (1-p)N-1 (1-p)N-1
= p (1-p)2(N-1)
hellip choosing optimum p and then letting n
= 1(2e) = 18
even worse than slotted Aloha
Link Layer 5-29
CSMA (carrier sense multiple access)
CSMA listen before transmit if channel sensed idle transmit entire frame
if channel sensed busy defer transmission
human analogy donrsquot interrupt others
Link Layer 5-30
CSMA collisions
collisions can still occur propagation delay means two nodes may not hear each otherrsquos transmission
collision entire packet transmission time wasted distance amp propagation
delay play role in in determining collision probability
spatial layout of nodes
Link Layer 5-31
CSMACD (collision detection)
CSMACD carrier sensing deferral as in CSMA collisions detected within short time
colliding transmissions aborted reducing channel wastage
collision detection easy in wired LANs measure signal strengths compare
transmitted received signals
difficult in wireless LANs received signal strength overwhelmed by local transmission strength
human analogy the polite conversationalist
Link Layer 5-32
CSMACD (collision detection)
spatial layout of nodes
Link Layer 5-33
Ethernet CSMACD algorithm
1 NIC receives datagram from network layer creates frame
2 If NIC senses channel idle starts frame transmission If NIC senses channel busy waits until channel idle then transmits
3 If NIC transmits entire frame without detecting another transmission NIC is done with frame
4 If NIC detects another transmission while transmitting aborts and sends jam signal
5 After aborting NIC enters binary (exponential) backoff
after mth collision NIC chooses K at random from 012 hellip 2m-1 NIC waits K512 bit times returns to Step 2
longer backoff interval with more collisions
Link Layer 5-34
CSMACD efficiency
Tprop = max prop delay between 2 nodes in LAN
ttrans = time to transmit max-size frame
efficiency goes to 1
as tprop goes to 0
as ttrans goes to infinity
better performance than ALOHA and simple cheap decentralized
transpropttefficiency
51
1
Link Layer 5-35
ldquoTaking turnsrdquo MAC protocols
channel partitioning MAC protocols share channel efficiently and fairly at high load
inefficient at low load delay in channel access 1N bandwidth allocated even if only 1 active node
random access MAC protocols efficient at low load single node can fully utilize
channel
high load collision overhead
ldquotaking turnsrdquo protocols look for best of both worlds
Link Layer 5-36
polling master node ldquoinvitesrdquo
slave nodes to transmit in turn
typically used with ldquodumbrdquo slave devices
concerns
polling overhead
latency
single point of failure (master)
master
slaves
poll
data
data
ldquoTaking turnsrdquo MAC protocols
Link Layer 5-37
token passing control token passed
from one node to next sequentially
token message
concerns
token overhead
latency
single point of failure (token)
T
data
(nothing
to send)
T
ldquoTaking turnsrdquo MAC protocols
cable headend
CMTS
ISP
cable modem termination system
multiple 40Mbps downstream (broadcast) channels
single CMTS transmits into channels
multiple 30 Mbps upstream channels
multiple access all users contend for certain upstream channel time slots (others assigned)
Cable access network
cable modem
splitter
hellip
hellip
Internet framesTV channels control transmitted downstream at different frequencies
upstream Internet frames TV control transmitted upstream at different frequencies in time slots
Link Layer 5-39
DOCSIS data over cable service interface spec FDM over upstream downstream frequency channels
TDM upstream some slots assigned some have contention
downstream MAP frame assigns upstream slots
request for upstream slots (and data) transmitted random access (binary backoff) in selected slots
MAP frame for
Interval [t1 t2]
Residences with cable modems
Downstream channel i
Upstream channel j
t1 t2
Assigned minislots containing cable modem
upstream data frames
Minislots containing
minislots request frames
cable headend
CMTS
Cable access network
Link Layer 5-40
Summary of MAC protocols
channel partitioning by time frequency or code Time Division Frequency Division
random access (dynamic)
ALOHA S-ALOHA CSMA CSMACD
carrier sensing easy in some technologies (wire) hard in others (wireless)
CSMACD used in Ethernet
CSMACA used in 80211
taking turns
polling from central site token passing
bluetooth FDDI token ring
Link Layer 5-41
Link layer LANs outline
51 introduction services
52 error detection correction
53 multiple access protocols
54 LANs addressing ARP
Ethernet
switches
VLANS
55 link virtualization MPLS
56 data center networking
57 a day in the life of a web request
Link Layer 5-42
MAC addresses and ARP
32-bit IP address network-layer address for interface
used for layer 3 (network layer) forwarding
MAC (or LAN or physical or Ethernet) address function used lsquolocallyrdquo to get frame from one interface to
another physically-connected interface (same network in IP-addressing sense)
48 bit MAC address (for most LANs) burned in NIC ROM also sometimes software settable
eg 1A-2F-BB-76-09-AD
hexadecimal (base 16) notation
(each ldquonumberrdquo represents 4 bits)
Link Layer 5-43
LAN addresses and ARP
each adapter on LAN has unique LAN address
adapter
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN
(wired or
wireless)
Link Layer 5-44
LAN addresses (more)
MAC address allocation administered by IEEE
manufacturer buys portion of MAC address space (to assure uniqueness)
analogy MAC address like Social Security Number
IP address like postal address
MAC flat address portability can move LAN card from one LAN to another
IP hierarchical address not portable address depends on IP subnet to which node is
attached
Link Layer 5-45
ARP address resolution protocol
ARP table each IP node (host router) on LAN has table
IPMAC address mappings for some LAN nodes
lt IP address MAC address TTLgt
TTL (Time To Live) time after which address mapping will be forgotten (typically 20 min)
Question how to determine
interfacersquos MAC address
knowing its IP address
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN
137196723
137196778
137196714
137196788
Link Layer 5-46
ARP protocol same LAN
A wants to send datagram to B Brsquos MAC address not in
Arsquos ARP table
A broadcasts ARP query packet containing Bs IP address dest MAC address = FF-FF-
FF-FF-FF-FF
all nodes on LAN receive ARP query
B receives ARP packet replies to A with its (Bs) MAC address frame sent to Arsquos MAC
address (unicast)
A caches (saves) IP-to-MAC address pair in its ARP table until information becomes old (times out) soft state information that
times out (goes away) unless refreshed
ARP is ldquoplug-and-playrdquo nodes create their ARP
tables without intervention from net administrator
Link Layer 5-47
walkthrough send datagram from A to B via R
focus on addressing ndash at IP (datagram) and MAC layer (frame)
assume A knows Brsquos IP address
assume A knows IP address of first hop router R (how)
assume A knows Rrsquos MAC address (how)
Addressing routing to another LAN
R
1A-23-F9-CD-06-9B 222222222220
111111111110 E6-E9-00-17-BB-4B CC-49-DE-D0-AB-7D
111111111112
111111111111
74-29-9C-E8-FF-55
A
222222222222
49-BD-D2-C7-56-2A
222222222221 88-B2-2F-54-1A-0F
B
R
1A-23-F9-CD-06-9B 222222222220
111111111110 E6-E9-00-17-BB-4B CC-49-DE-D0-AB-7D
111111111112
111111111111
74-29-9C-E8-FF-55
A
222222222222
49-BD-D2-C7-56-2A
222222222221 88-B2-2F-54-1A-0F
B
Link Layer 5-48
Addressing routing to another LAN
IP
Eth
Phy
IP src 111111111111
IP dest 222222222222
A creates IP datagram with IP source A destination B
A creates link-layer frame with Rs MAC address as dest frame contains A-to-B IP datagram
MAC src 74-29-9C-E8-FF-55
MAC dest E6-E9-00-17-BB-4B
R
1A-23-F9-CD-06-9B 222222222220
111111111110 E6-E9-00-17-BB-4B CC-49-DE-D0-AB-7D
111111111112
111111111111
74-29-9C-E8-FF-55
A
222222222222
49-BD-D2-C7-56-2A
222222222221 88-B2-2F-54-1A-0F
B
Link Layer 5-49
Addressing routing to another LAN
IP
Eth
Phy
frame sent from A to R
IP
Eth
Phy
frame received at R datagram removed passed up to IP
MAC src 74-29-9C-E8-FF-55
MAC dest E6-E9-00-17-BB-4B
IP src 111111111111
IP dest 222222222222
IP src 111111111111
IP dest 222222222222
R
1A-23-F9-CD-06-9B 222222222220
111111111110 E6-E9-00-17-BB-4B CC-49-DE-D0-AB-7D
111111111112
111111111111
74-29-9C-E8-FF-55
A
222222222222
49-BD-D2-C7-56-2A
222222222221 88-B2-2F-54-1A-0F
B
Link Layer 5-50
Addressing routing to another LAN
IP src 111111111111
IP dest 222222222222
R forwards datagram with IP source A destination B
R creates link-layer frame with Bs MAC address as dest frame contains A-to-B IP datagram
MAC src 1A-23-F9-CD-06-9B
MAC dest 49-BD-D2-C7-56-2A
IP
Eth
Phy
IP
Eth
Phy
R
1A-23-F9-CD-06-9B 222222222220
111111111110 E6-E9-00-17-BB-4B CC-49-DE-D0-AB-7D
111111111112
111111111111
74-29-9C-E8-FF-55
A
222222222222
49-BD-D2-C7-56-2A
222222222221 88-B2-2F-54-1A-0F
B
Link Layer 5-51
Addressing routing to another LAN R forwards datagram with IP source A destination B
R creates link-layer frame with Bs MAC address as dest frame contains A-to-B IP datagram
IP src 111111111111
IP dest 222222222222
MAC src 1A-23-F9-CD-06-9B
MAC dest 49-BD-D2-C7-56-2A
IP
Eth
Phy
IP
Eth
Phy
R
1A-23-F9-CD-06-9B 222222222220
111111111110 E6-E9-00-17-BB-4B CC-49-DE-D0-AB-7D
111111111112
111111111111
74-29-9C-E8-FF-55
A
222222222222
49-BD-D2-C7-56-2A
222222222221 88-B2-2F-54-1A-0F
B
Link Layer 5-52
Addressing routing to another LAN R forwards datagram with IP source A destination B
R creates link-layer frame with Bs MAC address as dest frame contains A-to-B IP datagram
IP src 111111111111
IP dest 222222222222
MAC src 1A-23-F9-CD-06-9B
MAC dest 49-BD-D2-C7-56-2A
IP
Eth
Phy
Link Layer 5-53
Link layer LANs outline
51 introduction services
52 error detection correction
53 multiple access protocols
54 LANs addressing ARP
Ethernet
switches
VLANS
55 link virtualization MPLS
56 data center networking
57 a day in the life of a web request
Link Layer 5-54
Ethernet
ldquodominantrdquo wired LAN technology
cheap $20 for NIC
first widely used LAN technology
simpler cheaper than token LANs and ATM
kept up with speed race 10 Mbps ndash 10 Gbps
Metcalfersquos Ethernet sketch
Link Layer 5-55
Ethernet physical topology
bus popular through mid 90s all nodes in same collision domain (can collide with each
other)
star prevails today active switch in center
each ldquospokerdquo runs a (separate) Ethernet protocol (nodes
do not collide with each other)
switch
bus coaxial cable star
Link Layer 5-56
Ethernet frame structure
sending adapter encapsulates IP datagram (or other network layer protocol packet) in Ethernet frame
preamble
7 bytes with pattern 10101010 followed by one byte with pattern 10101011
used to synchronize receiver sender clock rates
dest address
source address
data (payload) CRC preamble
type
Link Layer 5-57
Ethernet frame structure (more)
addresses 6 byte source destination MAC addresses if adapter receives frame with matching destination
address or with broadcast address (eg ARP packet) it passes data in frame to network layer protocol
otherwise adapter discards frame
type indicates higher layer protocol (mostly IP but others possible eg Novell IPX AppleTalk)
CRC cyclic redundancy check at receiver error detected frame is dropped
dest address
source address
data (payload) CRC preamble
type
Link Layer 5-58
Ethernet unreliable connectionless
connectionless no handshaking between sending and receiving NICs
unreliable receiving NIC doesnt send acks or nacks to sending NIC
data in dropped frames recovered only if initial sender uses higher layer rdt (eg TCP) otherwise dropped data lost
Ethernetrsquos MAC protocol unslotted CSMACD wth binary backoff
Link Layer 5-59
8023 Ethernet standards link amp physical layers
many different Ethernet standards
common MAC protocol and frame format
different speeds 2 Mbps 10 Mbps 100 Mbps 1Gbps 10G bps
different physical layer media fiber cable
application
transport
network
link
physical
MAC protocol
and frame format
100BASE-TX
100BASE-T4
100BASE-FX 100BASE-T2
100BASE-SX 100BASE-BX
fiber physical layer copper (twister
pair) physical layer
Link Layer 5-60
Link layer LANs outline
51 introduction services
52 error detection correction
53 multiple access protocols
54 LANs addressing ARP
Ethernet
switches
VLANS
55 link virtualization MPLS
56 data center networking
57 a day in the life of a web request
Link Layer 5-61
Ethernet switch link-layer device takes an active role
store forward Ethernet frames
examine incoming framersquos MAC address selectively forward frame to one-or-more outgoing links when frame is to be forwarded on segment uses CSMACD to access segment
transparent
hosts are unaware of presence of switches
plug-and-play self-learning
switches do not need to be configured
Link Layer 5-62
Switch multiple simultaneous transmissions
hosts have dedicated direct connection to switch
switches buffer packets
Ethernet protocol used on each incoming link but no collisions full duplex
each link is its own collision domain
switching A-to-Arsquo and B-to-Brsquo can transmit simultaneously without collisions switch with six interfaces
(123456)
A
Arsquo
B
Brsquo C
Crsquo
1 2
3 4 5
6
Link Layer 5-63
Switch forwarding table
Q how does switch know Arsquo reachable via interface 4 Brsquo reachable via interface 5
switch with six interfaces
(123456)
A
Arsquo
B
Brsquo C
Crsquo
1 2
3 4 5
6 A each switch has a switch table each entry
(MAC address of host interface to
reach host time stamp)
looks like a routing table
Q how are entries created maintained in switch table
something like a routing protocol
A
Arsquo
B
Brsquo C
Crsquo
1 2
3 4 5
6
Link Layer 5-64
Switch self-learning
switch learns which hosts can be reached through which interfaces
when frame received switch ldquolearnsrdquo location of sender incoming LAN segment
records senderlocation pair in switch table
A Arsquo
Source A
Dest Arsquo
MAC addr interface TTL
Switch table
(initially empty) A 1 60
Link Layer 5-65
Switch frame filteringforwarding
when frame received at switch 1 record incoming link MAC address of sending host
2 index switch table using MAC destination address
3 if entry found for destination then
if destination on segment from which frame arrived then drop frame
else forward frame on interface indicated by entry
else flood forward on all interfaces except arriving
interface
A
Arsquo
B
Brsquo C
Crsquo
1 2
3 4 5
6
Link Layer 5-66
Self-learning forwarding example
A Arsquo
Source A
Dest Arsquo
MAC addr interface TTL
switch table
(initially empty) A 1 60
A Arsquo A Arsquo A Arsquo A Arsquo A Arsquo
frame destination Arsquo locaton unknown flood
Arsquo A
destination A location
known
Arsquo 4 60
selectively send
on just one link
Link Layer 5-67
Interconnecting switches
switches can be connected together
Q sending from A to G - how does S1 know to forward frame destined to F via S4 and S3
A self learning (works exactly the same as in single-switch case)
A
B
S1
C D
E
F
S2
S4
S3
H
I
G
Link Layer 5-68
Self-learning multi-switch example
Suppose C sends frame to I I responds to C
Q show switch tables and packet forwarding in S1 S2 S3 S4
A
B
S1
C D
E
F
S2
S4
S3
H
I
G
Link Layer 5-69
Institutional network
to external
network
router
IP subnet
mail server
web server
Link Layer 5-70
Switches vs routers
both are store-and-forward
routers network-layer devices (examine network-layer headers)
switches link-layer devices (examine link-layer headers)
both have forwarding tables
routers compute tables using routing algorithms IP addresses
switches learn forwarding table using flooding learning MAC addresses
application
transport
network
link
physical
network
link
physical
link
physical
switch
datagram
application
transport
network
link
physical
frame
frame
frame
datagram
Link Layer 5-71
VLANs motivation
consider CS user moves office to
EE but wants connect to CS switch
single broadcast domain
all layer-2 broadcast traffic (ARP DHCP unknown location of destination MAC address) must cross entire LAN
securityprivacy efficiency issues
Computer
Science Electrical
Engineering
Computer
Engineering
Link Layer 5-72
VLANs port-based VLAN switch ports
grouped (by switch management software) so that single physical switch helliphellip
switch(es) supporting
VLAN capabilities can
be configured to
define multiple virtual
LANS over single
physical LAN
infrastructure
Virtual Local
Area Network 1
8
9
16 10 2
7
hellip
Electrical Engineering
(VLAN ports 1-8)
Computer Science
(VLAN ports 9-15)
15
hellip
Electrical Engineering
(VLAN ports 1-8)
hellip
1
8 2
7 9
16 10
15
hellip
Computer Science
(VLAN ports 9-16)
hellip operates as multiple virtual switches
Link Layer 5-73
Port-based VLAN
1
8
9
16 10 2
7
hellip
Electrical Engineering
(VLAN ports 1-8)
Computer Science
(VLAN ports 9-15)
15
hellip
traffic isolation frames tofrom ports 1-8 can only reach ports 1-8 can also define VLAN based on
MAC addresses of endpoints rather than switch port
dynamic membership ports can be dynamically assigned among VLANs
router
forwarding between VLANS done via routing (just as with separate switches) in practice vendors sell combined
switches plus routers
Link Layer 5-74
VLANS spanning multiple switches
trunk port carries frames between VLANS defined over multiple physical switches frames forwarded within VLAN between switches canrsquot be vanilla
8021 frames (must carry VLAN ID info)
8021q protocol addsremoved additional header fields for frames forwarded between trunk ports
1
8
9
10 2
7
hellip
Electrical Engineering
(VLAN ports 1-8)
Computer Science
(VLAN ports 9-15)
15
hellip
2
7 3
Ports 235 belong to EE VLAN
Ports 4678 belong to CS VLAN
5
4 6 8 16
1
Link Layer 5-75
type
2-byte Tag Protocol Identifier
(value 81-00)
Tag Control Information (12 bit VLAN ID field
3 bit priority field like IP TOS)
Recomputed CRC
8021Q VLAN frame format
8021 frame
8021Q frame
dest address
source address
data (payload) CRC preamble
dest address
source address
preamble data (payload) CRC
type
Link Layer 5-76
Link layer LANs outline
51 introduction services
52 error detection correction
53 multiple access protocols
54 LANs addressing ARP
Ethernet
switches
VLANS
55 link virtualization MPLS
56 data center networking
57 a day in the life of a web request
Link Layer 5-77
Multiprotocol label switching (MPLS)
initial goal high-speed IP forwarding using fixed length label (instead of IP address) fast lookup using fixed length identifier (rather than
shortest prefix matching)
borrowing ideas from Virtual Circuit (VC) approach
but IP datagram still keeps IP address
PPP or Ethernet
header IP header remainder of link-layer frame MPLS header
label Exp S TTL
20 3 1 5
Link Layer 5-78
MPLS capable routers
aka label-switched router
forward packets to outgoing interface based only on label value (donrsquot inspect IP address) MPLS forwarding table distinct from IP forwarding tables
flexibility MPLS forwarding decisions can differ from those of IP use destination and source addresses to route flows to
same destination differently (traffic engineering)
re-route flows quickly if link fails pre-computed backup paths (useful for VoIP)
Link Layer 5-79
R2
D
R3 R5
A
R6
MPLS versus IP paths
IP router IP routing path to destination determined
by destination address alone
R4
Link Layer 5-80
R2
D
R3 R4
R5
A
R6
MPLS versus IP paths
IP-only router
IP routing path to destination determined by destination address alone
MPLS and IP router
MPLS routing path to destination can be based on source and dest address fast reroute precompute backup routes in
case of link failure
entry router (R4) can use different MPLS
routes to A based eg on source address
Link Layer 5-81
MPLS signaling
modify OSPF IS-IS link-state flooding protocols to carry info used by MPLS routing eg link bandwidth amount of ldquoreservedrdquo link bandwidth
D
R4
R5
A
R6
entry MPLS router uses RSVP-TE signaling protocol to set up MPLS forwarding at downstream routers
modified link state flooding
RSVP-TE
Link Layer 5-82
R1 R2
D
R3 R4 R5
0
1
0 0
A
R6
in out out
label label dest interface
6 - A 0
in out out
label label dest interface
10 6 A 1
12 9 D 0
in out out
label label dest interface
10 A 0
12 D 0
1
in out out
label label dest interface
8 6 A 0
0
8 A 1
MPLS forwarding tables
Link Layer 5-83
Link layer LANs outline
51 introduction services
52 error detection correction
53 multiple access protocols
54 LANs addressing ARP
Ethernet
switches
VLANS
55 link virtualization MPLS
56 data center networking
57 a day in the life of a web request
Link Layer 5-84
Data center networks
10rsquos to 100rsquos of thousands of hosts often closely coupled in close proximity e-business (eg Amazon)
content-servers (eg YouTube Akamai Apple Microsoft)
search engines data mining (eg Google)
challenges
multiple applications each serving massive numbers of clients
managingbalancing load avoiding processing networking data bottlenecks
Inside a 40-ft Microsoft container
Chicago data center
Link Layer 5-85
Server racks
TOR switches
Tier-1 switches
Tier-2 switches
Load balancer
Load balancer
B
1 2 3 4 5 6 7 8
A C
Border router
Access router
Internet
Data center networks
load balancer application-layer routing receives external client requests
directs workload within data center
returns results to external client (hiding data
center internals from client)
Server racks
TOR switches
Tier-1 switches
Tier-2 switches
1 2 3 4 5 6 7 8
Data center networks
rich interconnection among switches racks
increased throughput between racks (multiple routing
paths possible)
increased reliability via redundancy
Link Layer 5-87
Link layer LANs outline
51 introduction services
52 error detection correction
53 multiple access protocols
54 LANs addressing ARP
Ethernet
switches
VLANS
55 link virtualization MPLS
56 data center networking
57 a day in the life of a web request
Link Layer 5-88
Synthesis a day in the life of a web request
journey down protocol stack complete application transport network link
putting-it-all-together synthesis goal identify review understand protocols (at all
layers) involved in seemingly simple scenario requesting www page
scenario student attaches laptop to campus network requestsreceives wwwgooglecom
Link Layer 5-89
A day in the life scenario
Comcast network
68800013
Googlersquos network
64233160019 64233169105
web server
DNS server
school network
68802024
web page
browser
router
(runs DHCP)
Link Layer 5-90
A day in the lifehellip connecting to the Internet
connecting laptop needs to get its own IP address addr of first-hop router addr of DNS server use DHCP
DHCP
UDP
IP
Eth
Phy
DHCP
DHCP
DHCP
DHCP
DHCP
DHCP
UDP
IP
Eth
Phy
DHCP
DHCP
DHCP
DHCP DHCP
DHCP request encapsulated in UDP encapsulated in IP encapsulated in 8023 Ethernet
Ethernet frame broadcast
(dest FFFFFFFFFFFF) on LAN received at router running DHCP server
Ethernet demuxed to IP demuxed UDP demuxed to DHCP
router
(runs DHCP)
Link Layer 5-91
DHCP server formulates DHCP ACK containing clientrsquos IP address IP address of first-hop router for client name amp IP address of DNS server
DHCP
UDP
IP
Eth
Phy
DHCP
DHCP
DHCP
DHCP
DHCP
UDP
IP
Eth
Phy
DHCP
DHCP
DHCP
DHCP
DHCP
encapsulation at DHCP server frame forwarded (switch learning) through LAN demultiplexing at client
Client now has IP address knows name amp addr of DNS
server IP address of its first-hop router
DHCP client receives DHCP ACK reply
A day in the lifehellip connecting to the Internet
router
(runs DHCP)
Link Layer 5-92
A day in the lifehellip ARP (before DNS before HTTP)
before sending HTTP request need IP address of wwwgooglecom DNS
DNS
UDP
IP
Eth
Phy
DNS
DNS
DNS
DNS query created encapsulated in UDP encapsulated in IP encapsulated in Eth To send frame to router need MAC address of router interface ARP
ARP query broadcast received by
router which replies with ARP reply giving MAC address of router interface
client now knows MAC address of first hop router so can now send frame containing DNS query
ARP query
Eth
Phy
ARP
ARP
ARP reply
router
(runs DHCP)
Link Layer 5-93
DNS
UDP
IP
Eth
Phy
DNS
DNS
DNS
DNS
DNS
IP datagram containing DNS query forwarded via LAN switch from client to 1st hop router
IP datagram forwarded from campus network into comcast network routed (tables created by RIP OSPF IS-IS andor BGP routing protocols) to DNS server
demuxrsquoed to DNS server
DNS server replies to client with IP address of wwwgooglecom
Comcast network
68800013
DNS server
DNS
UDP
IP
Eth
Phy
DNS
DNS
DNS
DNS
A day in the lifehellip using DNS
router
(runs DHCP)
Link Layer 5-94
A day in the lifehellipTCP connection carrying HTTP
HTTP
TCP
IP
Eth
Phy
HTTP
to send HTTP request client first opens TCP socket to web server
TCP SYN segment (step 1 in 3-way handshake) inter-domain routed to web server
TCP connection established 64233169105
web server
SYN
SYN
SYN
SYN
TCP
IP
Eth
Phy
SYN
SYN
SYN
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
web server responds with TCP SYNACK (step 2 in 3-way handshake)
router
(runs DHCP)
Link Layer 5-95
A day in the lifehellip HTTP requestreply
HTTP
TCP
IP
Eth
Phy
HTTP
HTTP request sent into TCP socket
IP datagram containing HTTP request routed to wwwgooglecom
IP datagram containing HTTP reply routed back to client
64233169105
web server
HTTP
TCP
IP
Eth
Phy web server responds with
HTTP reply (containing web page)
HTTP
HTTP
HTTP HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
web page finally () displayed
Link Layer 5-96
Chapter 5 Summary
principles behind data link layer services error detection correction
sharing a broadcast channel multiple access
link layer addressing
instantiation and implementation of various link layer technologies Ethernet
switched LANS VLANs
virtualized networks as a link layer MPLS
synthesis a day in the life of a web request
Link Layer 5-97
Chapter 5 letrsquos take a breath
journey down protocol stack complete (except PHY)
solid understanding of networking principles practice
hellip could stop here hellip but lots of interesting topics wireless
multimedia
security
network management
Link Layer 5-15
CRC example
want D2r XOR R = nG
equivalently D2r = nG XOR R
equivalently if we divide D2r by
G want remainder R to satisfy
R = remainder[ ] D2r
G
1001 101110000 1001
1
101
01000
000 1010 1001
010 000
100 000 1000 0000 1000
D G
R
r = 3
Link Layer 5-16
Link layer LANs outline
51 introduction services
52 error detection correction
53 multiple access protocols
54 LANs addressing ARP
Ethernet
switches
VLANS
55 link virtualization MPLS
56 data center networking
57 a day in the life of a web request
Link Layer 5-17
Multiple access links protocols
two types of ldquolinksrdquo
point-to-point PPP for dial-up access
point-to-point link between Ethernet switch host
broadcast (shared wire or medium) old-fashioned Ethernet
upstream HFC
80211 wireless LAN
shared wire (eg cabled Ethernet)
shared RF (eg 80211 WiFi)
shared RF (satellite)
humans at a cocktail party
(shared air acoustical)
Link Layer 5-18
Multiple access protocols
single shared broadcast channel
two or more simultaneous transmissions by nodes interference
collision if node receives two or more signals at the same time
multiple access protocol distributed algorithm that determines how nodes share
channel ie determine when node can transmit
communication about channel sharing must use channel itself no out-of-band channel for coordination
Link Layer 5-19
An ideal multiple access protocol
given broadcast channel of rate R bps
desiderata 1 when one node wants to transmit it can send at rate R
2 when M nodes want to transmit each can send at average rate RM
3 fully decentralized
bull no special node to coordinate transmissions
bull no synchronization of clocks slots
4 simple
Link Layer 5-20
MAC protocols taxonomy
three broad classes
channel partitioning divide channel into smaller ldquopiecesrdquo (time slots frequency code)
allocate piece to node for exclusive use
random access channel not divided allow collisions
ldquorecoverrdquo from collisions
ldquotaking turnsrdquo nodes take turns but nodes with more to send can take longer
turns
Link Layer 5-21
Channel partitioning MAC protocols TDMA
TDMA time division multiple access access to channel in rounds each station gets fixed length slot (length = pkt
trans time) in each round unused slots go idle example 6-station LAN 134 have pkt slots
256 idle
1 3 4 1 3 4
6-slot
frame
6-slot
frame
Link Layer 5-22
FDMA frequency division multiple access channel spectrum divided into frequency bands
each station assigned fixed frequency band
unused transmission time in frequency bands go idle
example 6-station LAN 134 have pkt frequency bands 256 idle
frequency b
ands
FDM cable
Channel partitioning MAC protocols FDMA
Link Layer 5-23
Random access protocols
when node has packet to send transmit at full channel data rate R no a priori coordination among nodes
two or more transmitting nodes ldquocollisionrdquo random access MAC protocol specifies
how to detect collisions how to recover from collisions (eg via delayed
retransmissions)
examples of random access MAC protocols slotted ALOHA ALOHA CSMA CSMACD CSMACA
Link Layer 5-24
Slotted ALOHA
assumptions all frames same size
time divided into equal size slots (time to transmit 1 frame)
nodes start to transmit only slot beginning
nodes are synchronized
if 2 or more nodes transmit in slot all nodes detect collision
operation
when node obtains fresh frame transmits in next slot
if no collision node can send new frame in next slot
if collision node retransmits frame in each subsequent slot with prob p until success
Link Layer 5-25
Pros single active node can
continuously transmit at full rate of channel
highly decentralized only slots in nodes need to be in sync
simple
Cons
collisions wasting slots
idle slots
nodes may be able to detect collision in less than time to transmit packet
clock synchronization
Slotted ALOHA
1 1 1 1
2
3
2 2
3 3
node 1
node 2
node 3
C C C S S S E E E
Link Layer 5-26
suppose N nodes with many frames to send each transmits in slot with probability p
prob that given node has success in a slot = p(1-p)N-1
prob that any node has a success = Np(1-p)N-1
max efficiency find p that maximizes Np(1-p)N-1
for many nodes take limit of Np(1-p)N-1 as N goes to infinity gives
max efficiency = 1e = 37
efficiency long-run fraction of successful slots (many nodes all with many frames to send)
at best channel used for useful transmissions 37 of time
Slotted ALOHA efficiency
Link Layer 5-27
Pure (unslotted) ALOHA
unslotted Aloha simpler no synchronization
when frame first arrives
transmit immediately
collision probability increases
frame sent at t0 collides with other frames sent in [t0-1t0+1]
Link Layer 5-28
Pure ALOHA efficiency
P(success by given node) = P(node transmits)
P(no other node transmits in [t0-1t0]
P(no other node transmits in [t0-1t0]
= p (1-p)N-1 (1-p)N-1
= p (1-p)2(N-1)
hellip choosing optimum p and then letting n
= 1(2e) = 18
even worse than slotted Aloha
Link Layer 5-29
CSMA (carrier sense multiple access)
CSMA listen before transmit if channel sensed idle transmit entire frame
if channel sensed busy defer transmission
human analogy donrsquot interrupt others
Link Layer 5-30
CSMA collisions
collisions can still occur propagation delay means two nodes may not hear each otherrsquos transmission
collision entire packet transmission time wasted distance amp propagation
delay play role in in determining collision probability
spatial layout of nodes
Link Layer 5-31
CSMACD (collision detection)
CSMACD carrier sensing deferral as in CSMA collisions detected within short time
colliding transmissions aborted reducing channel wastage
collision detection easy in wired LANs measure signal strengths compare
transmitted received signals
difficult in wireless LANs received signal strength overwhelmed by local transmission strength
human analogy the polite conversationalist
Link Layer 5-32
CSMACD (collision detection)
spatial layout of nodes
Link Layer 5-33
Ethernet CSMACD algorithm
1 NIC receives datagram from network layer creates frame
2 If NIC senses channel idle starts frame transmission If NIC senses channel busy waits until channel idle then transmits
3 If NIC transmits entire frame without detecting another transmission NIC is done with frame
4 If NIC detects another transmission while transmitting aborts and sends jam signal
5 After aborting NIC enters binary (exponential) backoff
after mth collision NIC chooses K at random from 012 hellip 2m-1 NIC waits K512 bit times returns to Step 2
longer backoff interval with more collisions
Link Layer 5-34
CSMACD efficiency
Tprop = max prop delay between 2 nodes in LAN
ttrans = time to transmit max-size frame
efficiency goes to 1
as tprop goes to 0
as ttrans goes to infinity
better performance than ALOHA and simple cheap decentralized
transpropttefficiency
51
1
Link Layer 5-35
ldquoTaking turnsrdquo MAC protocols
channel partitioning MAC protocols share channel efficiently and fairly at high load
inefficient at low load delay in channel access 1N bandwidth allocated even if only 1 active node
random access MAC protocols efficient at low load single node can fully utilize
channel
high load collision overhead
ldquotaking turnsrdquo protocols look for best of both worlds
Link Layer 5-36
polling master node ldquoinvitesrdquo
slave nodes to transmit in turn
typically used with ldquodumbrdquo slave devices
concerns
polling overhead
latency
single point of failure (master)
master
slaves
poll
data
data
ldquoTaking turnsrdquo MAC protocols
Link Layer 5-37
token passing control token passed
from one node to next sequentially
token message
concerns
token overhead
latency
single point of failure (token)
T
data
(nothing
to send)
T
ldquoTaking turnsrdquo MAC protocols
cable headend
CMTS
ISP
cable modem termination system
multiple 40Mbps downstream (broadcast) channels
single CMTS transmits into channels
multiple 30 Mbps upstream channels
multiple access all users contend for certain upstream channel time slots (others assigned)
Cable access network
cable modem
splitter
hellip
hellip
Internet framesTV channels control transmitted downstream at different frequencies
upstream Internet frames TV control transmitted upstream at different frequencies in time slots
Link Layer 5-39
DOCSIS data over cable service interface spec FDM over upstream downstream frequency channels
TDM upstream some slots assigned some have contention
downstream MAP frame assigns upstream slots
request for upstream slots (and data) transmitted random access (binary backoff) in selected slots
MAP frame for
Interval [t1 t2]
Residences with cable modems
Downstream channel i
Upstream channel j
t1 t2
Assigned minislots containing cable modem
upstream data frames
Minislots containing
minislots request frames
cable headend
CMTS
Cable access network
Link Layer 5-40
Summary of MAC protocols
channel partitioning by time frequency or code Time Division Frequency Division
random access (dynamic)
ALOHA S-ALOHA CSMA CSMACD
carrier sensing easy in some technologies (wire) hard in others (wireless)
CSMACD used in Ethernet
CSMACA used in 80211
taking turns
polling from central site token passing
bluetooth FDDI token ring
Link Layer 5-41
Link layer LANs outline
51 introduction services
52 error detection correction
53 multiple access protocols
54 LANs addressing ARP
Ethernet
switches
VLANS
55 link virtualization MPLS
56 data center networking
57 a day in the life of a web request
Link Layer 5-42
MAC addresses and ARP
32-bit IP address network-layer address for interface
used for layer 3 (network layer) forwarding
MAC (or LAN or physical or Ethernet) address function used lsquolocallyrdquo to get frame from one interface to
another physically-connected interface (same network in IP-addressing sense)
48 bit MAC address (for most LANs) burned in NIC ROM also sometimes software settable
eg 1A-2F-BB-76-09-AD
hexadecimal (base 16) notation
(each ldquonumberrdquo represents 4 bits)
Link Layer 5-43
LAN addresses and ARP
each adapter on LAN has unique LAN address
adapter
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN
(wired or
wireless)
Link Layer 5-44
LAN addresses (more)
MAC address allocation administered by IEEE
manufacturer buys portion of MAC address space (to assure uniqueness)
analogy MAC address like Social Security Number
IP address like postal address
MAC flat address portability can move LAN card from one LAN to another
IP hierarchical address not portable address depends on IP subnet to which node is
attached
Link Layer 5-45
ARP address resolution protocol
ARP table each IP node (host router) on LAN has table
IPMAC address mappings for some LAN nodes
lt IP address MAC address TTLgt
TTL (Time To Live) time after which address mapping will be forgotten (typically 20 min)
Question how to determine
interfacersquos MAC address
knowing its IP address
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN
137196723
137196778
137196714
137196788
Link Layer 5-46
ARP protocol same LAN
A wants to send datagram to B Brsquos MAC address not in
Arsquos ARP table
A broadcasts ARP query packet containing Bs IP address dest MAC address = FF-FF-
FF-FF-FF-FF
all nodes on LAN receive ARP query
B receives ARP packet replies to A with its (Bs) MAC address frame sent to Arsquos MAC
address (unicast)
A caches (saves) IP-to-MAC address pair in its ARP table until information becomes old (times out) soft state information that
times out (goes away) unless refreshed
ARP is ldquoplug-and-playrdquo nodes create their ARP
tables without intervention from net administrator
Link Layer 5-47
walkthrough send datagram from A to B via R
focus on addressing ndash at IP (datagram) and MAC layer (frame)
assume A knows Brsquos IP address
assume A knows IP address of first hop router R (how)
assume A knows Rrsquos MAC address (how)
Addressing routing to another LAN
R
1A-23-F9-CD-06-9B 222222222220
111111111110 E6-E9-00-17-BB-4B CC-49-DE-D0-AB-7D
111111111112
111111111111
74-29-9C-E8-FF-55
A
222222222222
49-BD-D2-C7-56-2A
222222222221 88-B2-2F-54-1A-0F
B
R
1A-23-F9-CD-06-9B 222222222220
111111111110 E6-E9-00-17-BB-4B CC-49-DE-D0-AB-7D
111111111112
111111111111
74-29-9C-E8-FF-55
A
222222222222
49-BD-D2-C7-56-2A
222222222221 88-B2-2F-54-1A-0F
B
Link Layer 5-48
Addressing routing to another LAN
IP
Eth
Phy
IP src 111111111111
IP dest 222222222222
A creates IP datagram with IP source A destination B
A creates link-layer frame with Rs MAC address as dest frame contains A-to-B IP datagram
MAC src 74-29-9C-E8-FF-55
MAC dest E6-E9-00-17-BB-4B
R
1A-23-F9-CD-06-9B 222222222220
111111111110 E6-E9-00-17-BB-4B CC-49-DE-D0-AB-7D
111111111112
111111111111
74-29-9C-E8-FF-55
A
222222222222
49-BD-D2-C7-56-2A
222222222221 88-B2-2F-54-1A-0F
B
Link Layer 5-49
Addressing routing to another LAN
IP
Eth
Phy
frame sent from A to R
IP
Eth
Phy
frame received at R datagram removed passed up to IP
MAC src 74-29-9C-E8-FF-55
MAC dest E6-E9-00-17-BB-4B
IP src 111111111111
IP dest 222222222222
IP src 111111111111
IP dest 222222222222
R
1A-23-F9-CD-06-9B 222222222220
111111111110 E6-E9-00-17-BB-4B CC-49-DE-D0-AB-7D
111111111112
111111111111
74-29-9C-E8-FF-55
A
222222222222
49-BD-D2-C7-56-2A
222222222221 88-B2-2F-54-1A-0F
B
Link Layer 5-50
Addressing routing to another LAN
IP src 111111111111
IP dest 222222222222
R forwards datagram with IP source A destination B
R creates link-layer frame with Bs MAC address as dest frame contains A-to-B IP datagram
MAC src 1A-23-F9-CD-06-9B
MAC dest 49-BD-D2-C7-56-2A
IP
Eth
Phy
IP
Eth
Phy
R
1A-23-F9-CD-06-9B 222222222220
111111111110 E6-E9-00-17-BB-4B CC-49-DE-D0-AB-7D
111111111112
111111111111
74-29-9C-E8-FF-55
A
222222222222
49-BD-D2-C7-56-2A
222222222221 88-B2-2F-54-1A-0F
B
Link Layer 5-51
Addressing routing to another LAN R forwards datagram with IP source A destination B
R creates link-layer frame with Bs MAC address as dest frame contains A-to-B IP datagram
IP src 111111111111
IP dest 222222222222
MAC src 1A-23-F9-CD-06-9B
MAC dest 49-BD-D2-C7-56-2A
IP
Eth
Phy
IP
Eth
Phy
R
1A-23-F9-CD-06-9B 222222222220
111111111110 E6-E9-00-17-BB-4B CC-49-DE-D0-AB-7D
111111111112
111111111111
74-29-9C-E8-FF-55
A
222222222222
49-BD-D2-C7-56-2A
222222222221 88-B2-2F-54-1A-0F
B
Link Layer 5-52
Addressing routing to another LAN R forwards datagram with IP source A destination B
R creates link-layer frame with Bs MAC address as dest frame contains A-to-B IP datagram
IP src 111111111111
IP dest 222222222222
MAC src 1A-23-F9-CD-06-9B
MAC dest 49-BD-D2-C7-56-2A
IP
Eth
Phy
Link Layer 5-53
Link layer LANs outline
51 introduction services
52 error detection correction
53 multiple access protocols
54 LANs addressing ARP
Ethernet
switches
VLANS
55 link virtualization MPLS
56 data center networking
57 a day in the life of a web request
Link Layer 5-54
Ethernet
ldquodominantrdquo wired LAN technology
cheap $20 for NIC
first widely used LAN technology
simpler cheaper than token LANs and ATM
kept up with speed race 10 Mbps ndash 10 Gbps
Metcalfersquos Ethernet sketch
Link Layer 5-55
Ethernet physical topology
bus popular through mid 90s all nodes in same collision domain (can collide with each
other)
star prevails today active switch in center
each ldquospokerdquo runs a (separate) Ethernet protocol (nodes
do not collide with each other)
switch
bus coaxial cable star
Link Layer 5-56
Ethernet frame structure
sending adapter encapsulates IP datagram (or other network layer protocol packet) in Ethernet frame
preamble
7 bytes with pattern 10101010 followed by one byte with pattern 10101011
used to synchronize receiver sender clock rates
dest address
source address
data (payload) CRC preamble
type
Link Layer 5-57
Ethernet frame structure (more)
addresses 6 byte source destination MAC addresses if adapter receives frame with matching destination
address or with broadcast address (eg ARP packet) it passes data in frame to network layer protocol
otherwise adapter discards frame
type indicates higher layer protocol (mostly IP but others possible eg Novell IPX AppleTalk)
CRC cyclic redundancy check at receiver error detected frame is dropped
dest address
source address
data (payload) CRC preamble
type
Link Layer 5-58
Ethernet unreliable connectionless
connectionless no handshaking between sending and receiving NICs
unreliable receiving NIC doesnt send acks or nacks to sending NIC
data in dropped frames recovered only if initial sender uses higher layer rdt (eg TCP) otherwise dropped data lost
Ethernetrsquos MAC protocol unslotted CSMACD wth binary backoff
Link Layer 5-59
8023 Ethernet standards link amp physical layers
many different Ethernet standards
common MAC protocol and frame format
different speeds 2 Mbps 10 Mbps 100 Mbps 1Gbps 10G bps
different physical layer media fiber cable
application
transport
network
link
physical
MAC protocol
and frame format
100BASE-TX
100BASE-T4
100BASE-FX 100BASE-T2
100BASE-SX 100BASE-BX
fiber physical layer copper (twister
pair) physical layer
Link Layer 5-60
Link layer LANs outline
51 introduction services
52 error detection correction
53 multiple access protocols
54 LANs addressing ARP
Ethernet
switches
VLANS
55 link virtualization MPLS
56 data center networking
57 a day in the life of a web request
Link Layer 5-61
Ethernet switch link-layer device takes an active role
store forward Ethernet frames
examine incoming framersquos MAC address selectively forward frame to one-or-more outgoing links when frame is to be forwarded on segment uses CSMACD to access segment
transparent
hosts are unaware of presence of switches
plug-and-play self-learning
switches do not need to be configured
Link Layer 5-62
Switch multiple simultaneous transmissions
hosts have dedicated direct connection to switch
switches buffer packets
Ethernet protocol used on each incoming link but no collisions full duplex
each link is its own collision domain
switching A-to-Arsquo and B-to-Brsquo can transmit simultaneously without collisions switch with six interfaces
(123456)
A
Arsquo
B
Brsquo C
Crsquo
1 2
3 4 5
6
Link Layer 5-63
Switch forwarding table
Q how does switch know Arsquo reachable via interface 4 Brsquo reachable via interface 5
switch with six interfaces
(123456)
A
Arsquo
B
Brsquo C
Crsquo
1 2
3 4 5
6 A each switch has a switch table each entry
(MAC address of host interface to
reach host time stamp)
looks like a routing table
Q how are entries created maintained in switch table
something like a routing protocol
A
Arsquo
B
Brsquo C
Crsquo
1 2
3 4 5
6
Link Layer 5-64
Switch self-learning
switch learns which hosts can be reached through which interfaces
when frame received switch ldquolearnsrdquo location of sender incoming LAN segment
records senderlocation pair in switch table
A Arsquo
Source A
Dest Arsquo
MAC addr interface TTL
Switch table
(initially empty) A 1 60
Link Layer 5-65
Switch frame filteringforwarding
when frame received at switch 1 record incoming link MAC address of sending host
2 index switch table using MAC destination address
3 if entry found for destination then
if destination on segment from which frame arrived then drop frame
else forward frame on interface indicated by entry
else flood forward on all interfaces except arriving
interface
A
Arsquo
B
Brsquo C
Crsquo
1 2
3 4 5
6
Link Layer 5-66
Self-learning forwarding example
A Arsquo
Source A
Dest Arsquo
MAC addr interface TTL
switch table
(initially empty) A 1 60
A Arsquo A Arsquo A Arsquo A Arsquo A Arsquo
frame destination Arsquo locaton unknown flood
Arsquo A
destination A location
known
Arsquo 4 60
selectively send
on just one link
Link Layer 5-67
Interconnecting switches
switches can be connected together
Q sending from A to G - how does S1 know to forward frame destined to F via S4 and S3
A self learning (works exactly the same as in single-switch case)
A
B
S1
C D
E
F
S2
S4
S3
H
I
G
Link Layer 5-68
Self-learning multi-switch example
Suppose C sends frame to I I responds to C
Q show switch tables and packet forwarding in S1 S2 S3 S4
A
B
S1
C D
E
F
S2
S4
S3
H
I
G
Link Layer 5-69
Institutional network
to external
network
router
IP subnet
mail server
web server
Link Layer 5-70
Switches vs routers
both are store-and-forward
routers network-layer devices (examine network-layer headers)
switches link-layer devices (examine link-layer headers)
both have forwarding tables
routers compute tables using routing algorithms IP addresses
switches learn forwarding table using flooding learning MAC addresses
application
transport
network
link
physical
network
link
physical
link
physical
switch
datagram
application
transport
network
link
physical
frame
frame
frame
datagram
Link Layer 5-71
VLANs motivation
consider CS user moves office to
EE but wants connect to CS switch
single broadcast domain
all layer-2 broadcast traffic (ARP DHCP unknown location of destination MAC address) must cross entire LAN
securityprivacy efficiency issues
Computer
Science Electrical
Engineering
Computer
Engineering
Link Layer 5-72
VLANs port-based VLAN switch ports
grouped (by switch management software) so that single physical switch helliphellip
switch(es) supporting
VLAN capabilities can
be configured to
define multiple virtual
LANS over single
physical LAN
infrastructure
Virtual Local
Area Network 1
8
9
16 10 2
7
hellip
Electrical Engineering
(VLAN ports 1-8)
Computer Science
(VLAN ports 9-15)
15
hellip
Electrical Engineering
(VLAN ports 1-8)
hellip
1
8 2
7 9
16 10
15
hellip
Computer Science
(VLAN ports 9-16)
hellip operates as multiple virtual switches
Link Layer 5-73
Port-based VLAN
1
8
9
16 10 2
7
hellip
Electrical Engineering
(VLAN ports 1-8)
Computer Science
(VLAN ports 9-15)
15
hellip
traffic isolation frames tofrom ports 1-8 can only reach ports 1-8 can also define VLAN based on
MAC addresses of endpoints rather than switch port
dynamic membership ports can be dynamically assigned among VLANs
router
forwarding between VLANS done via routing (just as with separate switches) in practice vendors sell combined
switches plus routers
Link Layer 5-74
VLANS spanning multiple switches
trunk port carries frames between VLANS defined over multiple physical switches frames forwarded within VLAN between switches canrsquot be vanilla
8021 frames (must carry VLAN ID info)
8021q protocol addsremoved additional header fields for frames forwarded between trunk ports
1
8
9
10 2
7
hellip
Electrical Engineering
(VLAN ports 1-8)
Computer Science
(VLAN ports 9-15)
15
hellip
2
7 3
Ports 235 belong to EE VLAN
Ports 4678 belong to CS VLAN
5
4 6 8 16
1
Link Layer 5-75
type
2-byte Tag Protocol Identifier
(value 81-00)
Tag Control Information (12 bit VLAN ID field
3 bit priority field like IP TOS)
Recomputed CRC
8021Q VLAN frame format
8021 frame
8021Q frame
dest address
source address
data (payload) CRC preamble
dest address
source address
preamble data (payload) CRC
type
Link Layer 5-76
Link layer LANs outline
51 introduction services
52 error detection correction
53 multiple access protocols
54 LANs addressing ARP
Ethernet
switches
VLANS
55 link virtualization MPLS
56 data center networking
57 a day in the life of a web request
Link Layer 5-77
Multiprotocol label switching (MPLS)
initial goal high-speed IP forwarding using fixed length label (instead of IP address) fast lookup using fixed length identifier (rather than
shortest prefix matching)
borrowing ideas from Virtual Circuit (VC) approach
but IP datagram still keeps IP address
PPP or Ethernet
header IP header remainder of link-layer frame MPLS header
label Exp S TTL
20 3 1 5
Link Layer 5-78
MPLS capable routers
aka label-switched router
forward packets to outgoing interface based only on label value (donrsquot inspect IP address) MPLS forwarding table distinct from IP forwarding tables
flexibility MPLS forwarding decisions can differ from those of IP use destination and source addresses to route flows to
same destination differently (traffic engineering)
re-route flows quickly if link fails pre-computed backup paths (useful for VoIP)
Link Layer 5-79
R2
D
R3 R5
A
R6
MPLS versus IP paths
IP router IP routing path to destination determined
by destination address alone
R4
Link Layer 5-80
R2
D
R3 R4
R5
A
R6
MPLS versus IP paths
IP-only router
IP routing path to destination determined by destination address alone
MPLS and IP router
MPLS routing path to destination can be based on source and dest address fast reroute precompute backup routes in
case of link failure
entry router (R4) can use different MPLS
routes to A based eg on source address
Link Layer 5-81
MPLS signaling
modify OSPF IS-IS link-state flooding protocols to carry info used by MPLS routing eg link bandwidth amount of ldquoreservedrdquo link bandwidth
D
R4
R5
A
R6
entry MPLS router uses RSVP-TE signaling protocol to set up MPLS forwarding at downstream routers
modified link state flooding
RSVP-TE
Link Layer 5-82
R1 R2
D
R3 R4 R5
0
1
0 0
A
R6
in out out
label label dest interface
6 - A 0
in out out
label label dest interface
10 6 A 1
12 9 D 0
in out out
label label dest interface
10 A 0
12 D 0
1
in out out
label label dest interface
8 6 A 0
0
8 A 1
MPLS forwarding tables
Link Layer 5-83
Link layer LANs outline
51 introduction services
52 error detection correction
53 multiple access protocols
54 LANs addressing ARP
Ethernet
switches
VLANS
55 link virtualization MPLS
56 data center networking
57 a day in the life of a web request
Link Layer 5-84
Data center networks
10rsquos to 100rsquos of thousands of hosts often closely coupled in close proximity e-business (eg Amazon)
content-servers (eg YouTube Akamai Apple Microsoft)
search engines data mining (eg Google)
challenges
multiple applications each serving massive numbers of clients
managingbalancing load avoiding processing networking data bottlenecks
Inside a 40-ft Microsoft container
Chicago data center
Link Layer 5-85
Server racks
TOR switches
Tier-1 switches
Tier-2 switches
Load balancer
Load balancer
B
1 2 3 4 5 6 7 8
A C
Border router
Access router
Internet
Data center networks
load balancer application-layer routing receives external client requests
directs workload within data center
returns results to external client (hiding data
center internals from client)
Server racks
TOR switches
Tier-1 switches
Tier-2 switches
1 2 3 4 5 6 7 8
Data center networks
rich interconnection among switches racks
increased throughput between racks (multiple routing
paths possible)
increased reliability via redundancy
Link Layer 5-87
Link layer LANs outline
51 introduction services
52 error detection correction
53 multiple access protocols
54 LANs addressing ARP
Ethernet
switches
VLANS
55 link virtualization MPLS
56 data center networking
57 a day in the life of a web request
Link Layer 5-88
Synthesis a day in the life of a web request
journey down protocol stack complete application transport network link
putting-it-all-together synthesis goal identify review understand protocols (at all
layers) involved in seemingly simple scenario requesting www page
scenario student attaches laptop to campus network requestsreceives wwwgooglecom
Link Layer 5-89
A day in the life scenario
Comcast network
68800013
Googlersquos network
64233160019 64233169105
web server
DNS server
school network
68802024
web page
browser
router
(runs DHCP)
Link Layer 5-90
A day in the lifehellip connecting to the Internet
connecting laptop needs to get its own IP address addr of first-hop router addr of DNS server use DHCP
DHCP
UDP
IP
Eth
Phy
DHCP
DHCP
DHCP
DHCP
DHCP
DHCP
UDP
IP
Eth
Phy
DHCP
DHCP
DHCP
DHCP DHCP
DHCP request encapsulated in UDP encapsulated in IP encapsulated in 8023 Ethernet
Ethernet frame broadcast
(dest FFFFFFFFFFFF) on LAN received at router running DHCP server
Ethernet demuxed to IP demuxed UDP demuxed to DHCP
router
(runs DHCP)
Link Layer 5-91
DHCP server formulates DHCP ACK containing clientrsquos IP address IP address of first-hop router for client name amp IP address of DNS server
DHCP
UDP
IP
Eth
Phy
DHCP
DHCP
DHCP
DHCP
DHCP
UDP
IP
Eth
Phy
DHCP
DHCP
DHCP
DHCP
DHCP
encapsulation at DHCP server frame forwarded (switch learning) through LAN demultiplexing at client
Client now has IP address knows name amp addr of DNS
server IP address of its first-hop router
DHCP client receives DHCP ACK reply
A day in the lifehellip connecting to the Internet
router
(runs DHCP)
Link Layer 5-92
A day in the lifehellip ARP (before DNS before HTTP)
before sending HTTP request need IP address of wwwgooglecom DNS
DNS
UDP
IP
Eth
Phy
DNS
DNS
DNS
DNS query created encapsulated in UDP encapsulated in IP encapsulated in Eth To send frame to router need MAC address of router interface ARP
ARP query broadcast received by
router which replies with ARP reply giving MAC address of router interface
client now knows MAC address of first hop router so can now send frame containing DNS query
ARP query
Eth
Phy
ARP
ARP
ARP reply
router
(runs DHCP)
Link Layer 5-93
DNS
UDP
IP
Eth
Phy
DNS
DNS
DNS
DNS
DNS
IP datagram containing DNS query forwarded via LAN switch from client to 1st hop router
IP datagram forwarded from campus network into comcast network routed (tables created by RIP OSPF IS-IS andor BGP routing protocols) to DNS server
demuxrsquoed to DNS server
DNS server replies to client with IP address of wwwgooglecom
Comcast network
68800013
DNS server
DNS
UDP
IP
Eth
Phy
DNS
DNS
DNS
DNS
A day in the lifehellip using DNS
router
(runs DHCP)
Link Layer 5-94
A day in the lifehellipTCP connection carrying HTTP
HTTP
TCP
IP
Eth
Phy
HTTP
to send HTTP request client first opens TCP socket to web server
TCP SYN segment (step 1 in 3-way handshake) inter-domain routed to web server
TCP connection established 64233169105
web server
SYN
SYN
SYN
SYN
TCP
IP
Eth
Phy
SYN
SYN
SYN
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
web server responds with TCP SYNACK (step 2 in 3-way handshake)
router
(runs DHCP)
Link Layer 5-95
A day in the lifehellip HTTP requestreply
HTTP
TCP
IP
Eth
Phy
HTTP
HTTP request sent into TCP socket
IP datagram containing HTTP request routed to wwwgooglecom
IP datagram containing HTTP reply routed back to client
64233169105
web server
HTTP
TCP
IP
Eth
Phy web server responds with
HTTP reply (containing web page)
HTTP
HTTP
HTTP HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
web page finally () displayed
Link Layer 5-96
Chapter 5 Summary
principles behind data link layer services error detection correction
sharing a broadcast channel multiple access
link layer addressing
instantiation and implementation of various link layer technologies Ethernet
switched LANS VLANs
virtualized networks as a link layer MPLS
synthesis a day in the life of a web request
Link Layer 5-97
Chapter 5 letrsquos take a breath
journey down protocol stack complete (except PHY)
solid understanding of networking principles practice
hellip could stop here hellip but lots of interesting topics wireless
multimedia
security
network management
Link Layer 5-16
Link layer LANs outline
51 introduction services
52 error detection correction
53 multiple access protocols
54 LANs addressing ARP
Ethernet
switches
VLANS
55 link virtualization MPLS
56 data center networking
57 a day in the life of a web request
Link Layer 5-17
Multiple access links protocols
two types of ldquolinksrdquo
point-to-point PPP for dial-up access
point-to-point link between Ethernet switch host
broadcast (shared wire or medium) old-fashioned Ethernet
upstream HFC
80211 wireless LAN
shared wire (eg cabled Ethernet)
shared RF (eg 80211 WiFi)
shared RF (satellite)
humans at a cocktail party
(shared air acoustical)
Link Layer 5-18
Multiple access protocols
single shared broadcast channel
two or more simultaneous transmissions by nodes interference
collision if node receives two or more signals at the same time
multiple access protocol distributed algorithm that determines how nodes share
channel ie determine when node can transmit
communication about channel sharing must use channel itself no out-of-band channel for coordination
Link Layer 5-19
An ideal multiple access protocol
given broadcast channel of rate R bps
desiderata 1 when one node wants to transmit it can send at rate R
2 when M nodes want to transmit each can send at average rate RM
3 fully decentralized
bull no special node to coordinate transmissions
bull no synchronization of clocks slots
4 simple
Link Layer 5-20
MAC protocols taxonomy
three broad classes
channel partitioning divide channel into smaller ldquopiecesrdquo (time slots frequency code)
allocate piece to node for exclusive use
random access channel not divided allow collisions
ldquorecoverrdquo from collisions
ldquotaking turnsrdquo nodes take turns but nodes with more to send can take longer
turns
Link Layer 5-21
Channel partitioning MAC protocols TDMA
TDMA time division multiple access access to channel in rounds each station gets fixed length slot (length = pkt
trans time) in each round unused slots go idle example 6-station LAN 134 have pkt slots
256 idle
1 3 4 1 3 4
6-slot
frame
6-slot
frame
Link Layer 5-22
FDMA frequency division multiple access channel spectrum divided into frequency bands
each station assigned fixed frequency band
unused transmission time in frequency bands go idle
example 6-station LAN 134 have pkt frequency bands 256 idle
frequency b
ands
FDM cable
Channel partitioning MAC protocols FDMA
Link Layer 5-23
Random access protocols
when node has packet to send transmit at full channel data rate R no a priori coordination among nodes
two or more transmitting nodes ldquocollisionrdquo random access MAC protocol specifies
how to detect collisions how to recover from collisions (eg via delayed
retransmissions)
examples of random access MAC protocols slotted ALOHA ALOHA CSMA CSMACD CSMACA
Link Layer 5-24
Slotted ALOHA
assumptions all frames same size
time divided into equal size slots (time to transmit 1 frame)
nodes start to transmit only slot beginning
nodes are synchronized
if 2 or more nodes transmit in slot all nodes detect collision
operation
when node obtains fresh frame transmits in next slot
if no collision node can send new frame in next slot
if collision node retransmits frame in each subsequent slot with prob p until success
Link Layer 5-25
Pros single active node can
continuously transmit at full rate of channel
highly decentralized only slots in nodes need to be in sync
simple
Cons
collisions wasting slots
idle slots
nodes may be able to detect collision in less than time to transmit packet
clock synchronization
Slotted ALOHA
1 1 1 1
2
3
2 2
3 3
node 1
node 2
node 3
C C C S S S E E E
Link Layer 5-26
suppose N nodes with many frames to send each transmits in slot with probability p
prob that given node has success in a slot = p(1-p)N-1
prob that any node has a success = Np(1-p)N-1
max efficiency find p that maximizes Np(1-p)N-1
for many nodes take limit of Np(1-p)N-1 as N goes to infinity gives
max efficiency = 1e = 37
efficiency long-run fraction of successful slots (many nodes all with many frames to send)
at best channel used for useful transmissions 37 of time
Slotted ALOHA efficiency
Link Layer 5-27
Pure (unslotted) ALOHA
unslotted Aloha simpler no synchronization
when frame first arrives
transmit immediately
collision probability increases
frame sent at t0 collides with other frames sent in [t0-1t0+1]
Link Layer 5-28
Pure ALOHA efficiency
P(success by given node) = P(node transmits)
P(no other node transmits in [t0-1t0]
P(no other node transmits in [t0-1t0]
= p (1-p)N-1 (1-p)N-1
= p (1-p)2(N-1)
hellip choosing optimum p and then letting n
= 1(2e) = 18
even worse than slotted Aloha
Link Layer 5-29
CSMA (carrier sense multiple access)
CSMA listen before transmit if channel sensed idle transmit entire frame
if channel sensed busy defer transmission
human analogy donrsquot interrupt others
Link Layer 5-30
CSMA collisions
collisions can still occur propagation delay means two nodes may not hear each otherrsquos transmission
collision entire packet transmission time wasted distance amp propagation
delay play role in in determining collision probability
spatial layout of nodes
Link Layer 5-31
CSMACD (collision detection)
CSMACD carrier sensing deferral as in CSMA collisions detected within short time
colliding transmissions aborted reducing channel wastage
collision detection easy in wired LANs measure signal strengths compare
transmitted received signals
difficult in wireless LANs received signal strength overwhelmed by local transmission strength
human analogy the polite conversationalist
Link Layer 5-32
CSMACD (collision detection)
spatial layout of nodes
Link Layer 5-33
Ethernet CSMACD algorithm
1 NIC receives datagram from network layer creates frame
2 If NIC senses channel idle starts frame transmission If NIC senses channel busy waits until channel idle then transmits
3 If NIC transmits entire frame without detecting another transmission NIC is done with frame
4 If NIC detects another transmission while transmitting aborts and sends jam signal
5 After aborting NIC enters binary (exponential) backoff
after mth collision NIC chooses K at random from 012 hellip 2m-1 NIC waits K512 bit times returns to Step 2
longer backoff interval with more collisions
Link Layer 5-34
CSMACD efficiency
Tprop = max prop delay between 2 nodes in LAN
ttrans = time to transmit max-size frame
efficiency goes to 1
as tprop goes to 0
as ttrans goes to infinity
better performance than ALOHA and simple cheap decentralized
transpropttefficiency
51
1
Link Layer 5-35
ldquoTaking turnsrdquo MAC protocols
channel partitioning MAC protocols share channel efficiently and fairly at high load
inefficient at low load delay in channel access 1N bandwidth allocated even if only 1 active node
random access MAC protocols efficient at low load single node can fully utilize
channel
high load collision overhead
ldquotaking turnsrdquo protocols look for best of both worlds
Link Layer 5-36
polling master node ldquoinvitesrdquo
slave nodes to transmit in turn
typically used with ldquodumbrdquo slave devices
concerns
polling overhead
latency
single point of failure (master)
master
slaves
poll
data
data
ldquoTaking turnsrdquo MAC protocols
Link Layer 5-37
token passing control token passed
from one node to next sequentially
token message
concerns
token overhead
latency
single point of failure (token)
T
data
(nothing
to send)
T
ldquoTaking turnsrdquo MAC protocols
cable headend
CMTS
ISP
cable modem termination system
multiple 40Mbps downstream (broadcast) channels
single CMTS transmits into channels
multiple 30 Mbps upstream channels
multiple access all users contend for certain upstream channel time slots (others assigned)
Cable access network
cable modem
splitter
hellip
hellip
Internet framesTV channels control transmitted downstream at different frequencies
upstream Internet frames TV control transmitted upstream at different frequencies in time slots
Link Layer 5-39
DOCSIS data over cable service interface spec FDM over upstream downstream frequency channels
TDM upstream some slots assigned some have contention
downstream MAP frame assigns upstream slots
request for upstream slots (and data) transmitted random access (binary backoff) in selected slots
MAP frame for
Interval [t1 t2]
Residences with cable modems
Downstream channel i
Upstream channel j
t1 t2
Assigned minislots containing cable modem
upstream data frames
Minislots containing
minislots request frames
cable headend
CMTS
Cable access network
Link Layer 5-40
Summary of MAC protocols
channel partitioning by time frequency or code Time Division Frequency Division
random access (dynamic)
ALOHA S-ALOHA CSMA CSMACD
carrier sensing easy in some technologies (wire) hard in others (wireless)
CSMACD used in Ethernet
CSMACA used in 80211
taking turns
polling from central site token passing
bluetooth FDDI token ring
Link Layer 5-41
Link layer LANs outline
51 introduction services
52 error detection correction
53 multiple access protocols
54 LANs addressing ARP
Ethernet
switches
VLANS
55 link virtualization MPLS
56 data center networking
57 a day in the life of a web request
Link Layer 5-42
MAC addresses and ARP
32-bit IP address network-layer address for interface
used for layer 3 (network layer) forwarding
MAC (or LAN or physical or Ethernet) address function used lsquolocallyrdquo to get frame from one interface to
another physically-connected interface (same network in IP-addressing sense)
48 bit MAC address (for most LANs) burned in NIC ROM also sometimes software settable
eg 1A-2F-BB-76-09-AD
hexadecimal (base 16) notation
(each ldquonumberrdquo represents 4 bits)
Link Layer 5-43
LAN addresses and ARP
each adapter on LAN has unique LAN address
adapter
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN
(wired or
wireless)
Link Layer 5-44
LAN addresses (more)
MAC address allocation administered by IEEE
manufacturer buys portion of MAC address space (to assure uniqueness)
analogy MAC address like Social Security Number
IP address like postal address
MAC flat address portability can move LAN card from one LAN to another
IP hierarchical address not portable address depends on IP subnet to which node is
attached
Link Layer 5-45
ARP address resolution protocol
ARP table each IP node (host router) on LAN has table
IPMAC address mappings for some LAN nodes
lt IP address MAC address TTLgt
TTL (Time To Live) time after which address mapping will be forgotten (typically 20 min)
Question how to determine
interfacersquos MAC address
knowing its IP address
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN
137196723
137196778
137196714
137196788
Link Layer 5-46
ARP protocol same LAN
A wants to send datagram to B Brsquos MAC address not in
Arsquos ARP table
A broadcasts ARP query packet containing Bs IP address dest MAC address = FF-FF-
FF-FF-FF-FF
all nodes on LAN receive ARP query
B receives ARP packet replies to A with its (Bs) MAC address frame sent to Arsquos MAC
address (unicast)
A caches (saves) IP-to-MAC address pair in its ARP table until information becomes old (times out) soft state information that
times out (goes away) unless refreshed
ARP is ldquoplug-and-playrdquo nodes create their ARP
tables without intervention from net administrator
Link Layer 5-47
walkthrough send datagram from A to B via R
focus on addressing ndash at IP (datagram) and MAC layer (frame)
assume A knows Brsquos IP address
assume A knows IP address of first hop router R (how)
assume A knows Rrsquos MAC address (how)
Addressing routing to another LAN
R
1A-23-F9-CD-06-9B 222222222220
111111111110 E6-E9-00-17-BB-4B CC-49-DE-D0-AB-7D
111111111112
111111111111
74-29-9C-E8-FF-55
A
222222222222
49-BD-D2-C7-56-2A
222222222221 88-B2-2F-54-1A-0F
B
R
1A-23-F9-CD-06-9B 222222222220
111111111110 E6-E9-00-17-BB-4B CC-49-DE-D0-AB-7D
111111111112
111111111111
74-29-9C-E8-FF-55
A
222222222222
49-BD-D2-C7-56-2A
222222222221 88-B2-2F-54-1A-0F
B
Link Layer 5-48
Addressing routing to another LAN
IP
Eth
Phy
IP src 111111111111
IP dest 222222222222
A creates IP datagram with IP source A destination B
A creates link-layer frame with Rs MAC address as dest frame contains A-to-B IP datagram
MAC src 74-29-9C-E8-FF-55
MAC dest E6-E9-00-17-BB-4B
R
1A-23-F9-CD-06-9B 222222222220
111111111110 E6-E9-00-17-BB-4B CC-49-DE-D0-AB-7D
111111111112
111111111111
74-29-9C-E8-FF-55
A
222222222222
49-BD-D2-C7-56-2A
222222222221 88-B2-2F-54-1A-0F
B
Link Layer 5-49
Addressing routing to another LAN
IP
Eth
Phy
frame sent from A to R
IP
Eth
Phy
frame received at R datagram removed passed up to IP
MAC src 74-29-9C-E8-FF-55
MAC dest E6-E9-00-17-BB-4B
IP src 111111111111
IP dest 222222222222
IP src 111111111111
IP dest 222222222222
R
1A-23-F9-CD-06-9B 222222222220
111111111110 E6-E9-00-17-BB-4B CC-49-DE-D0-AB-7D
111111111112
111111111111
74-29-9C-E8-FF-55
A
222222222222
49-BD-D2-C7-56-2A
222222222221 88-B2-2F-54-1A-0F
B
Link Layer 5-50
Addressing routing to another LAN
IP src 111111111111
IP dest 222222222222
R forwards datagram with IP source A destination B
R creates link-layer frame with Bs MAC address as dest frame contains A-to-B IP datagram
MAC src 1A-23-F9-CD-06-9B
MAC dest 49-BD-D2-C7-56-2A
IP
Eth
Phy
IP
Eth
Phy
R
1A-23-F9-CD-06-9B 222222222220
111111111110 E6-E9-00-17-BB-4B CC-49-DE-D0-AB-7D
111111111112
111111111111
74-29-9C-E8-FF-55
A
222222222222
49-BD-D2-C7-56-2A
222222222221 88-B2-2F-54-1A-0F
B
Link Layer 5-51
Addressing routing to another LAN R forwards datagram with IP source A destination B
R creates link-layer frame with Bs MAC address as dest frame contains A-to-B IP datagram
IP src 111111111111
IP dest 222222222222
MAC src 1A-23-F9-CD-06-9B
MAC dest 49-BD-D2-C7-56-2A
IP
Eth
Phy
IP
Eth
Phy
R
1A-23-F9-CD-06-9B 222222222220
111111111110 E6-E9-00-17-BB-4B CC-49-DE-D0-AB-7D
111111111112
111111111111
74-29-9C-E8-FF-55
A
222222222222
49-BD-D2-C7-56-2A
222222222221 88-B2-2F-54-1A-0F
B
Link Layer 5-52
Addressing routing to another LAN R forwards datagram with IP source A destination B
R creates link-layer frame with Bs MAC address as dest frame contains A-to-B IP datagram
IP src 111111111111
IP dest 222222222222
MAC src 1A-23-F9-CD-06-9B
MAC dest 49-BD-D2-C7-56-2A
IP
Eth
Phy
Link Layer 5-53
Link layer LANs outline
51 introduction services
52 error detection correction
53 multiple access protocols
54 LANs addressing ARP
Ethernet
switches
VLANS
55 link virtualization MPLS
56 data center networking
57 a day in the life of a web request
Link Layer 5-54
Ethernet
ldquodominantrdquo wired LAN technology
cheap $20 for NIC
first widely used LAN technology
simpler cheaper than token LANs and ATM
kept up with speed race 10 Mbps ndash 10 Gbps
Metcalfersquos Ethernet sketch
Link Layer 5-55
Ethernet physical topology
bus popular through mid 90s all nodes in same collision domain (can collide with each
other)
star prevails today active switch in center
each ldquospokerdquo runs a (separate) Ethernet protocol (nodes
do not collide with each other)
switch
bus coaxial cable star
Link Layer 5-56
Ethernet frame structure
sending adapter encapsulates IP datagram (or other network layer protocol packet) in Ethernet frame
preamble
7 bytes with pattern 10101010 followed by one byte with pattern 10101011
used to synchronize receiver sender clock rates
dest address
source address
data (payload) CRC preamble
type
Link Layer 5-57
Ethernet frame structure (more)
addresses 6 byte source destination MAC addresses if adapter receives frame with matching destination
address or with broadcast address (eg ARP packet) it passes data in frame to network layer protocol
otherwise adapter discards frame
type indicates higher layer protocol (mostly IP but others possible eg Novell IPX AppleTalk)
CRC cyclic redundancy check at receiver error detected frame is dropped
dest address
source address
data (payload) CRC preamble
type
Link Layer 5-58
Ethernet unreliable connectionless
connectionless no handshaking between sending and receiving NICs
unreliable receiving NIC doesnt send acks or nacks to sending NIC
data in dropped frames recovered only if initial sender uses higher layer rdt (eg TCP) otherwise dropped data lost
Ethernetrsquos MAC protocol unslotted CSMACD wth binary backoff
Link Layer 5-59
8023 Ethernet standards link amp physical layers
many different Ethernet standards
common MAC protocol and frame format
different speeds 2 Mbps 10 Mbps 100 Mbps 1Gbps 10G bps
different physical layer media fiber cable
application
transport
network
link
physical
MAC protocol
and frame format
100BASE-TX
100BASE-T4
100BASE-FX 100BASE-T2
100BASE-SX 100BASE-BX
fiber physical layer copper (twister
pair) physical layer
Link Layer 5-60
Link layer LANs outline
51 introduction services
52 error detection correction
53 multiple access protocols
54 LANs addressing ARP
Ethernet
switches
VLANS
55 link virtualization MPLS
56 data center networking
57 a day in the life of a web request
Link Layer 5-61
Ethernet switch link-layer device takes an active role
store forward Ethernet frames
examine incoming framersquos MAC address selectively forward frame to one-or-more outgoing links when frame is to be forwarded on segment uses CSMACD to access segment
transparent
hosts are unaware of presence of switches
plug-and-play self-learning
switches do not need to be configured
Link Layer 5-62
Switch multiple simultaneous transmissions
hosts have dedicated direct connection to switch
switches buffer packets
Ethernet protocol used on each incoming link but no collisions full duplex
each link is its own collision domain
switching A-to-Arsquo and B-to-Brsquo can transmit simultaneously without collisions switch with six interfaces
(123456)
A
Arsquo
B
Brsquo C
Crsquo
1 2
3 4 5
6
Link Layer 5-63
Switch forwarding table
Q how does switch know Arsquo reachable via interface 4 Brsquo reachable via interface 5
switch with six interfaces
(123456)
A
Arsquo
B
Brsquo C
Crsquo
1 2
3 4 5
6 A each switch has a switch table each entry
(MAC address of host interface to
reach host time stamp)
looks like a routing table
Q how are entries created maintained in switch table
something like a routing protocol
A
Arsquo
B
Brsquo C
Crsquo
1 2
3 4 5
6
Link Layer 5-64
Switch self-learning
switch learns which hosts can be reached through which interfaces
when frame received switch ldquolearnsrdquo location of sender incoming LAN segment
records senderlocation pair in switch table
A Arsquo
Source A
Dest Arsquo
MAC addr interface TTL
Switch table
(initially empty) A 1 60
Link Layer 5-65
Switch frame filteringforwarding
when frame received at switch 1 record incoming link MAC address of sending host
2 index switch table using MAC destination address
3 if entry found for destination then
if destination on segment from which frame arrived then drop frame
else forward frame on interface indicated by entry
else flood forward on all interfaces except arriving
interface
A
Arsquo
B
Brsquo C
Crsquo
1 2
3 4 5
6
Link Layer 5-66
Self-learning forwarding example
A Arsquo
Source A
Dest Arsquo
MAC addr interface TTL
switch table
(initially empty) A 1 60
A Arsquo A Arsquo A Arsquo A Arsquo A Arsquo
frame destination Arsquo locaton unknown flood
Arsquo A
destination A location
known
Arsquo 4 60
selectively send
on just one link
Link Layer 5-67
Interconnecting switches
switches can be connected together
Q sending from A to G - how does S1 know to forward frame destined to F via S4 and S3
A self learning (works exactly the same as in single-switch case)
A
B
S1
C D
E
F
S2
S4
S3
H
I
G
Link Layer 5-68
Self-learning multi-switch example
Suppose C sends frame to I I responds to C
Q show switch tables and packet forwarding in S1 S2 S3 S4
A
B
S1
C D
E
F
S2
S4
S3
H
I
G
Link Layer 5-69
Institutional network
to external
network
router
IP subnet
mail server
web server
Link Layer 5-70
Switches vs routers
both are store-and-forward
routers network-layer devices (examine network-layer headers)
switches link-layer devices (examine link-layer headers)
both have forwarding tables
routers compute tables using routing algorithms IP addresses
switches learn forwarding table using flooding learning MAC addresses
application
transport
network
link
physical
network
link
physical
link
physical
switch
datagram
application
transport
network
link
physical
frame
frame
frame
datagram
Link Layer 5-71
VLANs motivation
consider CS user moves office to
EE but wants connect to CS switch
single broadcast domain
all layer-2 broadcast traffic (ARP DHCP unknown location of destination MAC address) must cross entire LAN
securityprivacy efficiency issues
Computer
Science Electrical
Engineering
Computer
Engineering
Link Layer 5-72
VLANs port-based VLAN switch ports
grouped (by switch management software) so that single physical switch helliphellip
switch(es) supporting
VLAN capabilities can
be configured to
define multiple virtual
LANS over single
physical LAN
infrastructure
Virtual Local
Area Network 1
8
9
16 10 2
7
hellip
Electrical Engineering
(VLAN ports 1-8)
Computer Science
(VLAN ports 9-15)
15
hellip
Electrical Engineering
(VLAN ports 1-8)
hellip
1
8 2
7 9
16 10
15
hellip
Computer Science
(VLAN ports 9-16)
hellip operates as multiple virtual switches
Link Layer 5-73
Port-based VLAN
1
8
9
16 10 2
7
hellip
Electrical Engineering
(VLAN ports 1-8)
Computer Science
(VLAN ports 9-15)
15
hellip
traffic isolation frames tofrom ports 1-8 can only reach ports 1-8 can also define VLAN based on
MAC addresses of endpoints rather than switch port
dynamic membership ports can be dynamically assigned among VLANs
router
forwarding between VLANS done via routing (just as with separate switches) in practice vendors sell combined
switches plus routers
Link Layer 5-74
VLANS spanning multiple switches
trunk port carries frames between VLANS defined over multiple physical switches frames forwarded within VLAN between switches canrsquot be vanilla
8021 frames (must carry VLAN ID info)
8021q protocol addsremoved additional header fields for frames forwarded between trunk ports
1
8
9
10 2
7
hellip
Electrical Engineering
(VLAN ports 1-8)
Computer Science
(VLAN ports 9-15)
15
hellip
2
7 3
Ports 235 belong to EE VLAN
Ports 4678 belong to CS VLAN
5
4 6 8 16
1
Link Layer 5-75
type
2-byte Tag Protocol Identifier
(value 81-00)
Tag Control Information (12 bit VLAN ID field
3 bit priority field like IP TOS)
Recomputed CRC
8021Q VLAN frame format
8021 frame
8021Q frame
dest address
source address
data (payload) CRC preamble
dest address
source address
preamble data (payload) CRC
type
Link Layer 5-76
Link layer LANs outline
51 introduction services
52 error detection correction
53 multiple access protocols
54 LANs addressing ARP
Ethernet
switches
VLANS
55 link virtualization MPLS
56 data center networking
57 a day in the life of a web request
Link Layer 5-77
Multiprotocol label switching (MPLS)
initial goal high-speed IP forwarding using fixed length label (instead of IP address) fast lookup using fixed length identifier (rather than
shortest prefix matching)
borrowing ideas from Virtual Circuit (VC) approach
but IP datagram still keeps IP address
PPP or Ethernet
header IP header remainder of link-layer frame MPLS header
label Exp S TTL
20 3 1 5
Link Layer 5-78
MPLS capable routers
aka label-switched router
forward packets to outgoing interface based only on label value (donrsquot inspect IP address) MPLS forwarding table distinct from IP forwarding tables
flexibility MPLS forwarding decisions can differ from those of IP use destination and source addresses to route flows to
same destination differently (traffic engineering)
re-route flows quickly if link fails pre-computed backup paths (useful for VoIP)
Link Layer 5-79
R2
D
R3 R5
A
R6
MPLS versus IP paths
IP router IP routing path to destination determined
by destination address alone
R4
Link Layer 5-80
R2
D
R3 R4
R5
A
R6
MPLS versus IP paths
IP-only router
IP routing path to destination determined by destination address alone
MPLS and IP router
MPLS routing path to destination can be based on source and dest address fast reroute precompute backup routes in
case of link failure
entry router (R4) can use different MPLS
routes to A based eg on source address
Link Layer 5-81
MPLS signaling
modify OSPF IS-IS link-state flooding protocols to carry info used by MPLS routing eg link bandwidth amount of ldquoreservedrdquo link bandwidth
D
R4
R5
A
R6
entry MPLS router uses RSVP-TE signaling protocol to set up MPLS forwarding at downstream routers
modified link state flooding
RSVP-TE
Link Layer 5-82
R1 R2
D
R3 R4 R5
0
1
0 0
A
R6
in out out
label label dest interface
6 - A 0
in out out
label label dest interface
10 6 A 1
12 9 D 0
in out out
label label dest interface
10 A 0
12 D 0
1
in out out
label label dest interface
8 6 A 0
0
8 A 1
MPLS forwarding tables
Link Layer 5-83
Link layer LANs outline
51 introduction services
52 error detection correction
53 multiple access protocols
54 LANs addressing ARP
Ethernet
switches
VLANS
55 link virtualization MPLS
56 data center networking
57 a day in the life of a web request
Link Layer 5-84
Data center networks
10rsquos to 100rsquos of thousands of hosts often closely coupled in close proximity e-business (eg Amazon)
content-servers (eg YouTube Akamai Apple Microsoft)
search engines data mining (eg Google)
challenges
multiple applications each serving massive numbers of clients
managingbalancing load avoiding processing networking data bottlenecks
Inside a 40-ft Microsoft container
Chicago data center
Link Layer 5-85
Server racks
TOR switches
Tier-1 switches
Tier-2 switches
Load balancer
Load balancer
B
1 2 3 4 5 6 7 8
A C
Border router
Access router
Internet
Data center networks
load balancer application-layer routing receives external client requests
directs workload within data center
returns results to external client (hiding data
center internals from client)
Server racks
TOR switches
Tier-1 switches
Tier-2 switches
1 2 3 4 5 6 7 8
Data center networks
rich interconnection among switches racks
increased throughput between racks (multiple routing
paths possible)
increased reliability via redundancy
Link Layer 5-87
Link layer LANs outline
51 introduction services
52 error detection correction
53 multiple access protocols
54 LANs addressing ARP
Ethernet
switches
VLANS
55 link virtualization MPLS
56 data center networking
57 a day in the life of a web request
Link Layer 5-88
Synthesis a day in the life of a web request
journey down protocol stack complete application transport network link
putting-it-all-together synthesis goal identify review understand protocols (at all
layers) involved in seemingly simple scenario requesting www page
scenario student attaches laptop to campus network requestsreceives wwwgooglecom
Link Layer 5-89
A day in the life scenario
Comcast network
68800013
Googlersquos network
64233160019 64233169105
web server
DNS server
school network
68802024
web page
browser
router
(runs DHCP)
Link Layer 5-90
A day in the lifehellip connecting to the Internet
connecting laptop needs to get its own IP address addr of first-hop router addr of DNS server use DHCP
DHCP
UDP
IP
Eth
Phy
DHCP
DHCP
DHCP
DHCP
DHCP
DHCP
UDP
IP
Eth
Phy
DHCP
DHCP
DHCP
DHCP DHCP
DHCP request encapsulated in UDP encapsulated in IP encapsulated in 8023 Ethernet
Ethernet frame broadcast
(dest FFFFFFFFFFFF) on LAN received at router running DHCP server
Ethernet demuxed to IP demuxed UDP demuxed to DHCP
router
(runs DHCP)
Link Layer 5-91
DHCP server formulates DHCP ACK containing clientrsquos IP address IP address of first-hop router for client name amp IP address of DNS server
DHCP
UDP
IP
Eth
Phy
DHCP
DHCP
DHCP
DHCP
DHCP
UDP
IP
Eth
Phy
DHCP
DHCP
DHCP
DHCP
DHCP
encapsulation at DHCP server frame forwarded (switch learning) through LAN demultiplexing at client
Client now has IP address knows name amp addr of DNS
server IP address of its first-hop router
DHCP client receives DHCP ACK reply
A day in the lifehellip connecting to the Internet
router
(runs DHCP)
Link Layer 5-92
A day in the lifehellip ARP (before DNS before HTTP)
before sending HTTP request need IP address of wwwgooglecom DNS
DNS
UDP
IP
Eth
Phy
DNS
DNS
DNS
DNS query created encapsulated in UDP encapsulated in IP encapsulated in Eth To send frame to router need MAC address of router interface ARP
ARP query broadcast received by
router which replies with ARP reply giving MAC address of router interface
client now knows MAC address of first hop router so can now send frame containing DNS query
ARP query
Eth
Phy
ARP
ARP
ARP reply
router
(runs DHCP)
Link Layer 5-93
DNS
UDP
IP
Eth
Phy
DNS
DNS
DNS
DNS
DNS
IP datagram containing DNS query forwarded via LAN switch from client to 1st hop router
IP datagram forwarded from campus network into comcast network routed (tables created by RIP OSPF IS-IS andor BGP routing protocols) to DNS server
demuxrsquoed to DNS server
DNS server replies to client with IP address of wwwgooglecom
Comcast network
68800013
DNS server
DNS
UDP
IP
Eth
Phy
DNS
DNS
DNS
DNS
A day in the lifehellip using DNS
router
(runs DHCP)
Link Layer 5-94
A day in the lifehellipTCP connection carrying HTTP
HTTP
TCP
IP
Eth
Phy
HTTP
to send HTTP request client first opens TCP socket to web server
TCP SYN segment (step 1 in 3-way handshake) inter-domain routed to web server
TCP connection established 64233169105
web server
SYN
SYN
SYN
SYN
TCP
IP
Eth
Phy
SYN
SYN
SYN
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
web server responds with TCP SYNACK (step 2 in 3-way handshake)
router
(runs DHCP)
Link Layer 5-95
A day in the lifehellip HTTP requestreply
HTTP
TCP
IP
Eth
Phy
HTTP
HTTP request sent into TCP socket
IP datagram containing HTTP request routed to wwwgooglecom
IP datagram containing HTTP reply routed back to client
64233169105
web server
HTTP
TCP
IP
Eth
Phy web server responds with
HTTP reply (containing web page)
HTTP
HTTP
HTTP HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
web page finally () displayed
Link Layer 5-96
Chapter 5 Summary
principles behind data link layer services error detection correction
sharing a broadcast channel multiple access
link layer addressing
instantiation and implementation of various link layer technologies Ethernet
switched LANS VLANs
virtualized networks as a link layer MPLS
synthesis a day in the life of a web request
Link Layer 5-97
Chapter 5 letrsquos take a breath
journey down protocol stack complete (except PHY)
solid understanding of networking principles practice
hellip could stop here hellip but lots of interesting topics wireless
multimedia
security
network management
Link Layer 5-17
Multiple access links protocols
two types of ldquolinksrdquo
point-to-point PPP for dial-up access
point-to-point link between Ethernet switch host
broadcast (shared wire or medium) old-fashioned Ethernet
upstream HFC
80211 wireless LAN
shared wire (eg cabled Ethernet)
shared RF (eg 80211 WiFi)
shared RF (satellite)
humans at a cocktail party
(shared air acoustical)
Link Layer 5-18
Multiple access protocols
single shared broadcast channel
two or more simultaneous transmissions by nodes interference
collision if node receives two or more signals at the same time
multiple access protocol distributed algorithm that determines how nodes share
channel ie determine when node can transmit
communication about channel sharing must use channel itself no out-of-band channel for coordination
Link Layer 5-19
An ideal multiple access protocol
given broadcast channel of rate R bps
desiderata 1 when one node wants to transmit it can send at rate R
2 when M nodes want to transmit each can send at average rate RM
3 fully decentralized
bull no special node to coordinate transmissions
bull no synchronization of clocks slots
4 simple
Link Layer 5-20
MAC protocols taxonomy
three broad classes
channel partitioning divide channel into smaller ldquopiecesrdquo (time slots frequency code)
allocate piece to node for exclusive use
random access channel not divided allow collisions
ldquorecoverrdquo from collisions
ldquotaking turnsrdquo nodes take turns but nodes with more to send can take longer
turns
Link Layer 5-21
Channel partitioning MAC protocols TDMA
TDMA time division multiple access access to channel in rounds each station gets fixed length slot (length = pkt
trans time) in each round unused slots go idle example 6-station LAN 134 have pkt slots
256 idle
1 3 4 1 3 4
6-slot
frame
6-slot
frame
Link Layer 5-22
FDMA frequency division multiple access channel spectrum divided into frequency bands
each station assigned fixed frequency band
unused transmission time in frequency bands go idle
example 6-station LAN 134 have pkt frequency bands 256 idle
frequency b
ands
FDM cable
Channel partitioning MAC protocols FDMA
Link Layer 5-23
Random access protocols
when node has packet to send transmit at full channel data rate R no a priori coordination among nodes
two or more transmitting nodes ldquocollisionrdquo random access MAC protocol specifies
how to detect collisions how to recover from collisions (eg via delayed
retransmissions)
examples of random access MAC protocols slotted ALOHA ALOHA CSMA CSMACD CSMACA
Link Layer 5-24
Slotted ALOHA
assumptions all frames same size
time divided into equal size slots (time to transmit 1 frame)
nodes start to transmit only slot beginning
nodes are synchronized
if 2 or more nodes transmit in slot all nodes detect collision
operation
when node obtains fresh frame transmits in next slot
if no collision node can send new frame in next slot
if collision node retransmits frame in each subsequent slot with prob p until success
Link Layer 5-25
Pros single active node can
continuously transmit at full rate of channel
highly decentralized only slots in nodes need to be in sync
simple
Cons
collisions wasting slots
idle slots
nodes may be able to detect collision in less than time to transmit packet
clock synchronization
Slotted ALOHA
1 1 1 1
2
3
2 2
3 3
node 1
node 2
node 3
C C C S S S E E E
Link Layer 5-26
suppose N nodes with many frames to send each transmits in slot with probability p
prob that given node has success in a slot = p(1-p)N-1
prob that any node has a success = Np(1-p)N-1
max efficiency find p that maximizes Np(1-p)N-1
for many nodes take limit of Np(1-p)N-1 as N goes to infinity gives
max efficiency = 1e = 37
efficiency long-run fraction of successful slots (many nodes all with many frames to send)
at best channel used for useful transmissions 37 of time
Slotted ALOHA efficiency
Link Layer 5-27
Pure (unslotted) ALOHA
unslotted Aloha simpler no synchronization
when frame first arrives
transmit immediately
collision probability increases
frame sent at t0 collides with other frames sent in [t0-1t0+1]
Link Layer 5-28
Pure ALOHA efficiency
P(success by given node) = P(node transmits)
P(no other node transmits in [t0-1t0]
P(no other node transmits in [t0-1t0]
= p (1-p)N-1 (1-p)N-1
= p (1-p)2(N-1)
hellip choosing optimum p and then letting n
= 1(2e) = 18
even worse than slotted Aloha
Link Layer 5-29
CSMA (carrier sense multiple access)
CSMA listen before transmit if channel sensed idle transmit entire frame
if channel sensed busy defer transmission
human analogy donrsquot interrupt others
Link Layer 5-30
CSMA collisions
collisions can still occur propagation delay means two nodes may not hear each otherrsquos transmission
collision entire packet transmission time wasted distance amp propagation
delay play role in in determining collision probability
spatial layout of nodes
Link Layer 5-31
CSMACD (collision detection)
CSMACD carrier sensing deferral as in CSMA collisions detected within short time
colliding transmissions aborted reducing channel wastage
collision detection easy in wired LANs measure signal strengths compare
transmitted received signals
difficult in wireless LANs received signal strength overwhelmed by local transmission strength
human analogy the polite conversationalist
Link Layer 5-32
CSMACD (collision detection)
spatial layout of nodes
Link Layer 5-33
Ethernet CSMACD algorithm
1 NIC receives datagram from network layer creates frame
2 If NIC senses channel idle starts frame transmission If NIC senses channel busy waits until channel idle then transmits
3 If NIC transmits entire frame without detecting another transmission NIC is done with frame
4 If NIC detects another transmission while transmitting aborts and sends jam signal
5 After aborting NIC enters binary (exponential) backoff
after mth collision NIC chooses K at random from 012 hellip 2m-1 NIC waits K512 bit times returns to Step 2
longer backoff interval with more collisions
Link Layer 5-34
CSMACD efficiency
Tprop = max prop delay between 2 nodes in LAN
ttrans = time to transmit max-size frame
efficiency goes to 1
as tprop goes to 0
as ttrans goes to infinity
better performance than ALOHA and simple cheap decentralized
transpropttefficiency
51
1
Link Layer 5-35
ldquoTaking turnsrdquo MAC protocols
channel partitioning MAC protocols share channel efficiently and fairly at high load
inefficient at low load delay in channel access 1N bandwidth allocated even if only 1 active node
random access MAC protocols efficient at low load single node can fully utilize
channel
high load collision overhead
ldquotaking turnsrdquo protocols look for best of both worlds
Link Layer 5-36
polling master node ldquoinvitesrdquo
slave nodes to transmit in turn
typically used with ldquodumbrdquo slave devices
concerns
polling overhead
latency
single point of failure (master)
master
slaves
poll
data
data
ldquoTaking turnsrdquo MAC protocols
Link Layer 5-37
token passing control token passed
from one node to next sequentially
token message
concerns
token overhead
latency
single point of failure (token)
T
data
(nothing
to send)
T
ldquoTaking turnsrdquo MAC protocols
cable headend
CMTS
ISP
cable modem termination system
multiple 40Mbps downstream (broadcast) channels
single CMTS transmits into channels
multiple 30 Mbps upstream channels
multiple access all users contend for certain upstream channel time slots (others assigned)
Cable access network
cable modem
splitter
hellip
hellip
Internet framesTV channels control transmitted downstream at different frequencies
upstream Internet frames TV control transmitted upstream at different frequencies in time slots
Link Layer 5-39
DOCSIS data over cable service interface spec FDM over upstream downstream frequency channels
TDM upstream some slots assigned some have contention
downstream MAP frame assigns upstream slots
request for upstream slots (and data) transmitted random access (binary backoff) in selected slots
MAP frame for
Interval [t1 t2]
Residences with cable modems
Downstream channel i
Upstream channel j
t1 t2
Assigned minislots containing cable modem
upstream data frames
Minislots containing
minislots request frames
cable headend
CMTS
Cable access network
Link Layer 5-40
Summary of MAC protocols
channel partitioning by time frequency or code Time Division Frequency Division
random access (dynamic)
ALOHA S-ALOHA CSMA CSMACD
carrier sensing easy in some technologies (wire) hard in others (wireless)
CSMACD used in Ethernet
CSMACA used in 80211
taking turns
polling from central site token passing
bluetooth FDDI token ring
Link Layer 5-41
Link layer LANs outline
51 introduction services
52 error detection correction
53 multiple access protocols
54 LANs addressing ARP
Ethernet
switches
VLANS
55 link virtualization MPLS
56 data center networking
57 a day in the life of a web request
Link Layer 5-42
MAC addresses and ARP
32-bit IP address network-layer address for interface
used for layer 3 (network layer) forwarding
MAC (or LAN or physical or Ethernet) address function used lsquolocallyrdquo to get frame from one interface to
another physically-connected interface (same network in IP-addressing sense)
48 bit MAC address (for most LANs) burned in NIC ROM also sometimes software settable
eg 1A-2F-BB-76-09-AD
hexadecimal (base 16) notation
(each ldquonumberrdquo represents 4 bits)
Link Layer 5-43
LAN addresses and ARP
each adapter on LAN has unique LAN address
adapter
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN
(wired or
wireless)
Link Layer 5-44
LAN addresses (more)
MAC address allocation administered by IEEE
manufacturer buys portion of MAC address space (to assure uniqueness)
analogy MAC address like Social Security Number
IP address like postal address
MAC flat address portability can move LAN card from one LAN to another
IP hierarchical address not portable address depends on IP subnet to which node is
attached
Link Layer 5-45
ARP address resolution protocol
ARP table each IP node (host router) on LAN has table
IPMAC address mappings for some LAN nodes
lt IP address MAC address TTLgt
TTL (Time To Live) time after which address mapping will be forgotten (typically 20 min)
Question how to determine
interfacersquos MAC address
knowing its IP address
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN
137196723
137196778
137196714
137196788
Link Layer 5-46
ARP protocol same LAN
A wants to send datagram to B Brsquos MAC address not in
Arsquos ARP table
A broadcasts ARP query packet containing Bs IP address dest MAC address = FF-FF-
FF-FF-FF-FF
all nodes on LAN receive ARP query
B receives ARP packet replies to A with its (Bs) MAC address frame sent to Arsquos MAC
address (unicast)
A caches (saves) IP-to-MAC address pair in its ARP table until information becomes old (times out) soft state information that
times out (goes away) unless refreshed
ARP is ldquoplug-and-playrdquo nodes create their ARP
tables without intervention from net administrator
Link Layer 5-47
walkthrough send datagram from A to B via R
focus on addressing ndash at IP (datagram) and MAC layer (frame)
assume A knows Brsquos IP address
assume A knows IP address of first hop router R (how)
assume A knows Rrsquos MAC address (how)
Addressing routing to another LAN
R
1A-23-F9-CD-06-9B 222222222220
111111111110 E6-E9-00-17-BB-4B CC-49-DE-D0-AB-7D
111111111112
111111111111
74-29-9C-E8-FF-55
A
222222222222
49-BD-D2-C7-56-2A
222222222221 88-B2-2F-54-1A-0F
B
R
1A-23-F9-CD-06-9B 222222222220
111111111110 E6-E9-00-17-BB-4B CC-49-DE-D0-AB-7D
111111111112
111111111111
74-29-9C-E8-FF-55
A
222222222222
49-BD-D2-C7-56-2A
222222222221 88-B2-2F-54-1A-0F
B
Link Layer 5-48
Addressing routing to another LAN
IP
Eth
Phy
IP src 111111111111
IP dest 222222222222
A creates IP datagram with IP source A destination B
A creates link-layer frame with Rs MAC address as dest frame contains A-to-B IP datagram
MAC src 74-29-9C-E8-FF-55
MAC dest E6-E9-00-17-BB-4B
R
1A-23-F9-CD-06-9B 222222222220
111111111110 E6-E9-00-17-BB-4B CC-49-DE-D0-AB-7D
111111111112
111111111111
74-29-9C-E8-FF-55
A
222222222222
49-BD-D2-C7-56-2A
222222222221 88-B2-2F-54-1A-0F
B
Link Layer 5-49
Addressing routing to another LAN
IP
Eth
Phy
frame sent from A to R
IP
Eth
Phy
frame received at R datagram removed passed up to IP
MAC src 74-29-9C-E8-FF-55
MAC dest E6-E9-00-17-BB-4B
IP src 111111111111
IP dest 222222222222
IP src 111111111111
IP dest 222222222222
R
1A-23-F9-CD-06-9B 222222222220
111111111110 E6-E9-00-17-BB-4B CC-49-DE-D0-AB-7D
111111111112
111111111111
74-29-9C-E8-FF-55
A
222222222222
49-BD-D2-C7-56-2A
222222222221 88-B2-2F-54-1A-0F
B
Link Layer 5-50
Addressing routing to another LAN
IP src 111111111111
IP dest 222222222222
R forwards datagram with IP source A destination B
R creates link-layer frame with Bs MAC address as dest frame contains A-to-B IP datagram
MAC src 1A-23-F9-CD-06-9B
MAC dest 49-BD-D2-C7-56-2A
IP
Eth
Phy
IP
Eth
Phy
R
1A-23-F9-CD-06-9B 222222222220
111111111110 E6-E9-00-17-BB-4B CC-49-DE-D0-AB-7D
111111111112
111111111111
74-29-9C-E8-FF-55
A
222222222222
49-BD-D2-C7-56-2A
222222222221 88-B2-2F-54-1A-0F
B
Link Layer 5-51
Addressing routing to another LAN R forwards datagram with IP source A destination B
R creates link-layer frame with Bs MAC address as dest frame contains A-to-B IP datagram
IP src 111111111111
IP dest 222222222222
MAC src 1A-23-F9-CD-06-9B
MAC dest 49-BD-D2-C7-56-2A
IP
Eth
Phy
IP
Eth
Phy
R
1A-23-F9-CD-06-9B 222222222220
111111111110 E6-E9-00-17-BB-4B CC-49-DE-D0-AB-7D
111111111112
111111111111
74-29-9C-E8-FF-55
A
222222222222
49-BD-D2-C7-56-2A
222222222221 88-B2-2F-54-1A-0F
B
Link Layer 5-52
Addressing routing to another LAN R forwards datagram with IP source A destination B
R creates link-layer frame with Bs MAC address as dest frame contains A-to-B IP datagram
IP src 111111111111
IP dest 222222222222
MAC src 1A-23-F9-CD-06-9B
MAC dest 49-BD-D2-C7-56-2A
IP
Eth
Phy
Link Layer 5-53
Link layer LANs outline
51 introduction services
52 error detection correction
53 multiple access protocols
54 LANs addressing ARP
Ethernet
switches
VLANS
55 link virtualization MPLS
56 data center networking
57 a day in the life of a web request
Link Layer 5-54
Ethernet
ldquodominantrdquo wired LAN technology
cheap $20 for NIC
first widely used LAN technology
simpler cheaper than token LANs and ATM
kept up with speed race 10 Mbps ndash 10 Gbps
Metcalfersquos Ethernet sketch
Link Layer 5-55
Ethernet physical topology
bus popular through mid 90s all nodes in same collision domain (can collide with each
other)
star prevails today active switch in center
each ldquospokerdquo runs a (separate) Ethernet protocol (nodes
do not collide with each other)
switch
bus coaxial cable star
Link Layer 5-56
Ethernet frame structure
sending adapter encapsulates IP datagram (or other network layer protocol packet) in Ethernet frame
preamble
7 bytes with pattern 10101010 followed by one byte with pattern 10101011
used to synchronize receiver sender clock rates
dest address
source address
data (payload) CRC preamble
type
Link Layer 5-57
Ethernet frame structure (more)
addresses 6 byte source destination MAC addresses if adapter receives frame with matching destination
address or with broadcast address (eg ARP packet) it passes data in frame to network layer protocol
otherwise adapter discards frame
type indicates higher layer protocol (mostly IP but others possible eg Novell IPX AppleTalk)
CRC cyclic redundancy check at receiver error detected frame is dropped
dest address
source address
data (payload) CRC preamble
type
Link Layer 5-58
Ethernet unreliable connectionless
connectionless no handshaking between sending and receiving NICs
unreliable receiving NIC doesnt send acks or nacks to sending NIC
data in dropped frames recovered only if initial sender uses higher layer rdt (eg TCP) otherwise dropped data lost
Ethernetrsquos MAC protocol unslotted CSMACD wth binary backoff
Link Layer 5-59
8023 Ethernet standards link amp physical layers
many different Ethernet standards
common MAC protocol and frame format
different speeds 2 Mbps 10 Mbps 100 Mbps 1Gbps 10G bps
different physical layer media fiber cable
application
transport
network
link
physical
MAC protocol
and frame format
100BASE-TX
100BASE-T4
100BASE-FX 100BASE-T2
100BASE-SX 100BASE-BX
fiber physical layer copper (twister
pair) physical layer
Link Layer 5-60
Link layer LANs outline
51 introduction services
52 error detection correction
53 multiple access protocols
54 LANs addressing ARP
Ethernet
switches
VLANS
55 link virtualization MPLS
56 data center networking
57 a day in the life of a web request
Link Layer 5-61
Ethernet switch link-layer device takes an active role
store forward Ethernet frames
examine incoming framersquos MAC address selectively forward frame to one-or-more outgoing links when frame is to be forwarded on segment uses CSMACD to access segment
transparent
hosts are unaware of presence of switches
plug-and-play self-learning
switches do not need to be configured
Link Layer 5-62
Switch multiple simultaneous transmissions
hosts have dedicated direct connection to switch
switches buffer packets
Ethernet protocol used on each incoming link but no collisions full duplex
each link is its own collision domain
switching A-to-Arsquo and B-to-Brsquo can transmit simultaneously without collisions switch with six interfaces
(123456)
A
Arsquo
B
Brsquo C
Crsquo
1 2
3 4 5
6
Link Layer 5-63
Switch forwarding table
Q how does switch know Arsquo reachable via interface 4 Brsquo reachable via interface 5
switch with six interfaces
(123456)
A
Arsquo
B
Brsquo C
Crsquo
1 2
3 4 5
6 A each switch has a switch table each entry
(MAC address of host interface to
reach host time stamp)
looks like a routing table
Q how are entries created maintained in switch table
something like a routing protocol
A
Arsquo
B
Brsquo C
Crsquo
1 2
3 4 5
6
Link Layer 5-64
Switch self-learning
switch learns which hosts can be reached through which interfaces
when frame received switch ldquolearnsrdquo location of sender incoming LAN segment
records senderlocation pair in switch table
A Arsquo
Source A
Dest Arsquo
MAC addr interface TTL
Switch table
(initially empty) A 1 60
Link Layer 5-65
Switch frame filteringforwarding
when frame received at switch 1 record incoming link MAC address of sending host
2 index switch table using MAC destination address
3 if entry found for destination then
if destination on segment from which frame arrived then drop frame
else forward frame on interface indicated by entry
else flood forward on all interfaces except arriving
interface
A
Arsquo
B
Brsquo C
Crsquo
1 2
3 4 5
6
Link Layer 5-66
Self-learning forwarding example
A Arsquo
Source A
Dest Arsquo
MAC addr interface TTL
switch table
(initially empty) A 1 60
A Arsquo A Arsquo A Arsquo A Arsquo A Arsquo
frame destination Arsquo locaton unknown flood
Arsquo A
destination A location
known
Arsquo 4 60
selectively send
on just one link
Link Layer 5-67
Interconnecting switches
switches can be connected together
Q sending from A to G - how does S1 know to forward frame destined to F via S4 and S3
A self learning (works exactly the same as in single-switch case)
A
B
S1
C D
E
F
S2
S4
S3
H
I
G
Link Layer 5-68
Self-learning multi-switch example
Suppose C sends frame to I I responds to C
Q show switch tables and packet forwarding in S1 S2 S3 S4
A
B
S1
C D
E
F
S2
S4
S3
H
I
G
Link Layer 5-69
Institutional network
to external
network
router
IP subnet
mail server
web server
Link Layer 5-70
Switches vs routers
both are store-and-forward
routers network-layer devices (examine network-layer headers)
switches link-layer devices (examine link-layer headers)
both have forwarding tables
routers compute tables using routing algorithms IP addresses
switches learn forwarding table using flooding learning MAC addresses
application
transport
network
link
physical
network
link
physical
link
physical
switch
datagram
application
transport
network
link
physical
frame
frame
frame
datagram
Link Layer 5-71
VLANs motivation
consider CS user moves office to
EE but wants connect to CS switch
single broadcast domain
all layer-2 broadcast traffic (ARP DHCP unknown location of destination MAC address) must cross entire LAN
securityprivacy efficiency issues
Computer
Science Electrical
Engineering
Computer
Engineering
Link Layer 5-72
VLANs port-based VLAN switch ports
grouped (by switch management software) so that single physical switch helliphellip
switch(es) supporting
VLAN capabilities can
be configured to
define multiple virtual
LANS over single
physical LAN
infrastructure
Virtual Local
Area Network 1
8
9
16 10 2
7
hellip
Electrical Engineering
(VLAN ports 1-8)
Computer Science
(VLAN ports 9-15)
15
hellip
Electrical Engineering
(VLAN ports 1-8)
hellip
1
8 2
7 9
16 10
15
hellip
Computer Science
(VLAN ports 9-16)
hellip operates as multiple virtual switches
Link Layer 5-73
Port-based VLAN
1
8
9
16 10 2
7
hellip
Electrical Engineering
(VLAN ports 1-8)
Computer Science
(VLAN ports 9-15)
15
hellip
traffic isolation frames tofrom ports 1-8 can only reach ports 1-8 can also define VLAN based on
MAC addresses of endpoints rather than switch port
dynamic membership ports can be dynamically assigned among VLANs
router
forwarding between VLANS done via routing (just as with separate switches) in practice vendors sell combined
switches plus routers
Link Layer 5-74
VLANS spanning multiple switches
trunk port carries frames between VLANS defined over multiple physical switches frames forwarded within VLAN between switches canrsquot be vanilla
8021 frames (must carry VLAN ID info)
8021q protocol addsremoved additional header fields for frames forwarded between trunk ports
1
8
9
10 2
7
hellip
Electrical Engineering
(VLAN ports 1-8)
Computer Science
(VLAN ports 9-15)
15
hellip
2
7 3
Ports 235 belong to EE VLAN
Ports 4678 belong to CS VLAN
5
4 6 8 16
1
Link Layer 5-75
type
2-byte Tag Protocol Identifier
(value 81-00)
Tag Control Information (12 bit VLAN ID field
3 bit priority field like IP TOS)
Recomputed CRC
8021Q VLAN frame format
8021 frame
8021Q frame
dest address
source address
data (payload) CRC preamble
dest address
source address
preamble data (payload) CRC
type
Link Layer 5-76
Link layer LANs outline
51 introduction services
52 error detection correction
53 multiple access protocols
54 LANs addressing ARP
Ethernet
switches
VLANS
55 link virtualization MPLS
56 data center networking
57 a day in the life of a web request
Link Layer 5-77
Multiprotocol label switching (MPLS)
initial goal high-speed IP forwarding using fixed length label (instead of IP address) fast lookup using fixed length identifier (rather than
shortest prefix matching)
borrowing ideas from Virtual Circuit (VC) approach
but IP datagram still keeps IP address
PPP or Ethernet
header IP header remainder of link-layer frame MPLS header
label Exp S TTL
20 3 1 5
Link Layer 5-78
MPLS capable routers
aka label-switched router
forward packets to outgoing interface based only on label value (donrsquot inspect IP address) MPLS forwarding table distinct from IP forwarding tables
flexibility MPLS forwarding decisions can differ from those of IP use destination and source addresses to route flows to
same destination differently (traffic engineering)
re-route flows quickly if link fails pre-computed backup paths (useful for VoIP)
Link Layer 5-79
R2
D
R3 R5
A
R6
MPLS versus IP paths
IP router IP routing path to destination determined
by destination address alone
R4
Link Layer 5-80
R2
D
R3 R4
R5
A
R6
MPLS versus IP paths
IP-only router
IP routing path to destination determined by destination address alone
MPLS and IP router
MPLS routing path to destination can be based on source and dest address fast reroute precompute backup routes in
case of link failure
entry router (R4) can use different MPLS
routes to A based eg on source address
Link Layer 5-81
MPLS signaling
modify OSPF IS-IS link-state flooding protocols to carry info used by MPLS routing eg link bandwidth amount of ldquoreservedrdquo link bandwidth
D
R4
R5
A
R6
entry MPLS router uses RSVP-TE signaling protocol to set up MPLS forwarding at downstream routers
modified link state flooding
RSVP-TE
Link Layer 5-82
R1 R2
D
R3 R4 R5
0
1
0 0
A
R6
in out out
label label dest interface
6 - A 0
in out out
label label dest interface
10 6 A 1
12 9 D 0
in out out
label label dest interface
10 A 0
12 D 0
1
in out out
label label dest interface
8 6 A 0
0
8 A 1
MPLS forwarding tables
Link Layer 5-83
Link layer LANs outline
51 introduction services
52 error detection correction
53 multiple access protocols
54 LANs addressing ARP
Ethernet
switches
VLANS
55 link virtualization MPLS
56 data center networking
57 a day in the life of a web request
Link Layer 5-84
Data center networks
10rsquos to 100rsquos of thousands of hosts often closely coupled in close proximity e-business (eg Amazon)
content-servers (eg YouTube Akamai Apple Microsoft)
search engines data mining (eg Google)
challenges
multiple applications each serving massive numbers of clients
managingbalancing load avoiding processing networking data bottlenecks
Inside a 40-ft Microsoft container
Chicago data center
Link Layer 5-85
Server racks
TOR switches
Tier-1 switches
Tier-2 switches
Load balancer
Load balancer
B
1 2 3 4 5 6 7 8
A C
Border router
Access router
Internet
Data center networks
load balancer application-layer routing receives external client requests
directs workload within data center
returns results to external client (hiding data
center internals from client)
Server racks
TOR switches
Tier-1 switches
Tier-2 switches
1 2 3 4 5 6 7 8
Data center networks
rich interconnection among switches racks
increased throughput between racks (multiple routing
paths possible)
increased reliability via redundancy
Link Layer 5-87
Link layer LANs outline
51 introduction services
52 error detection correction
53 multiple access protocols
54 LANs addressing ARP
Ethernet
switches
VLANS
55 link virtualization MPLS
56 data center networking
57 a day in the life of a web request
Link Layer 5-88
Synthesis a day in the life of a web request
journey down protocol stack complete application transport network link
putting-it-all-together synthesis goal identify review understand protocols (at all
layers) involved in seemingly simple scenario requesting www page
scenario student attaches laptop to campus network requestsreceives wwwgooglecom
Link Layer 5-89
A day in the life scenario
Comcast network
68800013
Googlersquos network
64233160019 64233169105
web server
DNS server
school network
68802024
web page
browser
router
(runs DHCP)
Link Layer 5-90
A day in the lifehellip connecting to the Internet
connecting laptop needs to get its own IP address addr of first-hop router addr of DNS server use DHCP
DHCP
UDP
IP
Eth
Phy
DHCP
DHCP
DHCP
DHCP
DHCP
DHCP
UDP
IP
Eth
Phy
DHCP
DHCP
DHCP
DHCP DHCP
DHCP request encapsulated in UDP encapsulated in IP encapsulated in 8023 Ethernet
Ethernet frame broadcast
(dest FFFFFFFFFFFF) on LAN received at router running DHCP server
Ethernet demuxed to IP demuxed UDP demuxed to DHCP
router
(runs DHCP)
Link Layer 5-91
DHCP server formulates DHCP ACK containing clientrsquos IP address IP address of first-hop router for client name amp IP address of DNS server
DHCP
UDP
IP
Eth
Phy
DHCP
DHCP
DHCP
DHCP
DHCP
UDP
IP
Eth
Phy
DHCP
DHCP
DHCP
DHCP
DHCP
encapsulation at DHCP server frame forwarded (switch learning) through LAN demultiplexing at client
Client now has IP address knows name amp addr of DNS
server IP address of its first-hop router
DHCP client receives DHCP ACK reply
A day in the lifehellip connecting to the Internet
router
(runs DHCP)
Link Layer 5-92
A day in the lifehellip ARP (before DNS before HTTP)
before sending HTTP request need IP address of wwwgooglecom DNS
DNS
UDP
IP
Eth
Phy
DNS
DNS
DNS
DNS query created encapsulated in UDP encapsulated in IP encapsulated in Eth To send frame to router need MAC address of router interface ARP
ARP query broadcast received by
router which replies with ARP reply giving MAC address of router interface
client now knows MAC address of first hop router so can now send frame containing DNS query
ARP query
Eth
Phy
ARP
ARP
ARP reply
router
(runs DHCP)
Link Layer 5-93
DNS
UDP
IP
Eth
Phy
DNS
DNS
DNS
DNS
DNS
IP datagram containing DNS query forwarded via LAN switch from client to 1st hop router
IP datagram forwarded from campus network into comcast network routed (tables created by RIP OSPF IS-IS andor BGP routing protocols) to DNS server
demuxrsquoed to DNS server
DNS server replies to client with IP address of wwwgooglecom
Comcast network
68800013
DNS server
DNS
UDP
IP
Eth
Phy
DNS
DNS
DNS
DNS
A day in the lifehellip using DNS
router
(runs DHCP)
Link Layer 5-94
A day in the lifehellipTCP connection carrying HTTP
HTTP
TCP
IP
Eth
Phy
HTTP
to send HTTP request client first opens TCP socket to web server
TCP SYN segment (step 1 in 3-way handshake) inter-domain routed to web server
TCP connection established 64233169105
web server
SYN
SYN
SYN
SYN
TCP
IP
Eth
Phy
SYN
SYN
SYN
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
web server responds with TCP SYNACK (step 2 in 3-way handshake)
router
(runs DHCP)
Link Layer 5-95
A day in the lifehellip HTTP requestreply
HTTP
TCP
IP
Eth
Phy
HTTP
HTTP request sent into TCP socket
IP datagram containing HTTP request routed to wwwgooglecom
IP datagram containing HTTP reply routed back to client
64233169105
web server
HTTP
TCP
IP
Eth
Phy web server responds with
HTTP reply (containing web page)
HTTP
HTTP
HTTP HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
web page finally () displayed
Link Layer 5-96
Chapter 5 Summary
principles behind data link layer services error detection correction
sharing a broadcast channel multiple access
link layer addressing
instantiation and implementation of various link layer technologies Ethernet
switched LANS VLANs
virtualized networks as a link layer MPLS
synthesis a day in the life of a web request
Link Layer 5-97
Chapter 5 letrsquos take a breath
journey down protocol stack complete (except PHY)
solid understanding of networking principles practice
hellip could stop here hellip but lots of interesting topics wireless
multimedia
security
network management
Link Layer 5-18
Multiple access protocols
single shared broadcast channel
two or more simultaneous transmissions by nodes interference
collision if node receives two or more signals at the same time
multiple access protocol distributed algorithm that determines how nodes share
channel ie determine when node can transmit
communication about channel sharing must use channel itself no out-of-band channel for coordination
Link Layer 5-19
An ideal multiple access protocol
given broadcast channel of rate R bps
desiderata 1 when one node wants to transmit it can send at rate R
2 when M nodes want to transmit each can send at average rate RM
3 fully decentralized
bull no special node to coordinate transmissions
bull no synchronization of clocks slots
4 simple
Link Layer 5-20
MAC protocols taxonomy
three broad classes
channel partitioning divide channel into smaller ldquopiecesrdquo (time slots frequency code)
allocate piece to node for exclusive use
random access channel not divided allow collisions
ldquorecoverrdquo from collisions
ldquotaking turnsrdquo nodes take turns but nodes with more to send can take longer
turns
Link Layer 5-21
Channel partitioning MAC protocols TDMA
TDMA time division multiple access access to channel in rounds each station gets fixed length slot (length = pkt
trans time) in each round unused slots go idle example 6-station LAN 134 have pkt slots
256 idle
1 3 4 1 3 4
6-slot
frame
6-slot
frame
Link Layer 5-22
FDMA frequency division multiple access channel spectrum divided into frequency bands
each station assigned fixed frequency band
unused transmission time in frequency bands go idle
example 6-station LAN 134 have pkt frequency bands 256 idle
frequency b
ands
FDM cable
Channel partitioning MAC protocols FDMA
Link Layer 5-23
Random access protocols
when node has packet to send transmit at full channel data rate R no a priori coordination among nodes
two or more transmitting nodes ldquocollisionrdquo random access MAC protocol specifies
how to detect collisions how to recover from collisions (eg via delayed
retransmissions)
examples of random access MAC protocols slotted ALOHA ALOHA CSMA CSMACD CSMACA
Link Layer 5-24
Slotted ALOHA
assumptions all frames same size
time divided into equal size slots (time to transmit 1 frame)
nodes start to transmit only slot beginning
nodes are synchronized
if 2 or more nodes transmit in slot all nodes detect collision
operation
when node obtains fresh frame transmits in next slot
if no collision node can send new frame in next slot
if collision node retransmits frame in each subsequent slot with prob p until success
Link Layer 5-25
Pros single active node can
continuously transmit at full rate of channel
highly decentralized only slots in nodes need to be in sync
simple
Cons
collisions wasting slots
idle slots
nodes may be able to detect collision in less than time to transmit packet
clock synchronization
Slotted ALOHA
1 1 1 1
2
3
2 2
3 3
node 1
node 2
node 3
C C C S S S E E E
Link Layer 5-26
suppose N nodes with many frames to send each transmits in slot with probability p
prob that given node has success in a slot = p(1-p)N-1
prob that any node has a success = Np(1-p)N-1
max efficiency find p that maximizes Np(1-p)N-1
for many nodes take limit of Np(1-p)N-1 as N goes to infinity gives
max efficiency = 1e = 37
efficiency long-run fraction of successful slots (many nodes all with many frames to send)
at best channel used for useful transmissions 37 of time
Slotted ALOHA efficiency
Link Layer 5-27
Pure (unslotted) ALOHA
unslotted Aloha simpler no synchronization
when frame first arrives
transmit immediately
collision probability increases
frame sent at t0 collides with other frames sent in [t0-1t0+1]
Link Layer 5-28
Pure ALOHA efficiency
P(success by given node) = P(node transmits)
P(no other node transmits in [t0-1t0]
P(no other node transmits in [t0-1t0]
= p (1-p)N-1 (1-p)N-1
= p (1-p)2(N-1)
hellip choosing optimum p and then letting n
= 1(2e) = 18
even worse than slotted Aloha
Link Layer 5-29
CSMA (carrier sense multiple access)
CSMA listen before transmit if channel sensed idle transmit entire frame
if channel sensed busy defer transmission
human analogy donrsquot interrupt others
Link Layer 5-30
CSMA collisions
collisions can still occur propagation delay means two nodes may not hear each otherrsquos transmission
collision entire packet transmission time wasted distance amp propagation
delay play role in in determining collision probability
spatial layout of nodes
Link Layer 5-31
CSMACD (collision detection)
CSMACD carrier sensing deferral as in CSMA collisions detected within short time
colliding transmissions aborted reducing channel wastage
collision detection easy in wired LANs measure signal strengths compare
transmitted received signals
difficult in wireless LANs received signal strength overwhelmed by local transmission strength
human analogy the polite conversationalist
Link Layer 5-32
CSMACD (collision detection)
spatial layout of nodes
Link Layer 5-33
Ethernet CSMACD algorithm
1 NIC receives datagram from network layer creates frame
2 If NIC senses channel idle starts frame transmission If NIC senses channel busy waits until channel idle then transmits
3 If NIC transmits entire frame without detecting another transmission NIC is done with frame
4 If NIC detects another transmission while transmitting aborts and sends jam signal
5 After aborting NIC enters binary (exponential) backoff
after mth collision NIC chooses K at random from 012 hellip 2m-1 NIC waits K512 bit times returns to Step 2
longer backoff interval with more collisions
Link Layer 5-34
CSMACD efficiency
Tprop = max prop delay between 2 nodes in LAN
ttrans = time to transmit max-size frame
efficiency goes to 1
as tprop goes to 0
as ttrans goes to infinity
better performance than ALOHA and simple cheap decentralized
transpropttefficiency
51
1
Link Layer 5-35
ldquoTaking turnsrdquo MAC protocols
channel partitioning MAC protocols share channel efficiently and fairly at high load
inefficient at low load delay in channel access 1N bandwidth allocated even if only 1 active node
random access MAC protocols efficient at low load single node can fully utilize
channel
high load collision overhead
ldquotaking turnsrdquo protocols look for best of both worlds
Link Layer 5-36
polling master node ldquoinvitesrdquo
slave nodes to transmit in turn
typically used with ldquodumbrdquo slave devices
concerns
polling overhead
latency
single point of failure (master)
master
slaves
poll
data
data
ldquoTaking turnsrdquo MAC protocols
Link Layer 5-37
token passing control token passed
from one node to next sequentially
token message
concerns
token overhead
latency
single point of failure (token)
T
data
(nothing
to send)
T
ldquoTaking turnsrdquo MAC protocols
cable headend
CMTS
ISP
cable modem termination system
multiple 40Mbps downstream (broadcast) channels
single CMTS transmits into channels
multiple 30 Mbps upstream channels
multiple access all users contend for certain upstream channel time slots (others assigned)
Cable access network
cable modem
splitter
hellip
hellip
Internet framesTV channels control transmitted downstream at different frequencies
upstream Internet frames TV control transmitted upstream at different frequencies in time slots
Link Layer 5-39
DOCSIS data over cable service interface spec FDM over upstream downstream frequency channels
TDM upstream some slots assigned some have contention
downstream MAP frame assigns upstream slots
request for upstream slots (and data) transmitted random access (binary backoff) in selected slots
MAP frame for
Interval [t1 t2]
Residences with cable modems
Downstream channel i
Upstream channel j
t1 t2
Assigned minislots containing cable modem
upstream data frames
Minislots containing
minislots request frames
cable headend
CMTS
Cable access network
Link Layer 5-40
Summary of MAC protocols
channel partitioning by time frequency or code Time Division Frequency Division
random access (dynamic)
ALOHA S-ALOHA CSMA CSMACD
carrier sensing easy in some technologies (wire) hard in others (wireless)
CSMACD used in Ethernet
CSMACA used in 80211
taking turns
polling from central site token passing
bluetooth FDDI token ring
Link Layer 5-41
Link layer LANs outline
51 introduction services
52 error detection correction
53 multiple access protocols
54 LANs addressing ARP
Ethernet
switches
VLANS
55 link virtualization MPLS
56 data center networking
57 a day in the life of a web request
Link Layer 5-42
MAC addresses and ARP
32-bit IP address network-layer address for interface
used for layer 3 (network layer) forwarding
MAC (or LAN or physical or Ethernet) address function used lsquolocallyrdquo to get frame from one interface to
another physically-connected interface (same network in IP-addressing sense)
48 bit MAC address (for most LANs) burned in NIC ROM also sometimes software settable
eg 1A-2F-BB-76-09-AD
hexadecimal (base 16) notation
(each ldquonumberrdquo represents 4 bits)
Link Layer 5-43
LAN addresses and ARP
each adapter on LAN has unique LAN address
adapter
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN
(wired or
wireless)
Link Layer 5-44
LAN addresses (more)
MAC address allocation administered by IEEE
manufacturer buys portion of MAC address space (to assure uniqueness)
analogy MAC address like Social Security Number
IP address like postal address
MAC flat address portability can move LAN card from one LAN to another
IP hierarchical address not portable address depends on IP subnet to which node is
attached
Link Layer 5-45
ARP address resolution protocol
ARP table each IP node (host router) on LAN has table
IPMAC address mappings for some LAN nodes
lt IP address MAC address TTLgt
TTL (Time To Live) time after which address mapping will be forgotten (typically 20 min)
Question how to determine
interfacersquos MAC address
knowing its IP address
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN
137196723
137196778
137196714
137196788
Link Layer 5-46
ARP protocol same LAN
A wants to send datagram to B Brsquos MAC address not in
Arsquos ARP table
A broadcasts ARP query packet containing Bs IP address dest MAC address = FF-FF-
FF-FF-FF-FF
all nodes on LAN receive ARP query
B receives ARP packet replies to A with its (Bs) MAC address frame sent to Arsquos MAC
address (unicast)
A caches (saves) IP-to-MAC address pair in its ARP table until information becomes old (times out) soft state information that
times out (goes away) unless refreshed
ARP is ldquoplug-and-playrdquo nodes create their ARP
tables without intervention from net administrator
Link Layer 5-47
walkthrough send datagram from A to B via R
focus on addressing ndash at IP (datagram) and MAC layer (frame)
assume A knows Brsquos IP address
assume A knows IP address of first hop router R (how)
assume A knows Rrsquos MAC address (how)
Addressing routing to another LAN
R
1A-23-F9-CD-06-9B 222222222220
111111111110 E6-E9-00-17-BB-4B CC-49-DE-D0-AB-7D
111111111112
111111111111
74-29-9C-E8-FF-55
A
222222222222
49-BD-D2-C7-56-2A
222222222221 88-B2-2F-54-1A-0F
B
R
1A-23-F9-CD-06-9B 222222222220
111111111110 E6-E9-00-17-BB-4B CC-49-DE-D0-AB-7D
111111111112
111111111111
74-29-9C-E8-FF-55
A
222222222222
49-BD-D2-C7-56-2A
222222222221 88-B2-2F-54-1A-0F
B
Link Layer 5-48
Addressing routing to another LAN
IP
Eth
Phy
IP src 111111111111
IP dest 222222222222
A creates IP datagram with IP source A destination B
A creates link-layer frame with Rs MAC address as dest frame contains A-to-B IP datagram
MAC src 74-29-9C-E8-FF-55
MAC dest E6-E9-00-17-BB-4B
R
1A-23-F9-CD-06-9B 222222222220
111111111110 E6-E9-00-17-BB-4B CC-49-DE-D0-AB-7D
111111111112
111111111111
74-29-9C-E8-FF-55
A
222222222222
49-BD-D2-C7-56-2A
222222222221 88-B2-2F-54-1A-0F
B
Link Layer 5-49
Addressing routing to another LAN
IP
Eth
Phy
frame sent from A to R
IP
Eth
Phy
frame received at R datagram removed passed up to IP
MAC src 74-29-9C-E8-FF-55
MAC dest E6-E9-00-17-BB-4B
IP src 111111111111
IP dest 222222222222
IP src 111111111111
IP dest 222222222222
R
1A-23-F9-CD-06-9B 222222222220
111111111110 E6-E9-00-17-BB-4B CC-49-DE-D0-AB-7D
111111111112
111111111111
74-29-9C-E8-FF-55
A
222222222222
49-BD-D2-C7-56-2A
222222222221 88-B2-2F-54-1A-0F
B
Link Layer 5-50
Addressing routing to another LAN
IP src 111111111111
IP dest 222222222222
R forwards datagram with IP source A destination B
R creates link-layer frame with Bs MAC address as dest frame contains A-to-B IP datagram
MAC src 1A-23-F9-CD-06-9B
MAC dest 49-BD-D2-C7-56-2A
IP
Eth
Phy
IP
Eth
Phy
R
1A-23-F9-CD-06-9B 222222222220
111111111110 E6-E9-00-17-BB-4B CC-49-DE-D0-AB-7D
111111111112
111111111111
74-29-9C-E8-FF-55
A
222222222222
49-BD-D2-C7-56-2A
222222222221 88-B2-2F-54-1A-0F
B
Link Layer 5-51
Addressing routing to another LAN R forwards datagram with IP source A destination B
R creates link-layer frame with Bs MAC address as dest frame contains A-to-B IP datagram
IP src 111111111111
IP dest 222222222222
MAC src 1A-23-F9-CD-06-9B
MAC dest 49-BD-D2-C7-56-2A
IP
Eth
Phy
IP
Eth
Phy
R
1A-23-F9-CD-06-9B 222222222220
111111111110 E6-E9-00-17-BB-4B CC-49-DE-D0-AB-7D
111111111112
111111111111
74-29-9C-E8-FF-55
A
222222222222
49-BD-D2-C7-56-2A
222222222221 88-B2-2F-54-1A-0F
B
Link Layer 5-52
Addressing routing to another LAN R forwards datagram with IP source A destination B
R creates link-layer frame with Bs MAC address as dest frame contains A-to-B IP datagram
IP src 111111111111
IP dest 222222222222
MAC src 1A-23-F9-CD-06-9B
MAC dest 49-BD-D2-C7-56-2A
IP
Eth
Phy
Link Layer 5-53
Link layer LANs outline
51 introduction services
52 error detection correction
53 multiple access protocols
54 LANs addressing ARP
Ethernet
switches
VLANS
55 link virtualization MPLS
56 data center networking
57 a day in the life of a web request
Link Layer 5-54
Ethernet
ldquodominantrdquo wired LAN technology
cheap $20 for NIC
first widely used LAN technology
simpler cheaper than token LANs and ATM
kept up with speed race 10 Mbps ndash 10 Gbps
Metcalfersquos Ethernet sketch
Link Layer 5-55
Ethernet physical topology
bus popular through mid 90s all nodes in same collision domain (can collide with each
other)
star prevails today active switch in center
each ldquospokerdquo runs a (separate) Ethernet protocol (nodes
do not collide with each other)
switch
bus coaxial cable star
Link Layer 5-56
Ethernet frame structure
sending adapter encapsulates IP datagram (or other network layer protocol packet) in Ethernet frame
preamble
7 bytes with pattern 10101010 followed by one byte with pattern 10101011
used to synchronize receiver sender clock rates
dest address
source address
data (payload) CRC preamble
type
Link Layer 5-57
Ethernet frame structure (more)
addresses 6 byte source destination MAC addresses if adapter receives frame with matching destination
address or with broadcast address (eg ARP packet) it passes data in frame to network layer protocol
otherwise adapter discards frame
type indicates higher layer protocol (mostly IP but others possible eg Novell IPX AppleTalk)
CRC cyclic redundancy check at receiver error detected frame is dropped
dest address
source address
data (payload) CRC preamble
type
Link Layer 5-58
Ethernet unreliable connectionless
connectionless no handshaking between sending and receiving NICs
unreliable receiving NIC doesnt send acks or nacks to sending NIC
data in dropped frames recovered only if initial sender uses higher layer rdt (eg TCP) otherwise dropped data lost
Ethernetrsquos MAC protocol unslotted CSMACD wth binary backoff
Link Layer 5-59
8023 Ethernet standards link amp physical layers
many different Ethernet standards
common MAC protocol and frame format
different speeds 2 Mbps 10 Mbps 100 Mbps 1Gbps 10G bps
different physical layer media fiber cable
application
transport
network
link
physical
MAC protocol
and frame format
100BASE-TX
100BASE-T4
100BASE-FX 100BASE-T2
100BASE-SX 100BASE-BX
fiber physical layer copper (twister
pair) physical layer
Link Layer 5-60
Link layer LANs outline
51 introduction services
52 error detection correction
53 multiple access protocols
54 LANs addressing ARP
Ethernet
switches
VLANS
55 link virtualization MPLS
56 data center networking
57 a day in the life of a web request
Link Layer 5-61
Ethernet switch link-layer device takes an active role
store forward Ethernet frames
examine incoming framersquos MAC address selectively forward frame to one-or-more outgoing links when frame is to be forwarded on segment uses CSMACD to access segment
transparent
hosts are unaware of presence of switches
plug-and-play self-learning
switches do not need to be configured
Link Layer 5-62
Switch multiple simultaneous transmissions
hosts have dedicated direct connection to switch
switches buffer packets
Ethernet protocol used on each incoming link but no collisions full duplex
each link is its own collision domain
switching A-to-Arsquo and B-to-Brsquo can transmit simultaneously without collisions switch with six interfaces
(123456)
A
Arsquo
B
Brsquo C
Crsquo
1 2
3 4 5
6
Link Layer 5-63
Switch forwarding table
Q how does switch know Arsquo reachable via interface 4 Brsquo reachable via interface 5
switch with six interfaces
(123456)
A
Arsquo
B
Brsquo C
Crsquo
1 2
3 4 5
6 A each switch has a switch table each entry
(MAC address of host interface to
reach host time stamp)
looks like a routing table
Q how are entries created maintained in switch table
something like a routing protocol
A
Arsquo
B
Brsquo C
Crsquo
1 2
3 4 5
6
Link Layer 5-64
Switch self-learning
switch learns which hosts can be reached through which interfaces
when frame received switch ldquolearnsrdquo location of sender incoming LAN segment
records senderlocation pair in switch table
A Arsquo
Source A
Dest Arsquo
MAC addr interface TTL
Switch table
(initially empty) A 1 60
Link Layer 5-65
Switch frame filteringforwarding
when frame received at switch 1 record incoming link MAC address of sending host
2 index switch table using MAC destination address
3 if entry found for destination then
if destination on segment from which frame arrived then drop frame
else forward frame on interface indicated by entry
else flood forward on all interfaces except arriving
interface
A
Arsquo
B
Brsquo C
Crsquo
1 2
3 4 5
6
Link Layer 5-66
Self-learning forwarding example
A Arsquo
Source A
Dest Arsquo
MAC addr interface TTL
switch table
(initially empty) A 1 60
A Arsquo A Arsquo A Arsquo A Arsquo A Arsquo
frame destination Arsquo locaton unknown flood
Arsquo A
destination A location
known
Arsquo 4 60
selectively send
on just one link
Link Layer 5-67
Interconnecting switches
switches can be connected together
Q sending from A to G - how does S1 know to forward frame destined to F via S4 and S3
A self learning (works exactly the same as in single-switch case)
A
B
S1
C D
E
F
S2
S4
S3
H
I
G
Link Layer 5-68
Self-learning multi-switch example
Suppose C sends frame to I I responds to C
Q show switch tables and packet forwarding in S1 S2 S3 S4
A
B
S1
C D
E
F
S2
S4
S3
H
I
G
Link Layer 5-69
Institutional network
to external
network
router
IP subnet
mail server
web server
Link Layer 5-70
Switches vs routers
both are store-and-forward
routers network-layer devices (examine network-layer headers)
switches link-layer devices (examine link-layer headers)
both have forwarding tables
routers compute tables using routing algorithms IP addresses
switches learn forwarding table using flooding learning MAC addresses
application
transport
network
link
physical
network
link
physical
link
physical
switch
datagram
application
transport
network
link
physical
frame
frame
frame
datagram
Link Layer 5-71
VLANs motivation
consider CS user moves office to
EE but wants connect to CS switch
single broadcast domain
all layer-2 broadcast traffic (ARP DHCP unknown location of destination MAC address) must cross entire LAN
securityprivacy efficiency issues
Computer
Science Electrical
Engineering
Computer
Engineering
Link Layer 5-72
VLANs port-based VLAN switch ports
grouped (by switch management software) so that single physical switch helliphellip
switch(es) supporting
VLAN capabilities can
be configured to
define multiple virtual
LANS over single
physical LAN
infrastructure
Virtual Local
Area Network 1
8
9
16 10 2
7
hellip
Electrical Engineering
(VLAN ports 1-8)
Computer Science
(VLAN ports 9-15)
15
hellip
Electrical Engineering
(VLAN ports 1-8)
hellip
1
8 2
7 9
16 10
15
hellip
Computer Science
(VLAN ports 9-16)
hellip operates as multiple virtual switches
Link Layer 5-73
Port-based VLAN
1
8
9
16 10 2
7
hellip
Electrical Engineering
(VLAN ports 1-8)
Computer Science
(VLAN ports 9-15)
15
hellip
traffic isolation frames tofrom ports 1-8 can only reach ports 1-8 can also define VLAN based on
MAC addresses of endpoints rather than switch port
dynamic membership ports can be dynamically assigned among VLANs
router
forwarding between VLANS done via routing (just as with separate switches) in practice vendors sell combined
switches plus routers
Link Layer 5-74
VLANS spanning multiple switches
trunk port carries frames between VLANS defined over multiple physical switches frames forwarded within VLAN between switches canrsquot be vanilla
8021 frames (must carry VLAN ID info)
8021q protocol addsremoved additional header fields for frames forwarded between trunk ports
1
8
9
10 2
7
hellip
Electrical Engineering
(VLAN ports 1-8)
Computer Science
(VLAN ports 9-15)
15
hellip
2
7 3
Ports 235 belong to EE VLAN
Ports 4678 belong to CS VLAN
5
4 6 8 16
1
Link Layer 5-75
type
2-byte Tag Protocol Identifier
(value 81-00)
Tag Control Information (12 bit VLAN ID field
3 bit priority field like IP TOS)
Recomputed CRC
8021Q VLAN frame format
8021 frame
8021Q frame
dest address
source address
data (payload) CRC preamble
dest address
source address
preamble data (payload) CRC
type
Link Layer 5-76
Link layer LANs outline
51 introduction services
52 error detection correction
53 multiple access protocols
54 LANs addressing ARP
Ethernet
switches
VLANS
55 link virtualization MPLS
56 data center networking
57 a day in the life of a web request
Link Layer 5-77
Multiprotocol label switching (MPLS)
initial goal high-speed IP forwarding using fixed length label (instead of IP address) fast lookup using fixed length identifier (rather than
shortest prefix matching)
borrowing ideas from Virtual Circuit (VC) approach
but IP datagram still keeps IP address
PPP or Ethernet
header IP header remainder of link-layer frame MPLS header
label Exp S TTL
20 3 1 5
Link Layer 5-78
MPLS capable routers
aka label-switched router
forward packets to outgoing interface based only on label value (donrsquot inspect IP address) MPLS forwarding table distinct from IP forwarding tables
flexibility MPLS forwarding decisions can differ from those of IP use destination and source addresses to route flows to
same destination differently (traffic engineering)
re-route flows quickly if link fails pre-computed backup paths (useful for VoIP)
Link Layer 5-79
R2
D
R3 R5
A
R6
MPLS versus IP paths
IP router IP routing path to destination determined
by destination address alone
R4
Link Layer 5-80
R2
D
R3 R4
R5
A
R6
MPLS versus IP paths
IP-only router
IP routing path to destination determined by destination address alone
MPLS and IP router
MPLS routing path to destination can be based on source and dest address fast reroute precompute backup routes in
case of link failure
entry router (R4) can use different MPLS
routes to A based eg on source address
Link Layer 5-81
MPLS signaling
modify OSPF IS-IS link-state flooding protocols to carry info used by MPLS routing eg link bandwidth amount of ldquoreservedrdquo link bandwidth
D
R4
R5
A
R6
entry MPLS router uses RSVP-TE signaling protocol to set up MPLS forwarding at downstream routers
modified link state flooding
RSVP-TE
Link Layer 5-82
R1 R2
D
R3 R4 R5
0
1
0 0
A
R6
in out out
label label dest interface
6 - A 0
in out out
label label dest interface
10 6 A 1
12 9 D 0
in out out
label label dest interface
10 A 0
12 D 0
1
in out out
label label dest interface
8 6 A 0
0
8 A 1
MPLS forwarding tables
Link Layer 5-83
Link layer LANs outline
51 introduction services
52 error detection correction
53 multiple access protocols
54 LANs addressing ARP
Ethernet
switches
VLANS
55 link virtualization MPLS
56 data center networking
57 a day in the life of a web request
Link Layer 5-84
Data center networks
10rsquos to 100rsquos of thousands of hosts often closely coupled in close proximity e-business (eg Amazon)
content-servers (eg YouTube Akamai Apple Microsoft)
search engines data mining (eg Google)
challenges
multiple applications each serving massive numbers of clients
managingbalancing load avoiding processing networking data bottlenecks
Inside a 40-ft Microsoft container
Chicago data center
Link Layer 5-85
Server racks
TOR switches
Tier-1 switches
Tier-2 switches
Load balancer
Load balancer
B
1 2 3 4 5 6 7 8
A C
Border router
Access router
Internet
Data center networks
load balancer application-layer routing receives external client requests
directs workload within data center
returns results to external client (hiding data
center internals from client)
Server racks
TOR switches
Tier-1 switches
Tier-2 switches
1 2 3 4 5 6 7 8
Data center networks
rich interconnection among switches racks
increased throughput between racks (multiple routing
paths possible)
increased reliability via redundancy
Link Layer 5-87
Link layer LANs outline
51 introduction services
52 error detection correction
53 multiple access protocols
54 LANs addressing ARP
Ethernet
switches
VLANS
55 link virtualization MPLS
56 data center networking
57 a day in the life of a web request
Link Layer 5-88
Synthesis a day in the life of a web request
journey down protocol stack complete application transport network link
putting-it-all-together synthesis goal identify review understand protocols (at all
layers) involved in seemingly simple scenario requesting www page
scenario student attaches laptop to campus network requestsreceives wwwgooglecom
Link Layer 5-89
A day in the life scenario
Comcast network
68800013
Googlersquos network
64233160019 64233169105
web server
DNS server
school network
68802024
web page
browser
router
(runs DHCP)
Link Layer 5-90
A day in the lifehellip connecting to the Internet
connecting laptop needs to get its own IP address addr of first-hop router addr of DNS server use DHCP
DHCP
UDP
IP
Eth
Phy
DHCP
DHCP
DHCP
DHCP
DHCP
DHCP
UDP
IP
Eth
Phy
DHCP
DHCP
DHCP
DHCP DHCP
DHCP request encapsulated in UDP encapsulated in IP encapsulated in 8023 Ethernet
Ethernet frame broadcast
(dest FFFFFFFFFFFF) on LAN received at router running DHCP server
Ethernet demuxed to IP demuxed UDP demuxed to DHCP
router
(runs DHCP)
Link Layer 5-91
DHCP server formulates DHCP ACK containing clientrsquos IP address IP address of first-hop router for client name amp IP address of DNS server
DHCP
UDP
IP
Eth
Phy
DHCP
DHCP
DHCP
DHCP
DHCP
UDP
IP
Eth
Phy
DHCP
DHCP
DHCP
DHCP
DHCP
encapsulation at DHCP server frame forwarded (switch learning) through LAN demultiplexing at client
Client now has IP address knows name amp addr of DNS
server IP address of its first-hop router
DHCP client receives DHCP ACK reply
A day in the lifehellip connecting to the Internet
router
(runs DHCP)
Link Layer 5-92
A day in the lifehellip ARP (before DNS before HTTP)
before sending HTTP request need IP address of wwwgooglecom DNS
DNS
UDP
IP
Eth
Phy
DNS
DNS
DNS
DNS query created encapsulated in UDP encapsulated in IP encapsulated in Eth To send frame to router need MAC address of router interface ARP
ARP query broadcast received by
router which replies with ARP reply giving MAC address of router interface
client now knows MAC address of first hop router so can now send frame containing DNS query
ARP query
Eth
Phy
ARP
ARP
ARP reply
router
(runs DHCP)
Link Layer 5-93
DNS
UDP
IP
Eth
Phy
DNS
DNS
DNS
DNS
DNS
IP datagram containing DNS query forwarded via LAN switch from client to 1st hop router
IP datagram forwarded from campus network into comcast network routed (tables created by RIP OSPF IS-IS andor BGP routing protocols) to DNS server
demuxrsquoed to DNS server
DNS server replies to client with IP address of wwwgooglecom
Comcast network
68800013
DNS server
DNS
UDP
IP
Eth
Phy
DNS
DNS
DNS
DNS
A day in the lifehellip using DNS
router
(runs DHCP)
Link Layer 5-94
A day in the lifehellipTCP connection carrying HTTP
HTTP
TCP
IP
Eth
Phy
HTTP
to send HTTP request client first opens TCP socket to web server
TCP SYN segment (step 1 in 3-way handshake) inter-domain routed to web server
TCP connection established 64233169105
web server
SYN
SYN
SYN
SYN
TCP
IP
Eth
Phy
SYN
SYN
SYN
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
web server responds with TCP SYNACK (step 2 in 3-way handshake)
router
(runs DHCP)
Link Layer 5-95
A day in the lifehellip HTTP requestreply
HTTP
TCP
IP
Eth
Phy
HTTP
HTTP request sent into TCP socket
IP datagram containing HTTP request routed to wwwgooglecom
IP datagram containing HTTP reply routed back to client
64233169105
web server
HTTP
TCP
IP
Eth
Phy web server responds with
HTTP reply (containing web page)
HTTP
HTTP
HTTP HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
web page finally () displayed
Link Layer 5-96
Chapter 5 Summary
principles behind data link layer services error detection correction
sharing a broadcast channel multiple access
link layer addressing
instantiation and implementation of various link layer technologies Ethernet
switched LANS VLANs
virtualized networks as a link layer MPLS
synthesis a day in the life of a web request
Link Layer 5-97
Chapter 5 letrsquos take a breath
journey down protocol stack complete (except PHY)
solid understanding of networking principles practice
hellip could stop here hellip but lots of interesting topics wireless
multimedia
security
network management
Link Layer 5-19
An ideal multiple access protocol
given broadcast channel of rate R bps
desiderata 1 when one node wants to transmit it can send at rate R
2 when M nodes want to transmit each can send at average rate RM
3 fully decentralized
bull no special node to coordinate transmissions
bull no synchronization of clocks slots
4 simple
Link Layer 5-20
MAC protocols taxonomy
three broad classes
channel partitioning divide channel into smaller ldquopiecesrdquo (time slots frequency code)
allocate piece to node for exclusive use
random access channel not divided allow collisions
ldquorecoverrdquo from collisions
ldquotaking turnsrdquo nodes take turns but nodes with more to send can take longer
turns
Link Layer 5-21
Channel partitioning MAC protocols TDMA
TDMA time division multiple access access to channel in rounds each station gets fixed length slot (length = pkt
trans time) in each round unused slots go idle example 6-station LAN 134 have pkt slots
256 idle
1 3 4 1 3 4
6-slot
frame
6-slot
frame
Link Layer 5-22
FDMA frequency division multiple access channel spectrum divided into frequency bands
each station assigned fixed frequency band
unused transmission time in frequency bands go idle
example 6-station LAN 134 have pkt frequency bands 256 idle
frequency b
ands
FDM cable
Channel partitioning MAC protocols FDMA
Link Layer 5-23
Random access protocols
when node has packet to send transmit at full channel data rate R no a priori coordination among nodes
two or more transmitting nodes ldquocollisionrdquo random access MAC protocol specifies
how to detect collisions how to recover from collisions (eg via delayed
retransmissions)
examples of random access MAC protocols slotted ALOHA ALOHA CSMA CSMACD CSMACA
Link Layer 5-24
Slotted ALOHA
assumptions all frames same size
time divided into equal size slots (time to transmit 1 frame)
nodes start to transmit only slot beginning
nodes are synchronized
if 2 or more nodes transmit in slot all nodes detect collision
operation
when node obtains fresh frame transmits in next slot
if no collision node can send new frame in next slot
if collision node retransmits frame in each subsequent slot with prob p until success
Link Layer 5-25
Pros single active node can
continuously transmit at full rate of channel
highly decentralized only slots in nodes need to be in sync
simple
Cons
collisions wasting slots
idle slots
nodes may be able to detect collision in less than time to transmit packet
clock synchronization
Slotted ALOHA
1 1 1 1
2
3
2 2
3 3
node 1
node 2
node 3
C C C S S S E E E
Link Layer 5-26
suppose N nodes with many frames to send each transmits in slot with probability p
prob that given node has success in a slot = p(1-p)N-1
prob that any node has a success = Np(1-p)N-1
max efficiency find p that maximizes Np(1-p)N-1
for many nodes take limit of Np(1-p)N-1 as N goes to infinity gives
max efficiency = 1e = 37
efficiency long-run fraction of successful slots (many nodes all with many frames to send)
at best channel used for useful transmissions 37 of time
Slotted ALOHA efficiency
Link Layer 5-27
Pure (unslotted) ALOHA
unslotted Aloha simpler no synchronization
when frame first arrives
transmit immediately
collision probability increases
frame sent at t0 collides with other frames sent in [t0-1t0+1]
Link Layer 5-28
Pure ALOHA efficiency
P(success by given node) = P(node transmits)
P(no other node transmits in [t0-1t0]
P(no other node transmits in [t0-1t0]
= p (1-p)N-1 (1-p)N-1
= p (1-p)2(N-1)
hellip choosing optimum p and then letting n
= 1(2e) = 18
even worse than slotted Aloha
Link Layer 5-29
CSMA (carrier sense multiple access)
CSMA listen before transmit if channel sensed idle transmit entire frame
if channel sensed busy defer transmission
human analogy donrsquot interrupt others
Link Layer 5-30
CSMA collisions
collisions can still occur propagation delay means two nodes may not hear each otherrsquos transmission
collision entire packet transmission time wasted distance amp propagation
delay play role in in determining collision probability
spatial layout of nodes
Link Layer 5-31
CSMACD (collision detection)
CSMACD carrier sensing deferral as in CSMA collisions detected within short time
colliding transmissions aborted reducing channel wastage
collision detection easy in wired LANs measure signal strengths compare
transmitted received signals
difficult in wireless LANs received signal strength overwhelmed by local transmission strength
human analogy the polite conversationalist
Link Layer 5-32
CSMACD (collision detection)
spatial layout of nodes
Link Layer 5-33
Ethernet CSMACD algorithm
1 NIC receives datagram from network layer creates frame
2 If NIC senses channel idle starts frame transmission If NIC senses channel busy waits until channel idle then transmits
3 If NIC transmits entire frame without detecting another transmission NIC is done with frame
4 If NIC detects another transmission while transmitting aborts and sends jam signal
5 After aborting NIC enters binary (exponential) backoff
after mth collision NIC chooses K at random from 012 hellip 2m-1 NIC waits K512 bit times returns to Step 2
longer backoff interval with more collisions
Link Layer 5-34
CSMACD efficiency
Tprop = max prop delay between 2 nodes in LAN
ttrans = time to transmit max-size frame
efficiency goes to 1
as tprop goes to 0
as ttrans goes to infinity
better performance than ALOHA and simple cheap decentralized
transpropttefficiency
51
1
Link Layer 5-35
ldquoTaking turnsrdquo MAC protocols
channel partitioning MAC protocols share channel efficiently and fairly at high load
inefficient at low load delay in channel access 1N bandwidth allocated even if only 1 active node
random access MAC protocols efficient at low load single node can fully utilize
channel
high load collision overhead
ldquotaking turnsrdquo protocols look for best of both worlds
Link Layer 5-36
polling master node ldquoinvitesrdquo
slave nodes to transmit in turn
typically used with ldquodumbrdquo slave devices
concerns
polling overhead
latency
single point of failure (master)
master
slaves
poll
data
data
ldquoTaking turnsrdquo MAC protocols
Link Layer 5-37
token passing control token passed
from one node to next sequentially
token message
concerns
token overhead
latency
single point of failure (token)
T
data
(nothing
to send)
T
ldquoTaking turnsrdquo MAC protocols
cable headend
CMTS
ISP
cable modem termination system
multiple 40Mbps downstream (broadcast) channels
single CMTS transmits into channels
multiple 30 Mbps upstream channels
multiple access all users contend for certain upstream channel time slots (others assigned)
Cable access network
cable modem
splitter
hellip
hellip
Internet framesTV channels control transmitted downstream at different frequencies
upstream Internet frames TV control transmitted upstream at different frequencies in time slots
Link Layer 5-39
DOCSIS data over cable service interface spec FDM over upstream downstream frequency channels
TDM upstream some slots assigned some have contention
downstream MAP frame assigns upstream slots
request for upstream slots (and data) transmitted random access (binary backoff) in selected slots
MAP frame for
Interval [t1 t2]
Residences with cable modems
Downstream channel i
Upstream channel j
t1 t2
Assigned minislots containing cable modem
upstream data frames
Minislots containing
minislots request frames
cable headend
CMTS
Cable access network
Link Layer 5-40
Summary of MAC protocols
channel partitioning by time frequency or code Time Division Frequency Division
random access (dynamic)
ALOHA S-ALOHA CSMA CSMACD
carrier sensing easy in some technologies (wire) hard in others (wireless)
CSMACD used in Ethernet
CSMACA used in 80211
taking turns
polling from central site token passing
bluetooth FDDI token ring
Link Layer 5-41
Link layer LANs outline
51 introduction services
52 error detection correction
53 multiple access protocols
54 LANs addressing ARP
Ethernet
switches
VLANS
55 link virtualization MPLS
56 data center networking
57 a day in the life of a web request
Link Layer 5-42
MAC addresses and ARP
32-bit IP address network-layer address for interface
used for layer 3 (network layer) forwarding
MAC (or LAN or physical or Ethernet) address function used lsquolocallyrdquo to get frame from one interface to
another physically-connected interface (same network in IP-addressing sense)
48 bit MAC address (for most LANs) burned in NIC ROM also sometimes software settable
eg 1A-2F-BB-76-09-AD
hexadecimal (base 16) notation
(each ldquonumberrdquo represents 4 bits)
Link Layer 5-43
LAN addresses and ARP
each adapter on LAN has unique LAN address
adapter
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN
(wired or
wireless)
Link Layer 5-44
LAN addresses (more)
MAC address allocation administered by IEEE
manufacturer buys portion of MAC address space (to assure uniqueness)
analogy MAC address like Social Security Number
IP address like postal address
MAC flat address portability can move LAN card from one LAN to another
IP hierarchical address not portable address depends on IP subnet to which node is
attached
Link Layer 5-45
ARP address resolution protocol
ARP table each IP node (host router) on LAN has table
IPMAC address mappings for some LAN nodes
lt IP address MAC address TTLgt
TTL (Time To Live) time after which address mapping will be forgotten (typically 20 min)
Question how to determine
interfacersquos MAC address
knowing its IP address
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN
137196723
137196778
137196714
137196788
Link Layer 5-46
ARP protocol same LAN
A wants to send datagram to B Brsquos MAC address not in
Arsquos ARP table
A broadcasts ARP query packet containing Bs IP address dest MAC address = FF-FF-
FF-FF-FF-FF
all nodes on LAN receive ARP query
B receives ARP packet replies to A with its (Bs) MAC address frame sent to Arsquos MAC
address (unicast)
A caches (saves) IP-to-MAC address pair in its ARP table until information becomes old (times out) soft state information that
times out (goes away) unless refreshed
ARP is ldquoplug-and-playrdquo nodes create their ARP
tables without intervention from net administrator
Link Layer 5-47
walkthrough send datagram from A to B via R
focus on addressing ndash at IP (datagram) and MAC layer (frame)
assume A knows Brsquos IP address
assume A knows IP address of first hop router R (how)
assume A knows Rrsquos MAC address (how)
Addressing routing to another LAN
R
1A-23-F9-CD-06-9B 222222222220
111111111110 E6-E9-00-17-BB-4B CC-49-DE-D0-AB-7D
111111111112
111111111111
74-29-9C-E8-FF-55
A
222222222222
49-BD-D2-C7-56-2A
222222222221 88-B2-2F-54-1A-0F
B
R
1A-23-F9-CD-06-9B 222222222220
111111111110 E6-E9-00-17-BB-4B CC-49-DE-D0-AB-7D
111111111112
111111111111
74-29-9C-E8-FF-55
A
222222222222
49-BD-D2-C7-56-2A
222222222221 88-B2-2F-54-1A-0F
B
Link Layer 5-48
Addressing routing to another LAN
IP
Eth
Phy
IP src 111111111111
IP dest 222222222222
A creates IP datagram with IP source A destination B
A creates link-layer frame with Rs MAC address as dest frame contains A-to-B IP datagram
MAC src 74-29-9C-E8-FF-55
MAC dest E6-E9-00-17-BB-4B
R
1A-23-F9-CD-06-9B 222222222220
111111111110 E6-E9-00-17-BB-4B CC-49-DE-D0-AB-7D
111111111112
111111111111
74-29-9C-E8-FF-55
A
222222222222
49-BD-D2-C7-56-2A
222222222221 88-B2-2F-54-1A-0F
B
Link Layer 5-49
Addressing routing to another LAN
IP
Eth
Phy
frame sent from A to R
IP
Eth
Phy
frame received at R datagram removed passed up to IP
MAC src 74-29-9C-E8-FF-55
MAC dest E6-E9-00-17-BB-4B
IP src 111111111111
IP dest 222222222222
IP src 111111111111
IP dest 222222222222
R
1A-23-F9-CD-06-9B 222222222220
111111111110 E6-E9-00-17-BB-4B CC-49-DE-D0-AB-7D
111111111112
111111111111
74-29-9C-E8-FF-55
A
222222222222
49-BD-D2-C7-56-2A
222222222221 88-B2-2F-54-1A-0F
B
Link Layer 5-50
Addressing routing to another LAN
IP src 111111111111
IP dest 222222222222
R forwards datagram with IP source A destination B
R creates link-layer frame with Bs MAC address as dest frame contains A-to-B IP datagram
MAC src 1A-23-F9-CD-06-9B
MAC dest 49-BD-D2-C7-56-2A
IP
Eth
Phy
IP
Eth
Phy
R
1A-23-F9-CD-06-9B 222222222220
111111111110 E6-E9-00-17-BB-4B CC-49-DE-D0-AB-7D
111111111112
111111111111
74-29-9C-E8-FF-55
A
222222222222
49-BD-D2-C7-56-2A
222222222221 88-B2-2F-54-1A-0F
B
Link Layer 5-51
Addressing routing to another LAN R forwards datagram with IP source A destination B
R creates link-layer frame with Bs MAC address as dest frame contains A-to-B IP datagram
IP src 111111111111
IP dest 222222222222
MAC src 1A-23-F9-CD-06-9B
MAC dest 49-BD-D2-C7-56-2A
IP
Eth
Phy
IP
Eth
Phy
R
1A-23-F9-CD-06-9B 222222222220
111111111110 E6-E9-00-17-BB-4B CC-49-DE-D0-AB-7D
111111111112
111111111111
74-29-9C-E8-FF-55
A
222222222222
49-BD-D2-C7-56-2A
222222222221 88-B2-2F-54-1A-0F
B
Link Layer 5-52
Addressing routing to another LAN R forwards datagram with IP source A destination B
R creates link-layer frame with Bs MAC address as dest frame contains A-to-B IP datagram
IP src 111111111111
IP dest 222222222222
MAC src 1A-23-F9-CD-06-9B
MAC dest 49-BD-D2-C7-56-2A
IP
Eth
Phy
Link Layer 5-53
Link layer LANs outline
51 introduction services
52 error detection correction
53 multiple access protocols
54 LANs addressing ARP
Ethernet
switches
VLANS
55 link virtualization MPLS
56 data center networking
57 a day in the life of a web request
Link Layer 5-54
Ethernet
ldquodominantrdquo wired LAN technology
cheap $20 for NIC
first widely used LAN technology
simpler cheaper than token LANs and ATM
kept up with speed race 10 Mbps ndash 10 Gbps
Metcalfersquos Ethernet sketch
Link Layer 5-55
Ethernet physical topology
bus popular through mid 90s all nodes in same collision domain (can collide with each
other)
star prevails today active switch in center
each ldquospokerdquo runs a (separate) Ethernet protocol (nodes
do not collide with each other)
switch
bus coaxial cable star
Link Layer 5-56
Ethernet frame structure
sending adapter encapsulates IP datagram (or other network layer protocol packet) in Ethernet frame
preamble
7 bytes with pattern 10101010 followed by one byte with pattern 10101011
used to synchronize receiver sender clock rates
dest address
source address
data (payload) CRC preamble
type
Link Layer 5-57
Ethernet frame structure (more)
addresses 6 byte source destination MAC addresses if adapter receives frame with matching destination
address or with broadcast address (eg ARP packet) it passes data in frame to network layer protocol
otherwise adapter discards frame
type indicates higher layer protocol (mostly IP but others possible eg Novell IPX AppleTalk)
CRC cyclic redundancy check at receiver error detected frame is dropped
dest address
source address
data (payload) CRC preamble
type
Link Layer 5-58
Ethernet unreliable connectionless
connectionless no handshaking between sending and receiving NICs
unreliable receiving NIC doesnt send acks or nacks to sending NIC
data in dropped frames recovered only if initial sender uses higher layer rdt (eg TCP) otherwise dropped data lost
Ethernetrsquos MAC protocol unslotted CSMACD wth binary backoff
Link Layer 5-59
8023 Ethernet standards link amp physical layers
many different Ethernet standards
common MAC protocol and frame format
different speeds 2 Mbps 10 Mbps 100 Mbps 1Gbps 10G bps
different physical layer media fiber cable
application
transport
network
link
physical
MAC protocol
and frame format
100BASE-TX
100BASE-T4
100BASE-FX 100BASE-T2
100BASE-SX 100BASE-BX
fiber physical layer copper (twister
pair) physical layer
Link Layer 5-60
Link layer LANs outline
51 introduction services
52 error detection correction
53 multiple access protocols
54 LANs addressing ARP
Ethernet
switches
VLANS
55 link virtualization MPLS
56 data center networking
57 a day in the life of a web request
Link Layer 5-61
Ethernet switch link-layer device takes an active role
store forward Ethernet frames
examine incoming framersquos MAC address selectively forward frame to one-or-more outgoing links when frame is to be forwarded on segment uses CSMACD to access segment
transparent
hosts are unaware of presence of switches
plug-and-play self-learning
switches do not need to be configured
Link Layer 5-62
Switch multiple simultaneous transmissions
hosts have dedicated direct connection to switch
switches buffer packets
Ethernet protocol used on each incoming link but no collisions full duplex
each link is its own collision domain
switching A-to-Arsquo and B-to-Brsquo can transmit simultaneously without collisions switch with six interfaces
(123456)
A
Arsquo
B
Brsquo C
Crsquo
1 2
3 4 5
6
Link Layer 5-63
Switch forwarding table
Q how does switch know Arsquo reachable via interface 4 Brsquo reachable via interface 5
switch with six interfaces
(123456)
A
Arsquo
B
Brsquo C
Crsquo
1 2
3 4 5
6 A each switch has a switch table each entry
(MAC address of host interface to
reach host time stamp)
looks like a routing table
Q how are entries created maintained in switch table
something like a routing protocol
A
Arsquo
B
Brsquo C
Crsquo
1 2
3 4 5
6
Link Layer 5-64
Switch self-learning
switch learns which hosts can be reached through which interfaces
when frame received switch ldquolearnsrdquo location of sender incoming LAN segment
records senderlocation pair in switch table
A Arsquo
Source A
Dest Arsquo
MAC addr interface TTL
Switch table
(initially empty) A 1 60
Link Layer 5-65
Switch frame filteringforwarding
when frame received at switch 1 record incoming link MAC address of sending host
2 index switch table using MAC destination address
3 if entry found for destination then
if destination on segment from which frame arrived then drop frame
else forward frame on interface indicated by entry
else flood forward on all interfaces except arriving
interface
A
Arsquo
B
Brsquo C
Crsquo
1 2
3 4 5
6
Link Layer 5-66
Self-learning forwarding example
A Arsquo
Source A
Dest Arsquo
MAC addr interface TTL
switch table
(initially empty) A 1 60
A Arsquo A Arsquo A Arsquo A Arsquo A Arsquo
frame destination Arsquo locaton unknown flood
Arsquo A
destination A location
known
Arsquo 4 60
selectively send
on just one link
Link Layer 5-67
Interconnecting switches
switches can be connected together
Q sending from A to G - how does S1 know to forward frame destined to F via S4 and S3
A self learning (works exactly the same as in single-switch case)
A
B
S1
C D
E
F
S2
S4
S3
H
I
G
Link Layer 5-68
Self-learning multi-switch example
Suppose C sends frame to I I responds to C
Q show switch tables and packet forwarding in S1 S2 S3 S4
A
B
S1
C D
E
F
S2
S4
S3
H
I
G
Link Layer 5-69
Institutional network
to external
network
router
IP subnet
mail server
web server
Link Layer 5-70
Switches vs routers
both are store-and-forward
routers network-layer devices (examine network-layer headers)
switches link-layer devices (examine link-layer headers)
both have forwarding tables
routers compute tables using routing algorithms IP addresses
switches learn forwarding table using flooding learning MAC addresses
application
transport
network
link
physical
network
link
physical
link
physical
switch
datagram
application
transport
network
link
physical
frame
frame
frame
datagram
Link Layer 5-71
VLANs motivation
consider CS user moves office to
EE but wants connect to CS switch
single broadcast domain
all layer-2 broadcast traffic (ARP DHCP unknown location of destination MAC address) must cross entire LAN
securityprivacy efficiency issues
Computer
Science Electrical
Engineering
Computer
Engineering
Link Layer 5-72
VLANs port-based VLAN switch ports
grouped (by switch management software) so that single physical switch helliphellip
switch(es) supporting
VLAN capabilities can
be configured to
define multiple virtual
LANS over single
physical LAN
infrastructure
Virtual Local
Area Network 1
8
9
16 10 2
7
hellip
Electrical Engineering
(VLAN ports 1-8)
Computer Science
(VLAN ports 9-15)
15
hellip
Electrical Engineering
(VLAN ports 1-8)
hellip
1
8 2
7 9
16 10
15
hellip
Computer Science
(VLAN ports 9-16)
hellip operates as multiple virtual switches
Link Layer 5-73
Port-based VLAN
1
8
9
16 10 2
7
hellip
Electrical Engineering
(VLAN ports 1-8)
Computer Science
(VLAN ports 9-15)
15
hellip
traffic isolation frames tofrom ports 1-8 can only reach ports 1-8 can also define VLAN based on
MAC addresses of endpoints rather than switch port
dynamic membership ports can be dynamically assigned among VLANs
router
forwarding between VLANS done via routing (just as with separate switches) in practice vendors sell combined
switches plus routers
Link Layer 5-74
VLANS spanning multiple switches
trunk port carries frames between VLANS defined over multiple physical switches frames forwarded within VLAN between switches canrsquot be vanilla
8021 frames (must carry VLAN ID info)
8021q protocol addsremoved additional header fields for frames forwarded between trunk ports
1
8
9
10 2
7
hellip
Electrical Engineering
(VLAN ports 1-8)
Computer Science
(VLAN ports 9-15)
15
hellip
2
7 3
Ports 235 belong to EE VLAN
Ports 4678 belong to CS VLAN
5
4 6 8 16
1
Link Layer 5-75
type
2-byte Tag Protocol Identifier
(value 81-00)
Tag Control Information (12 bit VLAN ID field
3 bit priority field like IP TOS)
Recomputed CRC
8021Q VLAN frame format
8021 frame
8021Q frame
dest address
source address
data (payload) CRC preamble
dest address
source address
preamble data (payload) CRC
type
Link Layer 5-76
Link layer LANs outline
51 introduction services
52 error detection correction
53 multiple access protocols
54 LANs addressing ARP
Ethernet
switches
VLANS
55 link virtualization MPLS
56 data center networking
57 a day in the life of a web request
Link Layer 5-77
Multiprotocol label switching (MPLS)
initial goal high-speed IP forwarding using fixed length label (instead of IP address) fast lookup using fixed length identifier (rather than
shortest prefix matching)
borrowing ideas from Virtual Circuit (VC) approach
but IP datagram still keeps IP address
PPP or Ethernet
header IP header remainder of link-layer frame MPLS header
label Exp S TTL
20 3 1 5
Link Layer 5-78
MPLS capable routers
aka label-switched router
forward packets to outgoing interface based only on label value (donrsquot inspect IP address) MPLS forwarding table distinct from IP forwarding tables
flexibility MPLS forwarding decisions can differ from those of IP use destination and source addresses to route flows to
same destination differently (traffic engineering)
re-route flows quickly if link fails pre-computed backup paths (useful for VoIP)
Link Layer 5-79
R2
D
R3 R5
A
R6
MPLS versus IP paths
IP router IP routing path to destination determined
by destination address alone
R4
Link Layer 5-80
R2
D
R3 R4
R5
A
R6
MPLS versus IP paths
IP-only router
IP routing path to destination determined by destination address alone
MPLS and IP router
MPLS routing path to destination can be based on source and dest address fast reroute precompute backup routes in
case of link failure
entry router (R4) can use different MPLS
routes to A based eg on source address
Link Layer 5-81
MPLS signaling
modify OSPF IS-IS link-state flooding protocols to carry info used by MPLS routing eg link bandwidth amount of ldquoreservedrdquo link bandwidth
D
R4
R5
A
R6
entry MPLS router uses RSVP-TE signaling protocol to set up MPLS forwarding at downstream routers
modified link state flooding
RSVP-TE
Link Layer 5-82
R1 R2
D
R3 R4 R5
0
1
0 0
A
R6
in out out
label label dest interface
6 - A 0
in out out
label label dest interface
10 6 A 1
12 9 D 0
in out out
label label dest interface
10 A 0
12 D 0
1
in out out
label label dest interface
8 6 A 0
0
8 A 1
MPLS forwarding tables
Link Layer 5-83
Link layer LANs outline
51 introduction services
52 error detection correction
53 multiple access protocols
54 LANs addressing ARP
Ethernet
switches
VLANS
55 link virtualization MPLS
56 data center networking
57 a day in the life of a web request
Link Layer 5-84
Data center networks
10rsquos to 100rsquos of thousands of hosts often closely coupled in close proximity e-business (eg Amazon)
content-servers (eg YouTube Akamai Apple Microsoft)
search engines data mining (eg Google)
challenges
multiple applications each serving massive numbers of clients
managingbalancing load avoiding processing networking data bottlenecks
Inside a 40-ft Microsoft container
Chicago data center
Link Layer 5-85
Server racks
TOR switches
Tier-1 switches
Tier-2 switches
Load balancer
Load balancer
B
1 2 3 4 5 6 7 8
A C
Border router
Access router
Internet
Data center networks
load balancer application-layer routing receives external client requests
directs workload within data center
returns results to external client (hiding data
center internals from client)
Server racks
TOR switches
Tier-1 switches
Tier-2 switches
1 2 3 4 5 6 7 8
Data center networks
rich interconnection among switches racks
increased throughput between racks (multiple routing
paths possible)
increased reliability via redundancy
Link Layer 5-87
Link layer LANs outline
51 introduction services
52 error detection correction
53 multiple access protocols
54 LANs addressing ARP
Ethernet
switches
VLANS
55 link virtualization MPLS
56 data center networking
57 a day in the life of a web request
Link Layer 5-88
Synthesis a day in the life of a web request
journey down protocol stack complete application transport network link
putting-it-all-together synthesis goal identify review understand protocols (at all
layers) involved in seemingly simple scenario requesting www page
scenario student attaches laptop to campus network requestsreceives wwwgooglecom
Link Layer 5-89
A day in the life scenario
Comcast network
68800013
Googlersquos network
64233160019 64233169105
web server
DNS server
school network
68802024
web page
browser
router
(runs DHCP)
Link Layer 5-90
A day in the lifehellip connecting to the Internet
connecting laptop needs to get its own IP address addr of first-hop router addr of DNS server use DHCP
DHCP
UDP
IP
Eth
Phy
DHCP
DHCP
DHCP
DHCP
DHCP
DHCP
UDP
IP
Eth
Phy
DHCP
DHCP
DHCP
DHCP DHCP
DHCP request encapsulated in UDP encapsulated in IP encapsulated in 8023 Ethernet
Ethernet frame broadcast
(dest FFFFFFFFFFFF) on LAN received at router running DHCP server
Ethernet demuxed to IP demuxed UDP demuxed to DHCP
router
(runs DHCP)
Link Layer 5-91
DHCP server formulates DHCP ACK containing clientrsquos IP address IP address of first-hop router for client name amp IP address of DNS server
DHCP
UDP
IP
Eth
Phy
DHCP
DHCP
DHCP
DHCP
DHCP
UDP
IP
Eth
Phy
DHCP
DHCP
DHCP
DHCP
DHCP
encapsulation at DHCP server frame forwarded (switch learning) through LAN demultiplexing at client
Client now has IP address knows name amp addr of DNS
server IP address of its first-hop router
DHCP client receives DHCP ACK reply
A day in the lifehellip connecting to the Internet
router
(runs DHCP)
Link Layer 5-92
A day in the lifehellip ARP (before DNS before HTTP)
before sending HTTP request need IP address of wwwgooglecom DNS
DNS
UDP
IP
Eth
Phy
DNS
DNS
DNS
DNS query created encapsulated in UDP encapsulated in IP encapsulated in Eth To send frame to router need MAC address of router interface ARP
ARP query broadcast received by
router which replies with ARP reply giving MAC address of router interface
client now knows MAC address of first hop router so can now send frame containing DNS query
ARP query
Eth
Phy
ARP
ARP
ARP reply
router
(runs DHCP)
Link Layer 5-93
DNS
UDP
IP
Eth
Phy
DNS
DNS
DNS
DNS
DNS
IP datagram containing DNS query forwarded via LAN switch from client to 1st hop router
IP datagram forwarded from campus network into comcast network routed (tables created by RIP OSPF IS-IS andor BGP routing protocols) to DNS server
demuxrsquoed to DNS server
DNS server replies to client with IP address of wwwgooglecom
Comcast network
68800013
DNS server
DNS
UDP
IP
Eth
Phy
DNS
DNS
DNS
DNS
A day in the lifehellip using DNS
router
(runs DHCP)
Link Layer 5-94
A day in the lifehellipTCP connection carrying HTTP
HTTP
TCP
IP
Eth
Phy
HTTP
to send HTTP request client first opens TCP socket to web server
TCP SYN segment (step 1 in 3-way handshake) inter-domain routed to web server
TCP connection established 64233169105
web server
SYN
SYN
SYN
SYN
TCP
IP
Eth
Phy
SYN
SYN
SYN
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
web server responds with TCP SYNACK (step 2 in 3-way handshake)
router
(runs DHCP)
Link Layer 5-95
A day in the lifehellip HTTP requestreply
HTTP
TCP
IP
Eth
Phy
HTTP
HTTP request sent into TCP socket
IP datagram containing HTTP request routed to wwwgooglecom
IP datagram containing HTTP reply routed back to client
64233169105
web server
HTTP
TCP
IP
Eth
Phy web server responds with
HTTP reply (containing web page)
HTTP
HTTP
HTTP HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
web page finally () displayed
Link Layer 5-96
Chapter 5 Summary
principles behind data link layer services error detection correction
sharing a broadcast channel multiple access
link layer addressing
instantiation and implementation of various link layer technologies Ethernet
switched LANS VLANs
virtualized networks as a link layer MPLS
synthesis a day in the life of a web request
Link Layer 5-97
Chapter 5 letrsquos take a breath
journey down protocol stack complete (except PHY)
solid understanding of networking principles practice
hellip could stop here hellip but lots of interesting topics wireless
multimedia
security
network management
Link Layer 5-20
MAC protocols taxonomy
three broad classes
channel partitioning divide channel into smaller ldquopiecesrdquo (time slots frequency code)
allocate piece to node for exclusive use
random access channel not divided allow collisions
ldquorecoverrdquo from collisions
ldquotaking turnsrdquo nodes take turns but nodes with more to send can take longer
turns
Link Layer 5-21
Channel partitioning MAC protocols TDMA
TDMA time division multiple access access to channel in rounds each station gets fixed length slot (length = pkt
trans time) in each round unused slots go idle example 6-station LAN 134 have pkt slots
256 idle
1 3 4 1 3 4
6-slot
frame
6-slot
frame
Link Layer 5-22
FDMA frequency division multiple access channel spectrum divided into frequency bands
each station assigned fixed frequency band
unused transmission time in frequency bands go idle
example 6-station LAN 134 have pkt frequency bands 256 idle
frequency b
ands
FDM cable
Channel partitioning MAC protocols FDMA
Link Layer 5-23
Random access protocols
when node has packet to send transmit at full channel data rate R no a priori coordination among nodes
two or more transmitting nodes ldquocollisionrdquo random access MAC protocol specifies
how to detect collisions how to recover from collisions (eg via delayed
retransmissions)
examples of random access MAC protocols slotted ALOHA ALOHA CSMA CSMACD CSMACA
Link Layer 5-24
Slotted ALOHA
assumptions all frames same size
time divided into equal size slots (time to transmit 1 frame)
nodes start to transmit only slot beginning
nodes are synchronized
if 2 or more nodes transmit in slot all nodes detect collision
operation
when node obtains fresh frame transmits in next slot
if no collision node can send new frame in next slot
if collision node retransmits frame in each subsequent slot with prob p until success
Link Layer 5-25
Pros single active node can
continuously transmit at full rate of channel
highly decentralized only slots in nodes need to be in sync
simple
Cons
collisions wasting slots
idle slots
nodes may be able to detect collision in less than time to transmit packet
clock synchronization
Slotted ALOHA
1 1 1 1
2
3
2 2
3 3
node 1
node 2
node 3
C C C S S S E E E
Link Layer 5-26
suppose N nodes with many frames to send each transmits in slot with probability p
prob that given node has success in a slot = p(1-p)N-1
prob that any node has a success = Np(1-p)N-1
max efficiency find p that maximizes Np(1-p)N-1
for many nodes take limit of Np(1-p)N-1 as N goes to infinity gives
max efficiency = 1e = 37
efficiency long-run fraction of successful slots (many nodes all with many frames to send)
at best channel used for useful transmissions 37 of time
Slotted ALOHA efficiency
Link Layer 5-27
Pure (unslotted) ALOHA
unslotted Aloha simpler no synchronization
when frame first arrives
transmit immediately
collision probability increases
frame sent at t0 collides with other frames sent in [t0-1t0+1]
Link Layer 5-28
Pure ALOHA efficiency
P(success by given node) = P(node transmits)
P(no other node transmits in [t0-1t0]
P(no other node transmits in [t0-1t0]
= p (1-p)N-1 (1-p)N-1
= p (1-p)2(N-1)
hellip choosing optimum p and then letting n
= 1(2e) = 18
even worse than slotted Aloha
Link Layer 5-29
CSMA (carrier sense multiple access)
CSMA listen before transmit if channel sensed idle transmit entire frame
if channel sensed busy defer transmission
human analogy donrsquot interrupt others
Link Layer 5-30
CSMA collisions
collisions can still occur propagation delay means two nodes may not hear each otherrsquos transmission
collision entire packet transmission time wasted distance amp propagation
delay play role in in determining collision probability
spatial layout of nodes
Link Layer 5-31
CSMACD (collision detection)
CSMACD carrier sensing deferral as in CSMA collisions detected within short time
colliding transmissions aborted reducing channel wastage
collision detection easy in wired LANs measure signal strengths compare
transmitted received signals
difficult in wireless LANs received signal strength overwhelmed by local transmission strength
human analogy the polite conversationalist
Link Layer 5-32
CSMACD (collision detection)
spatial layout of nodes
Link Layer 5-33
Ethernet CSMACD algorithm
1 NIC receives datagram from network layer creates frame
2 If NIC senses channel idle starts frame transmission If NIC senses channel busy waits until channel idle then transmits
3 If NIC transmits entire frame without detecting another transmission NIC is done with frame
4 If NIC detects another transmission while transmitting aborts and sends jam signal
5 After aborting NIC enters binary (exponential) backoff
after mth collision NIC chooses K at random from 012 hellip 2m-1 NIC waits K512 bit times returns to Step 2
longer backoff interval with more collisions
Link Layer 5-34
CSMACD efficiency
Tprop = max prop delay between 2 nodes in LAN
ttrans = time to transmit max-size frame
efficiency goes to 1
as tprop goes to 0
as ttrans goes to infinity
better performance than ALOHA and simple cheap decentralized
transpropttefficiency
51
1
Link Layer 5-35
ldquoTaking turnsrdquo MAC protocols
channel partitioning MAC protocols share channel efficiently and fairly at high load
inefficient at low load delay in channel access 1N bandwidth allocated even if only 1 active node
random access MAC protocols efficient at low load single node can fully utilize
channel
high load collision overhead
ldquotaking turnsrdquo protocols look for best of both worlds
Link Layer 5-36
polling master node ldquoinvitesrdquo
slave nodes to transmit in turn
typically used with ldquodumbrdquo slave devices
concerns
polling overhead
latency
single point of failure (master)
master
slaves
poll
data
data
ldquoTaking turnsrdquo MAC protocols
Link Layer 5-37
token passing control token passed
from one node to next sequentially
token message
concerns
token overhead
latency
single point of failure (token)
T
data
(nothing
to send)
T
ldquoTaking turnsrdquo MAC protocols
cable headend
CMTS
ISP
cable modem termination system
multiple 40Mbps downstream (broadcast) channels
single CMTS transmits into channels
multiple 30 Mbps upstream channels
multiple access all users contend for certain upstream channel time slots (others assigned)
Cable access network
cable modem
splitter
hellip
hellip
Internet framesTV channels control transmitted downstream at different frequencies
upstream Internet frames TV control transmitted upstream at different frequencies in time slots
Link Layer 5-39
DOCSIS data over cable service interface spec FDM over upstream downstream frequency channels
TDM upstream some slots assigned some have contention
downstream MAP frame assigns upstream slots
request for upstream slots (and data) transmitted random access (binary backoff) in selected slots
MAP frame for
Interval [t1 t2]
Residences with cable modems
Downstream channel i
Upstream channel j
t1 t2
Assigned minislots containing cable modem
upstream data frames
Minislots containing
minislots request frames
cable headend
CMTS
Cable access network
Link Layer 5-40
Summary of MAC protocols
channel partitioning by time frequency or code Time Division Frequency Division
random access (dynamic)
ALOHA S-ALOHA CSMA CSMACD
carrier sensing easy in some technologies (wire) hard in others (wireless)
CSMACD used in Ethernet
CSMACA used in 80211
taking turns
polling from central site token passing
bluetooth FDDI token ring
Link Layer 5-41
Link layer LANs outline
51 introduction services
52 error detection correction
53 multiple access protocols
54 LANs addressing ARP
Ethernet
switches
VLANS
55 link virtualization MPLS
56 data center networking
57 a day in the life of a web request
Link Layer 5-42
MAC addresses and ARP
32-bit IP address network-layer address for interface
used for layer 3 (network layer) forwarding
MAC (or LAN or physical or Ethernet) address function used lsquolocallyrdquo to get frame from one interface to
another physically-connected interface (same network in IP-addressing sense)
48 bit MAC address (for most LANs) burned in NIC ROM also sometimes software settable
eg 1A-2F-BB-76-09-AD
hexadecimal (base 16) notation
(each ldquonumberrdquo represents 4 bits)
Link Layer 5-43
LAN addresses and ARP
each adapter on LAN has unique LAN address
adapter
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN
(wired or
wireless)
Link Layer 5-44
LAN addresses (more)
MAC address allocation administered by IEEE
manufacturer buys portion of MAC address space (to assure uniqueness)
analogy MAC address like Social Security Number
IP address like postal address
MAC flat address portability can move LAN card from one LAN to another
IP hierarchical address not portable address depends on IP subnet to which node is
attached
Link Layer 5-45
ARP address resolution protocol
ARP table each IP node (host router) on LAN has table
IPMAC address mappings for some LAN nodes
lt IP address MAC address TTLgt
TTL (Time To Live) time after which address mapping will be forgotten (typically 20 min)
Question how to determine
interfacersquos MAC address
knowing its IP address
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN
137196723
137196778
137196714
137196788
Link Layer 5-46
ARP protocol same LAN
A wants to send datagram to B Brsquos MAC address not in
Arsquos ARP table
A broadcasts ARP query packet containing Bs IP address dest MAC address = FF-FF-
FF-FF-FF-FF
all nodes on LAN receive ARP query
B receives ARP packet replies to A with its (Bs) MAC address frame sent to Arsquos MAC
address (unicast)
A caches (saves) IP-to-MAC address pair in its ARP table until information becomes old (times out) soft state information that
times out (goes away) unless refreshed
ARP is ldquoplug-and-playrdquo nodes create their ARP
tables without intervention from net administrator
Link Layer 5-47
walkthrough send datagram from A to B via R
focus on addressing ndash at IP (datagram) and MAC layer (frame)
assume A knows Brsquos IP address
assume A knows IP address of first hop router R (how)
assume A knows Rrsquos MAC address (how)
Addressing routing to another LAN
R
1A-23-F9-CD-06-9B 222222222220
111111111110 E6-E9-00-17-BB-4B CC-49-DE-D0-AB-7D
111111111112
111111111111
74-29-9C-E8-FF-55
A
222222222222
49-BD-D2-C7-56-2A
222222222221 88-B2-2F-54-1A-0F
B
R
1A-23-F9-CD-06-9B 222222222220
111111111110 E6-E9-00-17-BB-4B CC-49-DE-D0-AB-7D
111111111112
111111111111
74-29-9C-E8-FF-55
A
222222222222
49-BD-D2-C7-56-2A
222222222221 88-B2-2F-54-1A-0F
B
Link Layer 5-48
Addressing routing to another LAN
IP
Eth
Phy
IP src 111111111111
IP dest 222222222222
A creates IP datagram with IP source A destination B
A creates link-layer frame with Rs MAC address as dest frame contains A-to-B IP datagram
MAC src 74-29-9C-E8-FF-55
MAC dest E6-E9-00-17-BB-4B
R
1A-23-F9-CD-06-9B 222222222220
111111111110 E6-E9-00-17-BB-4B CC-49-DE-D0-AB-7D
111111111112
111111111111
74-29-9C-E8-FF-55
A
222222222222
49-BD-D2-C7-56-2A
222222222221 88-B2-2F-54-1A-0F
B
Link Layer 5-49
Addressing routing to another LAN
IP
Eth
Phy
frame sent from A to R
IP
Eth
Phy
frame received at R datagram removed passed up to IP
MAC src 74-29-9C-E8-FF-55
MAC dest E6-E9-00-17-BB-4B
IP src 111111111111
IP dest 222222222222
IP src 111111111111
IP dest 222222222222
R
1A-23-F9-CD-06-9B 222222222220
111111111110 E6-E9-00-17-BB-4B CC-49-DE-D0-AB-7D
111111111112
111111111111
74-29-9C-E8-FF-55
A
222222222222
49-BD-D2-C7-56-2A
222222222221 88-B2-2F-54-1A-0F
B
Link Layer 5-50
Addressing routing to another LAN
IP src 111111111111
IP dest 222222222222
R forwards datagram with IP source A destination B
R creates link-layer frame with Bs MAC address as dest frame contains A-to-B IP datagram
MAC src 1A-23-F9-CD-06-9B
MAC dest 49-BD-D2-C7-56-2A
IP
Eth
Phy
IP
Eth
Phy
R
1A-23-F9-CD-06-9B 222222222220
111111111110 E6-E9-00-17-BB-4B CC-49-DE-D0-AB-7D
111111111112
111111111111
74-29-9C-E8-FF-55
A
222222222222
49-BD-D2-C7-56-2A
222222222221 88-B2-2F-54-1A-0F
B
Link Layer 5-51
Addressing routing to another LAN R forwards datagram with IP source A destination B
R creates link-layer frame with Bs MAC address as dest frame contains A-to-B IP datagram
IP src 111111111111
IP dest 222222222222
MAC src 1A-23-F9-CD-06-9B
MAC dest 49-BD-D2-C7-56-2A
IP
Eth
Phy
IP
Eth
Phy
R
1A-23-F9-CD-06-9B 222222222220
111111111110 E6-E9-00-17-BB-4B CC-49-DE-D0-AB-7D
111111111112
111111111111
74-29-9C-E8-FF-55
A
222222222222
49-BD-D2-C7-56-2A
222222222221 88-B2-2F-54-1A-0F
B
Link Layer 5-52
Addressing routing to another LAN R forwards datagram with IP source A destination B
R creates link-layer frame with Bs MAC address as dest frame contains A-to-B IP datagram
IP src 111111111111
IP dest 222222222222
MAC src 1A-23-F9-CD-06-9B
MAC dest 49-BD-D2-C7-56-2A
IP
Eth
Phy
Link Layer 5-53
Link layer LANs outline
51 introduction services
52 error detection correction
53 multiple access protocols
54 LANs addressing ARP
Ethernet
switches
VLANS
55 link virtualization MPLS
56 data center networking
57 a day in the life of a web request
Link Layer 5-54
Ethernet
ldquodominantrdquo wired LAN technology
cheap $20 for NIC
first widely used LAN technology
simpler cheaper than token LANs and ATM
kept up with speed race 10 Mbps ndash 10 Gbps
Metcalfersquos Ethernet sketch
Link Layer 5-55
Ethernet physical topology
bus popular through mid 90s all nodes in same collision domain (can collide with each
other)
star prevails today active switch in center
each ldquospokerdquo runs a (separate) Ethernet protocol (nodes
do not collide with each other)
switch
bus coaxial cable star
Link Layer 5-56
Ethernet frame structure
sending adapter encapsulates IP datagram (or other network layer protocol packet) in Ethernet frame
preamble
7 bytes with pattern 10101010 followed by one byte with pattern 10101011
used to synchronize receiver sender clock rates
dest address
source address
data (payload) CRC preamble
type
Link Layer 5-57
Ethernet frame structure (more)
addresses 6 byte source destination MAC addresses if adapter receives frame with matching destination
address or with broadcast address (eg ARP packet) it passes data in frame to network layer protocol
otherwise adapter discards frame
type indicates higher layer protocol (mostly IP but others possible eg Novell IPX AppleTalk)
CRC cyclic redundancy check at receiver error detected frame is dropped
dest address
source address
data (payload) CRC preamble
type
Link Layer 5-58
Ethernet unreliable connectionless
connectionless no handshaking between sending and receiving NICs
unreliable receiving NIC doesnt send acks or nacks to sending NIC
data in dropped frames recovered only if initial sender uses higher layer rdt (eg TCP) otherwise dropped data lost
Ethernetrsquos MAC protocol unslotted CSMACD wth binary backoff
Link Layer 5-59
8023 Ethernet standards link amp physical layers
many different Ethernet standards
common MAC protocol and frame format
different speeds 2 Mbps 10 Mbps 100 Mbps 1Gbps 10G bps
different physical layer media fiber cable
application
transport
network
link
physical
MAC protocol
and frame format
100BASE-TX
100BASE-T4
100BASE-FX 100BASE-T2
100BASE-SX 100BASE-BX
fiber physical layer copper (twister
pair) physical layer
Link Layer 5-60
Link layer LANs outline
51 introduction services
52 error detection correction
53 multiple access protocols
54 LANs addressing ARP
Ethernet
switches
VLANS
55 link virtualization MPLS
56 data center networking
57 a day in the life of a web request
Link Layer 5-61
Ethernet switch link-layer device takes an active role
store forward Ethernet frames
examine incoming framersquos MAC address selectively forward frame to one-or-more outgoing links when frame is to be forwarded on segment uses CSMACD to access segment
transparent
hosts are unaware of presence of switches
plug-and-play self-learning
switches do not need to be configured
Link Layer 5-62
Switch multiple simultaneous transmissions
hosts have dedicated direct connection to switch
switches buffer packets
Ethernet protocol used on each incoming link but no collisions full duplex
each link is its own collision domain
switching A-to-Arsquo and B-to-Brsquo can transmit simultaneously without collisions switch with six interfaces
(123456)
A
Arsquo
B
Brsquo C
Crsquo
1 2
3 4 5
6
Link Layer 5-63
Switch forwarding table
Q how does switch know Arsquo reachable via interface 4 Brsquo reachable via interface 5
switch with six interfaces
(123456)
A
Arsquo
B
Brsquo C
Crsquo
1 2
3 4 5
6 A each switch has a switch table each entry
(MAC address of host interface to
reach host time stamp)
looks like a routing table
Q how are entries created maintained in switch table
something like a routing protocol
A
Arsquo
B
Brsquo C
Crsquo
1 2
3 4 5
6
Link Layer 5-64
Switch self-learning
switch learns which hosts can be reached through which interfaces
when frame received switch ldquolearnsrdquo location of sender incoming LAN segment
records senderlocation pair in switch table
A Arsquo
Source A
Dest Arsquo
MAC addr interface TTL
Switch table
(initially empty) A 1 60
Link Layer 5-65
Switch frame filteringforwarding
when frame received at switch 1 record incoming link MAC address of sending host
2 index switch table using MAC destination address
3 if entry found for destination then
if destination on segment from which frame arrived then drop frame
else forward frame on interface indicated by entry
else flood forward on all interfaces except arriving
interface
A
Arsquo
B
Brsquo C
Crsquo
1 2
3 4 5
6
Link Layer 5-66
Self-learning forwarding example
A Arsquo
Source A
Dest Arsquo
MAC addr interface TTL
switch table
(initially empty) A 1 60
A Arsquo A Arsquo A Arsquo A Arsquo A Arsquo
frame destination Arsquo locaton unknown flood
Arsquo A
destination A location
known
Arsquo 4 60
selectively send
on just one link
Link Layer 5-67
Interconnecting switches
switches can be connected together
Q sending from A to G - how does S1 know to forward frame destined to F via S4 and S3
A self learning (works exactly the same as in single-switch case)
A
B
S1
C D
E
F
S2
S4
S3
H
I
G
Link Layer 5-68
Self-learning multi-switch example
Suppose C sends frame to I I responds to C
Q show switch tables and packet forwarding in S1 S2 S3 S4
A
B
S1
C D
E
F
S2
S4
S3
H
I
G
Link Layer 5-69
Institutional network
to external
network
router
IP subnet
mail server
web server
Link Layer 5-70
Switches vs routers
both are store-and-forward
routers network-layer devices (examine network-layer headers)
switches link-layer devices (examine link-layer headers)
both have forwarding tables
routers compute tables using routing algorithms IP addresses
switches learn forwarding table using flooding learning MAC addresses
application
transport
network
link
physical
network
link
physical
link
physical
switch
datagram
application
transport
network
link
physical
frame
frame
frame
datagram
Link Layer 5-71
VLANs motivation
consider CS user moves office to
EE but wants connect to CS switch
single broadcast domain
all layer-2 broadcast traffic (ARP DHCP unknown location of destination MAC address) must cross entire LAN
securityprivacy efficiency issues
Computer
Science Electrical
Engineering
Computer
Engineering
Link Layer 5-72
VLANs port-based VLAN switch ports
grouped (by switch management software) so that single physical switch helliphellip
switch(es) supporting
VLAN capabilities can
be configured to
define multiple virtual
LANS over single
physical LAN
infrastructure
Virtual Local
Area Network 1
8
9
16 10 2
7
hellip
Electrical Engineering
(VLAN ports 1-8)
Computer Science
(VLAN ports 9-15)
15
hellip
Electrical Engineering
(VLAN ports 1-8)
hellip
1
8 2
7 9
16 10
15
hellip
Computer Science
(VLAN ports 9-16)
hellip operates as multiple virtual switches
Link Layer 5-73
Port-based VLAN
1
8
9
16 10 2
7
hellip
Electrical Engineering
(VLAN ports 1-8)
Computer Science
(VLAN ports 9-15)
15
hellip
traffic isolation frames tofrom ports 1-8 can only reach ports 1-8 can also define VLAN based on
MAC addresses of endpoints rather than switch port
dynamic membership ports can be dynamically assigned among VLANs
router
forwarding between VLANS done via routing (just as with separate switches) in practice vendors sell combined
switches plus routers
Link Layer 5-74
VLANS spanning multiple switches
trunk port carries frames between VLANS defined over multiple physical switches frames forwarded within VLAN between switches canrsquot be vanilla
8021 frames (must carry VLAN ID info)
8021q protocol addsremoved additional header fields for frames forwarded between trunk ports
1
8
9
10 2
7
hellip
Electrical Engineering
(VLAN ports 1-8)
Computer Science
(VLAN ports 9-15)
15
hellip
2
7 3
Ports 235 belong to EE VLAN
Ports 4678 belong to CS VLAN
5
4 6 8 16
1
Link Layer 5-75
type
2-byte Tag Protocol Identifier
(value 81-00)
Tag Control Information (12 bit VLAN ID field
3 bit priority field like IP TOS)
Recomputed CRC
8021Q VLAN frame format
8021 frame
8021Q frame
dest address
source address
data (payload) CRC preamble
dest address
source address
preamble data (payload) CRC
type
Link Layer 5-76
Link layer LANs outline
51 introduction services
52 error detection correction
53 multiple access protocols
54 LANs addressing ARP
Ethernet
switches
VLANS
55 link virtualization MPLS
56 data center networking
57 a day in the life of a web request
Link Layer 5-77
Multiprotocol label switching (MPLS)
initial goal high-speed IP forwarding using fixed length label (instead of IP address) fast lookup using fixed length identifier (rather than
shortest prefix matching)
borrowing ideas from Virtual Circuit (VC) approach
but IP datagram still keeps IP address
PPP or Ethernet
header IP header remainder of link-layer frame MPLS header
label Exp S TTL
20 3 1 5
Link Layer 5-78
MPLS capable routers
aka label-switched router
forward packets to outgoing interface based only on label value (donrsquot inspect IP address) MPLS forwarding table distinct from IP forwarding tables
flexibility MPLS forwarding decisions can differ from those of IP use destination and source addresses to route flows to
same destination differently (traffic engineering)
re-route flows quickly if link fails pre-computed backup paths (useful for VoIP)
Link Layer 5-79
R2
D
R3 R5
A
R6
MPLS versus IP paths
IP router IP routing path to destination determined
by destination address alone
R4
Link Layer 5-80
R2
D
R3 R4
R5
A
R6
MPLS versus IP paths
IP-only router
IP routing path to destination determined by destination address alone
MPLS and IP router
MPLS routing path to destination can be based on source and dest address fast reroute precompute backup routes in
case of link failure
entry router (R4) can use different MPLS
routes to A based eg on source address
Link Layer 5-81
MPLS signaling
modify OSPF IS-IS link-state flooding protocols to carry info used by MPLS routing eg link bandwidth amount of ldquoreservedrdquo link bandwidth
D
R4
R5
A
R6
entry MPLS router uses RSVP-TE signaling protocol to set up MPLS forwarding at downstream routers
modified link state flooding
RSVP-TE
Link Layer 5-82
R1 R2
D
R3 R4 R5
0
1
0 0
A
R6
in out out
label label dest interface
6 - A 0
in out out
label label dest interface
10 6 A 1
12 9 D 0
in out out
label label dest interface
10 A 0
12 D 0
1
in out out
label label dest interface
8 6 A 0
0
8 A 1
MPLS forwarding tables
Link Layer 5-83
Link layer LANs outline
51 introduction services
52 error detection correction
53 multiple access protocols
54 LANs addressing ARP
Ethernet
switches
VLANS
55 link virtualization MPLS
56 data center networking
57 a day in the life of a web request
Link Layer 5-84
Data center networks
10rsquos to 100rsquos of thousands of hosts often closely coupled in close proximity e-business (eg Amazon)
content-servers (eg YouTube Akamai Apple Microsoft)
search engines data mining (eg Google)
challenges
multiple applications each serving massive numbers of clients
managingbalancing load avoiding processing networking data bottlenecks
Inside a 40-ft Microsoft container
Chicago data center
Link Layer 5-85
Server racks
TOR switches
Tier-1 switches
Tier-2 switches
Load balancer
Load balancer
B
1 2 3 4 5 6 7 8
A C
Border router
Access router
Internet
Data center networks
load balancer application-layer routing receives external client requests
directs workload within data center
returns results to external client (hiding data
center internals from client)
Server racks
TOR switches
Tier-1 switches
Tier-2 switches
1 2 3 4 5 6 7 8
Data center networks
rich interconnection among switches racks
increased throughput between racks (multiple routing
paths possible)
increased reliability via redundancy
Link Layer 5-87
Link layer LANs outline
51 introduction services
52 error detection correction
53 multiple access protocols
54 LANs addressing ARP
Ethernet
switches
VLANS
55 link virtualization MPLS
56 data center networking
57 a day in the life of a web request
Link Layer 5-88
Synthesis a day in the life of a web request
journey down protocol stack complete application transport network link
putting-it-all-together synthesis goal identify review understand protocols (at all
layers) involved in seemingly simple scenario requesting www page
scenario student attaches laptop to campus network requestsreceives wwwgooglecom
Link Layer 5-89
A day in the life scenario
Comcast network
68800013
Googlersquos network
64233160019 64233169105
web server
DNS server
school network
68802024
web page
browser
router
(runs DHCP)
Link Layer 5-90
A day in the lifehellip connecting to the Internet
connecting laptop needs to get its own IP address addr of first-hop router addr of DNS server use DHCP
DHCP
UDP
IP
Eth
Phy
DHCP
DHCP
DHCP
DHCP
DHCP
DHCP
UDP
IP
Eth
Phy
DHCP
DHCP
DHCP
DHCP DHCP
DHCP request encapsulated in UDP encapsulated in IP encapsulated in 8023 Ethernet
Ethernet frame broadcast
(dest FFFFFFFFFFFF) on LAN received at router running DHCP server
Ethernet demuxed to IP demuxed UDP demuxed to DHCP
router
(runs DHCP)
Link Layer 5-91
DHCP server formulates DHCP ACK containing clientrsquos IP address IP address of first-hop router for client name amp IP address of DNS server
DHCP
UDP
IP
Eth
Phy
DHCP
DHCP
DHCP
DHCP
DHCP
UDP
IP
Eth
Phy
DHCP
DHCP
DHCP
DHCP
DHCP
encapsulation at DHCP server frame forwarded (switch learning) through LAN demultiplexing at client
Client now has IP address knows name amp addr of DNS
server IP address of its first-hop router
DHCP client receives DHCP ACK reply
A day in the lifehellip connecting to the Internet
router
(runs DHCP)
Link Layer 5-92
A day in the lifehellip ARP (before DNS before HTTP)
before sending HTTP request need IP address of wwwgooglecom DNS
DNS
UDP
IP
Eth
Phy
DNS
DNS
DNS
DNS query created encapsulated in UDP encapsulated in IP encapsulated in Eth To send frame to router need MAC address of router interface ARP
ARP query broadcast received by
router which replies with ARP reply giving MAC address of router interface
client now knows MAC address of first hop router so can now send frame containing DNS query
ARP query
Eth
Phy
ARP
ARP
ARP reply
router
(runs DHCP)
Link Layer 5-93
DNS
UDP
IP
Eth
Phy
DNS
DNS
DNS
DNS
DNS
IP datagram containing DNS query forwarded via LAN switch from client to 1st hop router
IP datagram forwarded from campus network into comcast network routed (tables created by RIP OSPF IS-IS andor BGP routing protocols) to DNS server
demuxrsquoed to DNS server
DNS server replies to client with IP address of wwwgooglecom
Comcast network
68800013
DNS server
DNS
UDP
IP
Eth
Phy
DNS
DNS
DNS
DNS
A day in the lifehellip using DNS
router
(runs DHCP)
Link Layer 5-94
A day in the lifehellipTCP connection carrying HTTP
HTTP
TCP
IP
Eth
Phy
HTTP
to send HTTP request client first opens TCP socket to web server
TCP SYN segment (step 1 in 3-way handshake) inter-domain routed to web server
TCP connection established 64233169105
web server
SYN
SYN
SYN
SYN
TCP
IP
Eth
Phy
SYN
SYN
SYN
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
web server responds with TCP SYNACK (step 2 in 3-way handshake)
router
(runs DHCP)
Link Layer 5-95
A day in the lifehellip HTTP requestreply
HTTP
TCP
IP
Eth
Phy
HTTP
HTTP request sent into TCP socket
IP datagram containing HTTP request routed to wwwgooglecom
IP datagram containing HTTP reply routed back to client
64233169105
web server
HTTP
TCP
IP
Eth
Phy web server responds with
HTTP reply (containing web page)
HTTP
HTTP
HTTP HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
web page finally () displayed
Link Layer 5-96
Chapter 5 Summary
principles behind data link layer services error detection correction
sharing a broadcast channel multiple access
link layer addressing
instantiation and implementation of various link layer technologies Ethernet
switched LANS VLANs
virtualized networks as a link layer MPLS
synthesis a day in the life of a web request
Link Layer 5-97
Chapter 5 letrsquos take a breath
journey down protocol stack complete (except PHY)
solid understanding of networking principles practice
hellip could stop here hellip but lots of interesting topics wireless
multimedia
security
network management
Link Layer 5-21
Channel partitioning MAC protocols TDMA
TDMA time division multiple access access to channel in rounds each station gets fixed length slot (length = pkt
trans time) in each round unused slots go idle example 6-station LAN 134 have pkt slots
256 idle
1 3 4 1 3 4
6-slot
frame
6-slot
frame
Link Layer 5-22
FDMA frequency division multiple access channel spectrum divided into frequency bands
each station assigned fixed frequency band
unused transmission time in frequency bands go idle
example 6-station LAN 134 have pkt frequency bands 256 idle
frequency b
ands
FDM cable
Channel partitioning MAC protocols FDMA
Link Layer 5-23
Random access protocols
when node has packet to send transmit at full channel data rate R no a priori coordination among nodes
two or more transmitting nodes ldquocollisionrdquo random access MAC protocol specifies
how to detect collisions how to recover from collisions (eg via delayed
retransmissions)
examples of random access MAC protocols slotted ALOHA ALOHA CSMA CSMACD CSMACA
Link Layer 5-24
Slotted ALOHA
assumptions all frames same size
time divided into equal size slots (time to transmit 1 frame)
nodes start to transmit only slot beginning
nodes are synchronized
if 2 or more nodes transmit in slot all nodes detect collision
operation
when node obtains fresh frame transmits in next slot
if no collision node can send new frame in next slot
if collision node retransmits frame in each subsequent slot with prob p until success
Link Layer 5-25
Pros single active node can
continuously transmit at full rate of channel
highly decentralized only slots in nodes need to be in sync
simple
Cons
collisions wasting slots
idle slots
nodes may be able to detect collision in less than time to transmit packet
clock synchronization
Slotted ALOHA
1 1 1 1
2
3
2 2
3 3
node 1
node 2
node 3
C C C S S S E E E
Link Layer 5-26
suppose N nodes with many frames to send each transmits in slot with probability p
prob that given node has success in a slot = p(1-p)N-1
prob that any node has a success = Np(1-p)N-1
max efficiency find p that maximizes Np(1-p)N-1
for many nodes take limit of Np(1-p)N-1 as N goes to infinity gives
max efficiency = 1e = 37
efficiency long-run fraction of successful slots (many nodes all with many frames to send)
at best channel used for useful transmissions 37 of time
Slotted ALOHA efficiency
Link Layer 5-27
Pure (unslotted) ALOHA
unslotted Aloha simpler no synchronization
when frame first arrives
transmit immediately
collision probability increases
frame sent at t0 collides with other frames sent in [t0-1t0+1]
Link Layer 5-28
Pure ALOHA efficiency
P(success by given node) = P(node transmits)
P(no other node transmits in [t0-1t0]
P(no other node transmits in [t0-1t0]
= p (1-p)N-1 (1-p)N-1
= p (1-p)2(N-1)
hellip choosing optimum p and then letting n
= 1(2e) = 18
even worse than slotted Aloha
Link Layer 5-29
CSMA (carrier sense multiple access)
CSMA listen before transmit if channel sensed idle transmit entire frame
if channel sensed busy defer transmission
human analogy donrsquot interrupt others
Link Layer 5-30
CSMA collisions
collisions can still occur propagation delay means two nodes may not hear each otherrsquos transmission
collision entire packet transmission time wasted distance amp propagation
delay play role in in determining collision probability
spatial layout of nodes
Link Layer 5-31
CSMACD (collision detection)
CSMACD carrier sensing deferral as in CSMA collisions detected within short time
colliding transmissions aborted reducing channel wastage
collision detection easy in wired LANs measure signal strengths compare
transmitted received signals
difficult in wireless LANs received signal strength overwhelmed by local transmission strength
human analogy the polite conversationalist
Link Layer 5-32
CSMACD (collision detection)
spatial layout of nodes
Link Layer 5-33
Ethernet CSMACD algorithm
1 NIC receives datagram from network layer creates frame
2 If NIC senses channel idle starts frame transmission If NIC senses channel busy waits until channel idle then transmits
3 If NIC transmits entire frame without detecting another transmission NIC is done with frame
4 If NIC detects another transmission while transmitting aborts and sends jam signal
5 After aborting NIC enters binary (exponential) backoff
after mth collision NIC chooses K at random from 012 hellip 2m-1 NIC waits K512 bit times returns to Step 2
longer backoff interval with more collisions
Link Layer 5-34
CSMACD efficiency
Tprop = max prop delay between 2 nodes in LAN
ttrans = time to transmit max-size frame
efficiency goes to 1
as tprop goes to 0
as ttrans goes to infinity
better performance than ALOHA and simple cheap decentralized
transpropttefficiency
51
1
Link Layer 5-35
ldquoTaking turnsrdquo MAC protocols
channel partitioning MAC protocols share channel efficiently and fairly at high load
inefficient at low load delay in channel access 1N bandwidth allocated even if only 1 active node
random access MAC protocols efficient at low load single node can fully utilize
channel
high load collision overhead
ldquotaking turnsrdquo protocols look for best of both worlds
Link Layer 5-36
polling master node ldquoinvitesrdquo
slave nodes to transmit in turn
typically used with ldquodumbrdquo slave devices
concerns
polling overhead
latency
single point of failure (master)
master
slaves
poll
data
data
ldquoTaking turnsrdquo MAC protocols
Link Layer 5-37
token passing control token passed
from one node to next sequentially
token message
concerns
token overhead
latency
single point of failure (token)
T
data
(nothing
to send)
T
ldquoTaking turnsrdquo MAC protocols
cable headend
CMTS
ISP
cable modem termination system
multiple 40Mbps downstream (broadcast) channels
single CMTS transmits into channels
multiple 30 Mbps upstream channels
multiple access all users contend for certain upstream channel time slots (others assigned)
Cable access network
cable modem
splitter
hellip
hellip
Internet framesTV channels control transmitted downstream at different frequencies
upstream Internet frames TV control transmitted upstream at different frequencies in time slots
Link Layer 5-39
DOCSIS data over cable service interface spec FDM over upstream downstream frequency channels
TDM upstream some slots assigned some have contention
downstream MAP frame assigns upstream slots
request for upstream slots (and data) transmitted random access (binary backoff) in selected slots
MAP frame for
Interval [t1 t2]
Residences with cable modems
Downstream channel i
Upstream channel j
t1 t2
Assigned minislots containing cable modem
upstream data frames
Minislots containing
minislots request frames
cable headend
CMTS
Cable access network
Link Layer 5-40
Summary of MAC protocols
channel partitioning by time frequency or code Time Division Frequency Division
random access (dynamic)
ALOHA S-ALOHA CSMA CSMACD
carrier sensing easy in some technologies (wire) hard in others (wireless)
CSMACD used in Ethernet
CSMACA used in 80211
taking turns
polling from central site token passing
bluetooth FDDI token ring
Link Layer 5-41
Link layer LANs outline
51 introduction services
52 error detection correction
53 multiple access protocols
54 LANs addressing ARP
Ethernet
switches
VLANS
55 link virtualization MPLS
56 data center networking
57 a day in the life of a web request
Link Layer 5-42
MAC addresses and ARP
32-bit IP address network-layer address for interface
used for layer 3 (network layer) forwarding
MAC (or LAN or physical or Ethernet) address function used lsquolocallyrdquo to get frame from one interface to
another physically-connected interface (same network in IP-addressing sense)
48 bit MAC address (for most LANs) burned in NIC ROM also sometimes software settable
eg 1A-2F-BB-76-09-AD
hexadecimal (base 16) notation
(each ldquonumberrdquo represents 4 bits)
Link Layer 5-43
LAN addresses and ARP
each adapter on LAN has unique LAN address
adapter
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN
(wired or
wireless)
Link Layer 5-44
LAN addresses (more)
MAC address allocation administered by IEEE
manufacturer buys portion of MAC address space (to assure uniqueness)
analogy MAC address like Social Security Number
IP address like postal address
MAC flat address portability can move LAN card from one LAN to another
IP hierarchical address not portable address depends on IP subnet to which node is
attached
Link Layer 5-45
ARP address resolution protocol
ARP table each IP node (host router) on LAN has table
IPMAC address mappings for some LAN nodes
lt IP address MAC address TTLgt
TTL (Time To Live) time after which address mapping will be forgotten (typically 20 min)
Question how to determine
interfacersquos MAC address
knowing its IP address
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN
137196723
137196778
137196714
137196788
Link Layer 5-46
ARP protocol same LAN
A wants to send datagram to B Brsquos MAC address not in
Arsquos ARP table
A broadcasts ARP query packet containing Bs IP address dest MAC address = FF-FF-
FF-FF-FF-FF
all nodes on LAN receive ARP query
B receives ARP packet replies to A with its (Bs) MAC address frame sent to Arsquos MAC
address (unicast)
A caches (saves) IP-to-MAC address pair in its ARP table until information becomes old (times out) soft state information that
times out (goes away) unless refreshed
ARP is ldquoplug-and-playrdquo nodes create their ARP
tables without intervention from net administrator
Link Layer 5-47
walkthrough send datagram from A to B via R
focus on addressing ndash at IP (datagram) and MAC layer (frame)
assume A knows Brsquos IP address
assume A knows IP address of first hop router R (how)
assume A knows Rrsquos MAC address (how)
Addressing routing to another LAN
R
1A-23-F9-CD-06-9B 222222222220
111111111110 E6-E9-00-17-BB-4B CC-49-DE-D0-AB-7D
111111111112
111111111111
74-29-9C-E8-FF-55
A
222222222222
49-BD-D2-C7-56-2A
222222222221 88-B2-2F-54-1A-0F
B
R
1A-23-F9-CD-06-9B 222222222220
111111111110 E6-E9-00-17-BB-4B CC-49-DE-D0-AB-7D
111111111112
111111111111
74-29-9C-E8-FF-55
A
222222222222
49-BD-D2-C7-56-2A
222222222221 88-B2-2F-54-1A-0F
B
Link Layer 5-48
Addressing routing to another LAN
IP
Eth
Phy
IP src 111111111111
IP dest 222222222222
A creates IP datagram with IP source A destination B
A creates link-layer frame with Rs MAC address as dest frame contains A-to-B IP datagram
MAC src 74-29-9C-E8-FF-55
MAC dest E6-E9-00-17-BB-4B
R
1A-23-F9-CD-06-9B 222222222220
111111111110 E6-E9-00-17-BB-4B CC-49-DE-D0-AB-7D
111111111112
111111111111
74-29-9C-E8-FF-55
A
222222222222
49-BD-D2-C7-56-2A
222222222221 88-B2-2F-54-1A-0F
B
Link Layer 5-49
Addressing routing to another LAN
IP
Eth
Phy
frame sent from A to R
IP
Eth
Phy
frame received at R datagram removed passed up to IP
MAC src 74-29-9C-E8-FF-55
MAC dest E6-E9-00-17-BB-4B
IP src 111111111111
IP dest 222222222222
IP src 111111111111
IP dest 222222222222
R
1A-23-F9-CD-06-9B 222222222220
111111111110 E6-E9-00-17-BB-4B CC-49-DE-D0-AB-7D
111111111112
111111111111
74-29-9C-E8-FF-55
A
222222222222
49-BD-D2-C7-56-2A
222222222221 88-B2-2F-54-1A-0F
B
Link Layer 5-50
Addressing routing to another LAN
IP src 111111111111
IP dest 222222222222
R forwards datagram with IP source A destination B
R creates link-layer frame with Bs MAC address as dest frame contains A-to-B IP datagram
MAC src 1A-23-F9-CD-06-9B
MAC dest 49-BD-D2-C7-56-2A
IP
Eth
Phy
IP
Eth
Phy
R
1A-23-F9-CD-06-9B 222222222220
111111111110 E6-E9-00-17-BB-4B CC-49-DE-D0-AB-7D
111111111112
111111111111
74-29-9C-E8-FF-55
A
222222222222
49-BD-D2-C7-56-2A
222222222221 88-B2-2F-54-1A-0F
B
Link Layer 5-51
Addressing routing to another LAN R forwards datagram with IP source A destination B
R creates link-layer frame with Bs MAC address as dest frame contains A-to-B IP datagram
IP src 111111111111
IP dest 222222222222
MAC src 1A-23-F9-CD-06-9B
MAC dest 49-BD-D2-C7-56-2A
IP
Eth
Phy
IP
Eth
Phy
R
1A-23-F9-CD-06-9B 222222222220
111111111110 E6-E9-00-17-BB-4B CC-49-DE-D0-AB-7D
111111111112
111111111111
74-29-9C-E8-FF-55
A
222222222222
49-BD-D2-C7-56-2A
222222222221 88-B2-2F-54-1A-0F
B
Link Layer 5-52
Addressing routing to another LAN R forwards datagram with IP source A destination B
R creates link-layer frame with Bs MAC address as dest frame contains A-to-B IP datagram
IP src 111111111111
IP dest 222222222222
MAC src 1A-23-F9-CD-06-9B
MAC dest 49-BD-D2-C7-56-2A
IP
Eth
Phy
Link Layer 5-53
Link layer LANs outline
51 introduction services
52 error detection correction
53 multiple access protocols
54 LANs addressing ARP
Ethernet
switches
VLANS
55 link virtualization MPLS
56 data center networking
57 a day in the life of a web request
Link Layer 5-54
Ethernet
ldquodominantrdquo wired LAN technology
cheap $20 for NIC
first widely used LAN technology
simpler cheaper than token LANs and ATM
kept up with speed race 10 Mbps ndash 10 Gbps
Metcalfersquos Ethernet sketch
Link Layer 5-55
Ethernet physical topology
bus popular through mid 90s all nodes in same collision domain (can collide with each
other)
star prevails today active switch in center
each ldquospokerdquo runs a (separate) Ethernet protocol (nodes
do not collide with each other)
switch
bus coaxial cable star
Link Layer 5-56
Ethernet frame structure
sending adapter encapsulates IP datagram (or other network layer protocol packet) in Ethernet frame
preamble
7 bytes with pattern 10101010 followed by one byte with pattern 10101011
used to synchronize receiver sender clock rates
dest address
source address
data (payload) CRC preamble
type
Link Layer 5-57
Ethernet frame structure (more)
addresses 6 byte source destination MAC addresses if adapter receives frame with matching destination
address or with broadcast address (eg ARP packet) it passes data in frame to network layer protocol
otherwise adapter discards frame
type indicates higher layer protocol (mostly IP but others possible eg Novell IPX AppleTalk)
CRC cyclic redundancy check at receiver error detected frame is dropped
dest address
source address
data (payload) CRC preamble
type
Link Layer 5-58
Ethernet unreliable connectionless
connectionless no handshaking between sending and receiving NICs
unreliable receiving NIC doesnt send acks or nacks to sending NIC
data in dropped frames recovered only if initial sender uses higher layer rdt (eg TCP) otherwise dropped data lost
Ethernetrsquos MAC protocol unslotted CSMACD wth binary backoff
Link Layer 5-59
8023 Ethernet standards link amp physical layers
many different Ethernet standards
common MAC protocol and frame format
different speeds 2 Mbps 10 Mbps 100 Mbps 1Gbps 10G bps
different physical layer media fiber cable
application
transport
network
link
physical
MAC protocol
and frame format
100BASE-TX
100BASE-T4
100BASE-FX 100BASE-T2
100BASE-SX 100BASE-BX
fiber physical layer copper (twister
pair) physical layer
Link Layer 5-60
Link layer LANs outline
51 introduction services
52 error detection correction
53 multiple access protocols
54 LANs addressing ARP
Ethernet
switches
VLANS
55 link virtualization MPLS
56 data center networking
57 a day in the life of a web request
Link Layer 5-61
Ethernet switch link-layer device takes an active role
store forward Ethernet frames
examine incoming framersquos MAC address selectively forward frame to one-or-more outgoing links when frame is to be forwarded on segment uses CSMACD to access segment
transparent
hosts are unaware of presence of switches
plug-and-play self-learning
switches do not need to be configured
Link Layer 5-62
Switch multiple simultaneous transmissions
hosts have dedicated direct connection to switch
switches buffer packets
Ethernet protocol used on each incoming link but no collisions full duplex
each link is its own collision domain
switching A-to-Arsquo and B-to-Brsquo can transmit simultaneously without collisions switch with six interfaces
(123456)
A
Arsquo
B
Brsquo C
Crsquo
1 2
3 4 5
6
Link Layer 5-63
Switch forwarding table
Q how does switch know Arsquo reachable via interface 4 Brsquo reachable via interface 5
switch with six interfaces
(123456)
A
Arsquo
B
Brsquo C
Crsquo
1 2
3 4 5
6 A each switch has a switch table each entry
(MAC address of host interface to
reach host time stamp)
looks like a routing table
Q how are entries created maintained in switch table
something like a routing protocol
A
Arsquo
B
Brsquo C
Crsquo
1 2
3 4 5
6
Link Layer 5-64
Switch self-learning
switch learns which hosts can be reached through which interfaces
when frame received switch ldquolearnsrdquo location of sender incoming LAN segment
records senderlocation pair in switch table
A Arsquo
Source A
Dest Arsquo
MAC addr interface TTL
Switch table
(initially empty) A 1 60
Link Layer 5-65
Switch frame filteringforwarding
when frame received at switch 1 record incoming link MAC address of sending host
2 index switch table using MAC destination address
3 if entry found for destination then
if destination on segment from which frame arrived then drop frame
else forward frame on interface indicated by entry
else flood forward on all interfaces except arriving
interface
A
Arsquo
B
Brsquo C
Crsquo
1 2
3 4 5
6
Link Layer 5-66
Self-learning forwarding example
A Arsquo
Source A
Dest Arsquo
MAC addr interface TTL
switch table
(initially empty) A 1 60
A Arsquo A Arsquo A Arsquo A Arsquo A Arsquo
frame destination Arsquo locaton unknown flood
Arsquo A
destination A location
known
Arsquo 4 60
selectively send
on just one link
Link Layer 5-67
Interconnecting switches
switches can be connected together
Q sending from A to G - how does S1 know to forward frame destined to F via S4 and S3
A self learning (works exactly the same as in single-switch case)
A
B
S1
C D
E
F
S2
S4
S3
H
I
G
Link Layer 5-68
Self-learning multi-switch example
Suppose C sends frame to I I responds to C
Q show switch tables and packet forwarding in S1 S2 S3 S4
A
B
S1
C D
E
F
S2
S4
S3
H
I
G
Link Layer 5-69
Institutional network
to external
network
router
IP subnet
mail server
web server
Link Layer 5-70
Switches vs routers
both are store-and-forward
routers network-layer devices (examine network-layer headers)
switches link-layer devices (examine link-layer headers)
both have forwarding tables
routers compute tables using routing algorithms IP addresses
switches learn forwarding table using flooding learning MAC addresses
application
transport
network
link
physical
network
link
physical
link
physical
switch
datagram
application
transport
network
link
physical
frame
frame
frame
datagram
Link Layer 5-71
VLANs motivation
consider CS user moves office to
EE but wants connect to CS switch
single broadcast domain
all layer-2 broadcast traffic (ARP DHCP unknown location of destination MAC address) must cross entire LAN
securityprivacy efficiency issues
Computer
Science Electrical
Engineering
Computer
Engineering
Link Layer 5-72
VLANs port-based VLAN switch ports
grouped (by switch management software) so that single physical switch helliphellip
switch(es) supporting
VLAN capabilities can
be configured to
define multiple virtual
LANS over single
physical LAN
infrastructure
Virtual Local
Area Network 1
8
9
16 10 2
7
hellip
Electrical Engineering
(VLAN ports 1-8)
Computer Science
(VLAN ports 9-15)
15
hellip
Electrical Engineering
(VLAN ports 1-8)
hellip
1
8 2
7 9
16 10
15
hellip
Computer Science
(VLAN ports 9-16)
hellip operates as multiple virtual switches
Link Layer 5-73
Port-based VLAN
1
8
9
16 10 2
7
hellip
Electrical Engineering
(VLAN ports 1-8)
Computer Science
(VLAN ports 9-15)
15
hellip
traffic isolation frames tofrom ports 1-8 can only reach ports 1-8 can also define VLAN based on
MAC addresses of endpoints rather than switch port
dynamic membership ports can be dynamically assigned among VLANs
router
forwarding between VLANS done via routing (just as with separate switches) in practice vendors sell combined
switches plus routers
Link Layer 5-74
VLANS spanning multiple switches
trunk port carries frames between VLANS defined over multiple physical switches frames forwarded within VLAN between switches canrsquot be vanilla
8021 frames (must carry VLAN ID info)
8021q protocol addsremoved additional header fields for frames forwarded between trunk ports
1
8
9
10 2
7
hellip
Electrical Engineering
(VLAN ports 1-8)
Computer Science
(VLAN ports 9-15)
15
hellip
2
7 3
Ports 235 belong to EE VLAN
Ports 4678 belong to CS VLAN
5
4 6 8 16
1
Link Layer 5-75
type
2-byte Tag Protocol Identifier
(value 81-00)
Tag Control Information (12 bit VLAN ID field
3 bit priority field like IP TOS)
Recomputed CRC
8021Q VLAN frame format
8021 frame
8021Q frame
dest address
source address
data (payload) CRC preamble
dest address
source address
preamble data (payload) CRC
type
Link Layer 5-76
Link layer LANs outline
51 introduction services
52 error detection correction
53 multiple access protocols
54 LANs addressing ARP
Ethernet
switches
VLANS
55 link virtualization MPLS
56 data center networking
57 a day in the life of a web request
Link Layer 5-77
Multiprotocol label switching (MPLS)
initial goal high-speed IP forwarding using fixed length label (instead of IP address) fast lookup using fixed length identifier (rather than
shortest prefix matching)
borrowing ideas from Virtual Circuit (VC) approach
but IP datagram still keeps IP address
PPP or Ethernet
header IP header remainder of link-layer frame MPLS header
label Exp S TTL
20 3 1 5
Link Layer 5-78
MPLS capable routers
aka label-switched router
forward packets to outgoing interface based only on label value (donrsquot inspect IP address) MPLS forwarding table distinct from IP forwarding tables
flexibility MPLS forwarding decisions can differ from those of IP use destination and source addresses to route flows to
same destination differently (traffic engineering)
re-route flows quickly if link fails pre-computed backup paths (useful for VoIP)
Link Layer 5-79
R2
D
R3 R5
A
R6
MPLS versus IP paths
IP router IP routing path to destination determined
by destination address alone
R4
Link Layer 5-80
R2
D
R3 R4
R5
A
R6
MPLS versus IP paths
IP-only router
IP routing path to destination determined by destination address alone
MPLS and IP router
MPLS routing path to destination can be based on source and dest address fast reroute precompute backup routes in
case of link failure
entry router (R4) can use different MPLS
routes to A based eg on source address
Link Layer 5-81
MPLS signaling
modify OSPF IS-IS link-state flooding protocols to carry info used by MPLS routing eg link bandwidth amount of ldquoreservedrdquo link bandwidth
D
R4
R5
A
R6
entry MPLS router uses RSVP-TE signaling protocol to set up MPLS forwarding at downstream routers
modified link state flooding
RSVP-TE
Link Layer 5-82
R1 R2
D
R3 R4 R5
0
1
0 0
A
R6
in out out
label label dest interface
6 - A 0
in out out
label label dest interface
10 6 A 1
12 9 D 0
in out out
label label dest interface
10 A 0
12 D 0
1
in out out
label label dest interface
8 6 A 0
0
8 A 1
MPLS forwarding tables
Link Layer 5-83
Link layer LANs outline
51 introduction services
52 error detection correction
53 multiple access protocols
54 LANs addressing ARP
Ethernet
switches
VLANS
55 link virtualization MPLS
56 data center networking
57 a day in the life of a web request
Link Layer 5-84
Data center networks
10rsquos to 100rsquos of thousands of hosts often closely coupled in close proximity e-business (eg Amazon)
content-servers (eg YouTube Akamai Apple Microsoft)
search engines data mining (eg Google)
challenges
multiple applications each serving massive numbers of clients
managingbalancing load avoiding processing networking data bottlenecks
Inside a 40-ft Microsoft container
Chicago data center
Link Layer 5-85
Server racks
TOR switches
Tier-1 switches
Tier-2 switches
Load balancer
Load balancer
B
1 2 3 4 5 6 7 8
A C
Border router
Access router
Internet
Data center networks
load balancer application-layer routing receives external client requests
directs workload within data center
returns results to external client (hiding data
center internals from client)
Server racks
TOR switches
Tier-1 switches
Tier-2 switches
1 2 3 4 5 6 7 8
Data center networks
rich interconnection among switches racks
increased throughput between racks (multiple routing
paths possible)
increased reliability via redundancy
Link Layer 5-87
Link layer LANs outline
51 introduction services
52 error detection correction
53 multiple access protocols
54 LANs addressing ARP
Ethernet
switches
VLANS
55 link virtualization MPLS
56 data center networking
57 a day in the life of a web request
Link Layer 5-88
Synthesis a day in the life of a web request
journey down protocol stack complete application transport network link
putting-it-all-together synthesis goal identify review understand protocols (at all
layers) involved in seemingly simple scenario requesting www page
scenario student attaches laptop to campus network requestsreceives wwwgooglecom
Link Layer 5-89
A day in the life scenario
Comcast network
68800013
Googlersquos network
64233160019 64233169105
web server
DNS server
school network
68802024
web page
browser
router
(runs DHCP)
Link Layer 5-90
A day in the lifehellip connecting to the Internet
connecting laptop needs to get its own IP address addr of first-hop router addr of DNS server use DHCP
DHCP
UDP
IP
Eth
Phy
DHCP
DHCP
DHCP
DHCP
DHCP
DHCP
UDP
IP
Eth
Phy
DHCP
DHCP
DHCP
DHCP DHCP
DHCP request encapsulated in UDP encapsulated in IP encapsulated in 8023 Ethernet
Ethernet frame broadcast
(dest FFFFFFFFFFFF) on LAN received at router running DHCP server
Ethernet demuxed to IP demuxed UDP demuxed to DHCP
router
(runs DHCP)
Link Layer 5-91
DHCP server formulates DHCP ACK containing clientrsquos IP address IP address of first-hop router for client name amp IP address of DNS server
DHCP
UDP
IP
Eth
Phy
DHCP
DHCP
DHCP
DHCP
DHCP
UDP
IP
Eth
Phy
DHCP
DHCP
DHCP
DHCP
DHCP
encapsulation at DHCP server frame forwarded (switch learning) through LAN demultiplexing at client
Client now has IP address knows name amp addr of DNS
server IP address of its first-hop router
DHCP client receives DHCP ACK reply
A day in the lifehellip connecting to the Internet
router
(runs DHCP)
Link Layer 5-92
A day in the lifehellip ARP (before DNS before HTTP)
before sending HTTP request need IP address of wwwgooglecom DNS
DNS
UDP
IP
Eth
Phy
DNS
DNS
DNS
DNS query created encapsulated in UDP encapsulated in IP encapsulated in Eth To send frame to router need MAC address of router interface ARP
ARP query broadcast received by
router which replies with ARP reply giving MAC address of router interface
client now knows MAC address of first hop router so can now send frame containing DNS query
ARP query
Eth
Phy
ARP
ARP
ARP reply
router
(runs DHCP)
Link Layer 5-93
DNS
UDP
IP
Eth
Phy
DNS
DNS
DNS
DNS
DNS
IP datagram containing DNS query forwarded via LAN switch from client to 1st hop router
IP datagram forwarded from campus network into comcast network routed (tables created by RIP OSPF IS-IS andor BGP routing protocols) to DNS server
demuxrsquoed to DNS server
DNS server replies to client with IP address of wwwgooglecom
Comcast network
68800013
DNS server
DNS
UDP
IP
Eth
Phy
DNS
DNS
DNS
DNS
A day in the lifehellip using DNS
router
(runs DHCP)
Link Layer 5-94
A day in the lifehellipTCP connection carrying HTTP
HTTP
TCP
IP
Eth
Phy
HTTP
to send HTTP request client first opens TCP socket to web server
TCP SYN segment (step 1 in 3-way handshake) inter-domain routed to web server
TCP connection established 64233169105
web server
SYN
SYN
SYN
SYN
TCP
IP
Eth
Phy
SYN
SYN
SYN
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
web server responds with TCP SYNACK (step 2 in 3-way handshake)
router
(runs DHCP)
Link Layer 5-95
A day in the lifehellip HTTP requestreply
HTTP
TCP
IP
Eth
Phy
HTTP
HTTP request sent into TCP socket
IP datagram containing HTTP request routed to wwwgooglecom
IP datagram containing HTTP reply routed back to client
64233169105
web server
HTTP
TCP
IP
Eth
Phy web server responds with
HTTP reply (containing web page)
HTTP
HTTP
HTTP HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
web page finally () displayed
Link Layer 5-96
Chapter 5 Summary
principles behind data link layer services error detection correction
sharing a broadcast channel multiple access
link layer addressing
instantiation and implementation of various link layer technologies Ethernet
switched LANS VLANs
virtualized networks as a link layer MPLS
synthesis a day in the life of a web request
Link Layer 5-97
Chapter 5 letrsquos take a breath
journey down protocol stack complete (except PHY)
solid understanding of networking principles practice
hellip could stop here hellip but lots of interesting topics wireless
multimedia
security
network management
Link Layer 5-22
FDMA frequency division multiple access channel spectrum divided into frequency bands
each station assigned fixed frequency band
unused transmission time in frequency bands go idle
example 6-station LAN 134 have pkt frequency bands 256 idle
frequency b
ands
FDM cable
Channel partitioning MAC protocols FDMA
Link Layer 5-23
Random access protocols
when node has packet to send transmit at full channel data rate R no a priori coordination among nodes
two or more transmitting nodes ldquocollisionrdquo random access MAC protocol specifies
how to detect collisions how to recover from collisions (eg via delayed
retransmissions)
examples of random access MAC protocols slotted ALOHA ALOHA CSMA CSMACD CSMACA
Link Layer 5-24
Slotted ALOHA
assumptions all frames same size
time divided into equal size slots (time to transmit 1 frame)
nodes start to transmit only slot beginning
nodes are synchronized
if 2 or more nodes transmit in slot all nodes detect collision
operation
when node obtains fresh frame transmits in next slot
if no collision node can send new frame in next slot
if collision node retransmits frame in each subsequent slot with prob p until success
Link Layer 5-25
Pros single active node can
continuously transmit at full rate of channel
highly decentralized only slots in nodes need to be in sync
simple
Cons
collisions wasting slots
idle slots
nodes may be able to detect collision in less than time to transmit packet
clock synchronization
Slotted ALOHA
1 1 1 1
2
3
2 2
3 3
node 1
node 2
node 3
C C C S S S E E E
Link Layer 5-26
suppose N nodes with many frames to send each transmits in slot with probability p
prob that given node has success in a slot = p(1-p)N-1
prob that any node has a success = Np(1-p)N-1
max efficiency find p that maximizes Np(1-p)N-1
for many nodes take limit of Np(1-p)N-1 as N goes to infinity gives
max efficiency = 1e = 37
efficiency long-run fraction of successful slots (many nodes all with many frames to send)
at best channel used for useful transmissions 37 of time
Slotted ALOHA efficiency
Link Layer 5-27
Pure (unslotted) ALOHA
unslotted Aloha simpler no synchronization
when frame first arrives
transmit immediately
collision probability increases
frame sent at t0 collides with other frames sent in [t0-1t0+1]
Link Layer 5-28
Pure ALOHA efficiency
P(success by given node) = P(node transmits)
P(no other node transmits in [t0-1t0]
P(no other node transmits in [t0-1t0]
= p (1-p)N-1 (1-p)N-1
= p (1-p)2(N-1)
hellip choosing optimum p and then letting n
= 1(2e) = 18
even worse than slotted Aloha
Link Layer 5-29
CSMA (carrier sense multiple access)
CSMA listen before transmit if channel sensed idle transmit entire frame
if channel sensed busy defer transmission
human analogy donrsquot interrupt others
Link Layer 5-30
CSMA collisions
collisions can still occur propagation delay means two nodes may not hear each otherrsquos transmission
collision entire packet transmission time wasted distance amp propagation
delay play role in in determining collision probability
spatial layout of nodes
Link Layer 5-31
CSMACD (collision detection)
CSMACD carrier sensing deferral as in CSMA collisions detected within short time
colliding transmissions aborted reducing channel wastage
collision detection easy in wired LANs measure signal strengths compare
transmitted received signals
difficult in wireless LANs received signal strength overwhelmed by local transmission strength
human analogy the polite conversationalist
Link Layer 5-32
CSMACD (collision detection)
spatial layout of nodes
Link Layer 5-33
Ethernet CSMACD algorithm
1 NIC receives datagram from network layer creates frame
2 If NIC senses channel idle starts frame transmission If NIC senses channel busy waits until channel idle then transmits
3 If NIC transmits entire frame without detecting another transmission NIC is done with frame
4 If NIC detects another transmission while transmitting aborts and sends jam signal
5 After aborting NIC enters binary (exponential) backoff
after mth collision NIC chooses K at random from 012 hellip 2m-1 NIC waits K512 bit times returns to Step 2
longer backoff interval with more collisions
Link Layer 5-34
CSMACD efficiency
Tprop = max prop delay between 2 nodes in LAN
ttrans = time to transmit max-size frame
efficiency goes to 1
as tprop goes to 0
as ttrans goes to infinity
better performance than ALOHA and simple cheap decentralized
transpropttefficiency
51
1
Link Layer 5-35
ldquoTaking turnsrdquo MAC protocols
channel partitioning MAC protocols share channel efficiently and fairly at high load
inefficient at low load delay in channel access 1N bandwidth allocated even if only 1 active node
random access MAC protocols efficient at low load single node can fully utilize
channel
high load collision overhead
ldquotaking turnsrdquo protocols look for best of both worlds
Link Layer 5-36
polling master node ldquoinvitesrdquo
slave nodes to transmit in turn
typically used with ldquodumbrdquo slave devices
concerns
polling overhead
latency
single point of failure (master)
master
slaves
poll
data
data
ldquoTaking turnsrdquo MAC protocols
Link Layer 5-37
token passing control token passed
from one node to next sequentially
token message
concerns
token overhead
latency
single point of failure (token)
T
data
(nothing
to send)
T
ldquoTaking turnsrdquo MAC protocols
cable headend
CMTS
ISP
cable modem termination system
multiple 40Mbps downstream (broadcast) channels
single CMTS transmits into channels
multiple 30 Mbps upstream channels
multiple access all users contend for certain upstream channel time slots (others assigned)
Cable access network
cable modem
splitter
hellip
hellip
Internet framesTV channels control transmitted downstream at different frequencies
upstream Internet frames TV control transmitted upstream at different frequencies in time slots
Link Layer 5-39
DOCSIS data over cable service interface spec FDM over upstream downstream frequency channels
TDM upstream some slots assigned some have contention
downstream MAP frame assigns upstream slots
request for upstream slots (and data) transmitted random access (binary backoff) in selected slots
MAP frame for
Interval [t1 t2]
Residences with cable modems
Downstream channel i
Upstream channel j
t1 t2
Assigned minislots containing cable modem
upstream data frames
Minislots containing
minislots request frames
cable headend
CMTS
Cable access network
Link Layer 5-40
Summary of MAC protocols
channel partitioning by time frequency or code Time Division Frequency Division
random access (dynamic)
ALOHA S-ALOHA CSMA CSMACD
carrier sensing easy in some technologies (wire) hard in others (wireless)
CSMACD used in Ethernet
CSMACA used in 80211
taking turns
polling from central site token passing
bluetooth FDDI token ring
Link Layer 5-41
Link layer LANs outline
51 introduction services
52 error detection correction
53 multiple access protocols
54 LANs addressing ARP
Ethernet
switches
VLANS
55 link virtualization MPLS
56 data center networking
57 a day in the life of a web request
Link Layer 5-42
MAC addresses and ARP
32-bit IP address network-layer address for interface
used for layer 3 (network layer) forwarding
MAC (or LAN or physical or Ethernet) address function used lsquolocallyrdquo to get frame from one interface to
another physically-connected interface (same network in IP-addressing sense)
48 bit MAC address (for most LANs) burned in NIC ROM also sometimes software settable
eg 1A-2F-BB-76-09-AD
hexadecimal (base 16) notation
(each ldquonumberrdquo represents 4 bits)
Link Layer 5-43
LAN addresses and ARP
each adapter on LAN has unique LAN address
adapter
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN
(wired or
wireless)
Link Layer 5-44
LAN addresses (more)
MAC address allocation administered by IEEE
manufacturer buys portion of MAC address space (to assure uniqueness)
analogy MAC address like Social Security Number
IP address like postal address
MAC flat address portability can move LAN card from one LAN to another
IP hierarchical address not portable address depends on IP subnet to which node is
attached
Link Layer 5-45
ARP address resolution protocol
ARP table each IP node (host router) on LAN has table
IPMAC address mappings for some LAN nodes
lt IP address MAC address TTLgt
TTL (Time To Live) time after which address mapping will be forgotten (typically 20 min)
Question how to determine
interfacersquos MAC address
knowing its IP address
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN
137196723
137196778
137196714
137196788
Link Layer 5-46
ARP protocol same LAN
A wants to send datagram to B Brsquos MAC address not in
Arsquos ARP table
A broadcasts ARP query packet containing Bs IP address dest MAC address = FF-FF-
FF-FF-FF-FF
all nodes on LAN receive ARP query
B receives ARP packet replies to A with its (Bs) MAC address frame sent to Arsquos MAC
address (unicast)
A caches (saves) IP-to-MAC address pair in its ARP table until information becomes old (times out) soft state information that
times out (goes away) unless refreshed
ARP is ldquoplug-and-playrdquo nodes create their ARP
tables without intervention from net administrator
Link Layer 5-47
walkthrough send datagram from A to B via R
focus on addressing ndash at IP (datagram) and MAC layer (frame)
assume A knows Brsquos IP address
assume A knows IP address of first hop router R (how)
assume A knows Rrsquos MAC address (how)
Addressing routing to another LAN
R
1A-23-F9-CD-06-9B 222222222220
111111111110 E6-E9-00-17-BB-4B CC-49-DE-D0-AB-7D
111111111112
111111111111
74-29-9C-E8-FF-55
A
222222222222
49-BD-D2-C7-56-2A
222222222221 88-B2-2F-54-1A-0F
B
R
1A-23-F9-CD-06-9B 222222222220
111111111110 E6-E9-00-17-BB-4B CC-49-DE-D0-AB-7D
111111111112
111111111111
74-29-9C-E8-FF-55
A
222222222222
49-BD-D2-C7-56-2A
222222222221 88-B2-2F-54-1A-0F
B
Link Layer 5-48
Addressing routing to another LAN
IP
Eth
Phy
IP src 111111111111
IP dest 222222222222
A creates IP datagram with IP source A destination B
A creates link-layer frame with Rs MAC address as dest frame contains A-to-B IP datagram
MAC src 74-29-9C-E8-FF-55
MAC dest E6-E9-00-17-BB-4B
R
1A-23-F9-CD-06-9B 222222222220
111111111110 E6-E9-00-17-BB-4B CC-49-DE-D0-AB-7D
111111111112
111111111111
74-29-9C-E8-FF-55
A
222222222222
49-BD-D2-C7-56-2A
222222222221 88-B2-2F-54-1A-0F
B
Link Layer 5-49
Addressing routing to another LAN
IP
Eth
Phy
frame sent from A to R
IP
Eth
Phy
frame received at R datagram removed passed up to IP
MAC src 74-29-9C-E8-FF-55
MAC dest E6-E9-00-17-BB-4B
IP src 111111111111
IP dest 222222222222
IP src 111111111111
IP dest 222222222222
R
1A-23-F9-CD-06-9B 222222222220
111111111110 E6-E9-00-17-BB-4B CC-49-DE-D0-AB-7D
111111111112
111111111111
74-29-9C-E8-FF-55
A
222222222222
49-BD-D2-C7-56-2A
222222222221 88-B2-2F-54-1A-0F
B
Link Layer 5-50
Addressing routing to another LAN
IP src 111111111111
IP dest 222222222222
R forwards datagram with IP source A destination B
R creates link-layer frame with Bs MAC address as dest frame contains A-to-B IP datagram
MAC src 1A-23-F9-CD-06-9B
MAC dest 49-BD-D2-C7-56-2A
IP
Eth
Phy
IP
Eth
Phy
R
1A-23-F9-CD-06-9B 222222222220
111111111110 E6-E9-00-17-BB-4B CC-49-DE-D0-AB-7D
111111111112
111111111111
74-29-9C-E8-FF-55
A
222222222222
49-BD-D2-C7-56-2A
222222222221 88-B2-2F-54-1A-0F
B
Link Layer 5-51
Addressing routing to another LAN R forwards datagram with IP source A destination B
R creates link-layer frame with Bs MAC address as dest frame contains A-to-B IP datagram
IP src 111111111111
IP dest 222222222222
MAC src 1A-23-F9-CD-06-9B
MAC dest 49-BD-D2-C7-56-2A
IP
Eth
Phy
IP
Eth
Phy
R
1A-23-F9-CD-06-9B 222222222220
111111111110 E6-E9-00-17-BB-4B CC-49-DE-D0-AB-7D
111111111112
111111111111
74-29-9C-E8-FF-55
A
222222222222
49-BD-D2-C7-56-2A
222222222221 88-B2-2F-54-1A-0F
B
Link Layer 5-52
Addressing routing to another LAN R forwards datagram with IP source A destination B
R creates link-layer frame with Bs MAC address as dest frame contains A-to-B IP datagram
IP src 111111111111
IP dest 222222222222
MAC src 1A-23-F9-CD-06-9B
MAC dest 49-BD-D2-C7-56-2A
IP
Eth
Phy
Link Layer 5-53
Link layer LANs outline
51 introduction services
52 error detection correction
53 multiple access protocols
54 LANs addressing ARP
Ethernet
switches
VLANS
55 link virtualization MPLS
56 data center networking
57 a day in the life of a web request
Link Layer 5-54
Ethernet
ldquodominantrdquo wired LAN technology
cheap $20 for NIC
first widely used LAN technology
simpler cheaper than token LANs and ATM
kept up with speed race 10 Mbps ndash 10 Gbps
Metcalfersquos Ethernet sketch
Link Layer 5-55
Ethernet physical topology
bus popular through mid 90s all nodes in same collision domain (can collide with each
other)
star prevails today active switch in center
each ldquospokerdquo runs a (separate) Ethernet protocol (nodes
do not collide with each other)
switch
bus coaxial cable star
Link Layer 5-56
Ethernet frame structure
sending adapter encapsulates IP datagram (or other network layer protocol packet) in Ethernet frame
preamble
7 bytes with pattern 10101010 followed by one byte with pattern 10101011
used to synchronize receiver sender clock rates
dest address
source address
data (payload) CRC preamble
type
Link Layer 5-57
Ethernet frame structure (more)
addresses 6 byte source destination MAC addresses if adapter receives frame with matching destination
address or with broadcast address (eg ARP packet) it passes data in frame to network layer protocol
otherwise adapter discards frame
type indicates higher layer protocol (mostly IP but others possible eg Novell IPX AppleTalk)
CRC cyclic redundancy check at receiver error detected frame is dropped
dest address
source address
data (payload) CRC preamble
type
Link Layer 5-58
Ethernet unreliable connectionless
connectionless no handshaking between sending and receiving NICs
unreliable receiving NIC doesnt send acks or nacks to sending NIC
data in dropped frames recovered only if initial sender uses higher layer rdt (eg TCP) otherwise dropped data lost
Ethernetrsquos MAC protocol unslotted CSMACD wth binary backoff
Link Layer 5-59
8023 Ethernet standards link amp physical layers
many different Ethernet standards
common MAC protocol and frame format
different speeds 2 Mbps 10 Mbps 100 Mbps 1Gbps 10G bps
different physical layer media fiber cable
application
transport
network
link
physical
MAC protocol
and frame format
100BASE-TX
100BASE-T4
100BASE-FX 100BASE-T2
100BASE-SX 100BASE-BX
fiber physical layer copper (twister
pair) physical layer
Link Layer 5-60
Link layer LANs outline
51 introduction services
52 error detection correction
53 multiple access protocols
54 LANs addressing ARP
Ethernet
switches
VLANS
55 link virtualization MPLS
56 data center networking
57 a day in the life of a web request
Link Layer 5-61
Ethernet switch link-layer device takes an active role
store forward Ethernet frames
examine incoming framersquos MAC address selectively forward frame to one-or-more outgoing links when frame is to be forwarded on segment uses CSMACD to access segment
transparent
hosts are unaware of presence of switches
plug-and-play self-learning
switches do not need to be configured
Link Layer 5-62
Switch multiple simultaneous transmissions
hosts have dedicated direct connection to switch
switches buffer packets
Ethernet protocol used on each incoming link but no collisions full duplex
each link is its own collision domain
switching A-to-Arsquo and B-to-Brsquo can transmit simultaneously without collisions switch with six interfaces
(123456)
A
Arsquo
B
Brsquo C
Crsquo
1 2
3 4 5
6
Link Layer 5-63
Switch forwarding table
Q how does switch know Arsquo reachable via interface 4 Brsquo reachable via interface 5
switch with six interfaces
(123456)
A
Arsquo
B
Brsquo C
Crsquo
1 2
3 4 5
6 A each switch has a switch table each entry
(MAC address of host interface to
reach host time stamp)
looks like a routing table
Q how are entries created maintained in switch table
something like a routing protocol
A
Arsquo
B
Brsquo C
Crsquo
1 2
3 4 5
6
Link Layer 5-64
Switch self-learning
switch learns which hosts can be reached through which interfaces
when frame received switch ldquolearnsrdquo location of sender incoming LAN segment
records senderlocation pair in switch table
A Arsquo
Source A
Dest Arsquo
MAC addr interface TTL
Switch table
(initially empty) A 1 60
Link Layer 5-65
Switch frame filteringforwarding
when frame received at switch 1 record incoming link MAC address of sending host
2 index switch table using MAC destination address
3 if entry found for destination then
if destination on segment from which frame arrived then drop frame
else forward frame on interface indicated by entry
else flood forward on all interfaces except arriving
interface
A
Arsquo
B
Brsquo C
Crsquo
1 2
3 4 5
6
Link Layer 5-66
Self-learning forwarding example
A Arsquo
Source A
Dest Arsquo
MAC addr interface TTL
switch table
(initially empty) A 1 60
A Arsquo A Arsquo A Arsquo A Arsquo A Arsquo
frame destination Arsquo locaton unknown flood
Arsquo A
destination A location
known
Arsquo 4 60
selectively send
on just one link
Link Layer 5-67
Interconnecting switches
switches can be connected together
Q sending from A to G - how does S1 know to forward frame destined to F via S4 and S3
A self learning (works exactly the same as in single-switch case)
A
B
S1
C D
E
F
S2
S4
S3
H
I
G
Link Layer 5-68
Self-learning multi-switch example
Suppose C sends frame to I I responds to C
Q show switch tables and packet forwarding in S1 S2 S3 S4
A
B
S1
C D
E
F
S2
S4
S3
H
I
G
Link Layer 5-69
Institutional network
to external
network
router
IP subnet
mail server
web server
Link Layer 5-70
Switches vs routers
both are store-and-forward
routers network-layer devices (examine network-layer headers)
switches link-layer devices (examine link-layer headers)
both have forwarding tables
routers compute tables using routing algorithms IP addresses
switches learn forwarding table using flooding learning MAC addresses
application
transport
network
link
physical
network
link
physical
link
physical
switch
datagram
application
transport
network
link
physical
frame
frame
frame
datagram
Link Layer 5-71
VLANs motivation
consider CS user moves office to
EE but wants connect to CS switch
single broadcast domain
all layer-2 broadcast traffic (ARP DHCP unknown location of destination MAC address) must cross entire LAN
securityprivacy efficiency issues
Computer
Science Electrical
Engineering
Computer
Engineering
Link Layer 5-72
VLANs port-based VLAN switch ports
grouped (by switch management software) so that single physical switch helliphellip
switch(es) supporting
VLAN capabilities can
be configured to
define multiple virtual
LANS over single
physical LAN
infrastructure
Virtual Local
Area Network 1
8
9
16 10 2
7
hellip
Electrical Engineering
(VLAN ports 1-8)
Computer Science
(VLAN ports 9-15)
15
hellip
Electrical Engineering
(VLAN ports 1-8)
hellip
1
8 2
7 9
16 10
15
hellip
Computer Science
(VLAN ports 9-16)
hellip operates as multiple virtual switches
Link Layer 5-73
Port-based VLAN
1
8
9
16 10 2
7
hellip
Electrical Engineering
(VLAN ports 1-8)
Computer Science
(VLAN ports 9-15)
15
hellip
traffic isolation frames tofrom ports 1-8 can only reach ports 1-8 can also define VLAN based on
MAC addresses of endpoints rather than switch port
dynamic membership ports can be dynamically assigned among VLANs
router
forwarding between VLANS done via routing (just as with separate switches) in practice vendors sell combined
switches plus routers
Link Layer 5-74
VLANS spanning multiple switches
trunk port carries frames between VLANS defined over multiple physical switches frames forwarded within VLAN between switches canrsquot be vanilla
8021 frames (must carry VLAN ID info)
8021q protocol addsremoved additional header fields for frames forwarded between trunk ports
1
8
9
10 2
7
hellip
Electrical Engineering
(VLAN ports 1-8)
Computer Science
(VLAN ports 9-15)
15
hellip
2
7 3
Ports 235 belong to EE VLAN
Ports 4678 belong to CS VLAN
5
4 6 8 16
1
Link Layer 5-75
type
2-byte Tag Protocol Identifier
(value 81-00)
Tag Control Information (12 bit VLAN ID field
3 bit priority field like IP TOS)
Recomputed CRC
8021Q VLAN frame format
8021 frame
8021Q frame
dest address
source address
data (payload) CRC preamble
dest address
source address
preamble data (payload) CRC
type
Link Layer 5-76
Link layer LANs outline
51 introduction services
52 error detection correction
53 multiple access protocols
54 LANs addressing ARP
Ethernet
switches
VLANS
55 link virtualization MPLS
56 data center networking
57 a day in the life of a web request
Link Layer 5-77
Multiprotocol label switching (MPLS)
initial goal high-speed IP forwarding using fixed length label (instead of IP address) fast lookup using fixed length identifier (rather than
shortest prefix matching)
borrowing ideas from Virtual Circuit (VC) approach
but IP datagram still keeps IP address
PPP or Ethernet
header IP header remainder of link-layer frame MPLS header
label Exp S TTL
20 3 1 5
Link Layer 5-78
MPLS capable routers
aka label-switched router
forward packets to outgoing interface based only on label value (donrsquot inspect IP address) MPLS forwarding table distinct from IP forwarding tables
flexibility MPLS forwarding decisions can differ from those of IP use destination and source addresses to route flows to
same destination differently (traffic engineering)
re-route flows quickly if link fails pre-computed backup paths (useful for VoIP)
Link Layer 5-79
R2
D
R3 R5
A
R6
MPLS versus IP paths
IP router IP routing path to destination determined
by destination address alone
R4
Link Layer 5-80
R2
D
R3 R4
R5
A
R6
MPLS versus IP paths
IP-only router
IP routing path to destination determined by destination address alone
MPLS and IP router
MPLS routing path to destination can be based on source and dest address fast reroute precompute backup routes in
case of link failure
entry router (R4) can use different MPLS
routes to A based eg on source address
Link Layer 5-81
MPLS signaling
modify OSPF IS-IS link-state flooding protocols to carry info used by MPLS routing eg link bandwidth amount of ldquoreservedrdquo link bandwidth
D
R4
R5
A
R6
entry MPLS router uses RSVP-TE signaling protocol to set up MPLS forwarding at downstream routers
modified link state flooding
RSVP-TE
Link Layer 5-82
R1 R2
D
R3 R4 R5
0
1
0 0
A
R6
in out out
label label dest interface
6 - A 0
in out out
label label dest interface
10 6 A 1
12 9 D 0
in out out
label label dest interface
10 A 0
12 D 0
1
in out out
label label dest interface
8 6 A 0
0
8 A 1
MPLS forwarding tables
Link Layer 5-83
Link layer LANs outline
51 introduction services
52 error detection correction
53 multiple access protocols
54 LANs addressing ARP
Ethernet
switches
VLANS
55 link virtualization MPLS
56 data center networking
57 a day in the life of a web request
Link Layer 5-84
Data center networks
10rsquos to 100rsquos of thousands of hosts often closely coupled in close proximity e-business (eg Amazon)
content-servers (eg YouTube Akamai Apple Microsoft)
search engines data mining (eg Google)
challenges
multiple applications each serving massive numbers of clients
managingbalancing load avoiding processing networking data bottlenecks
Inside a 40-ft Microsoft container
Chicago data center
Link Layer 5-85
Server racks
TOR switches
Tier-1 switches
Tier-2 switches
Load balancer
Load balancer
B
1 2 3 4 5 6 7 8
A C
Border router
Access router
Internet
Data center networks
load balancer application-layer routing receives external client requests
directs workload within data center
returns results to external client (hiding data
center internals from client)
Server racks
TOR switches
Tier-1 switches
Tier-2 switches
1 2 3 4 5 6 7 8
Data center networks
rich interconnection among switches racks
increased throughput between racks (multiple routing
paths possible)
increased reliability via redundancy
Link Layer 5-87
Link layer LANs outline
51 introduction services
52 error detection correction
53 multiple access protocols
54 LANs addressing ARP
Ethernet
switches
VLANS
55 link virtualization MPLS
56 data center networking
57 a day in the life of a web request
Link Layer 5-88
Synthesis a day in the life of a web request
journey down protocol stack complete application transport network link
putting-it-all-together synthesis goal identify review understand protocols (at all
layers) involved in seemingly simple scenario requesting www page
scenario student attaches laptop to campus network requestsreceives wwwgooglecom
Link Layer 5-89
A day in the life scenario
Comcast network
68800013
Googlersquos network
64233160019 64233169105
web server
DNS server
school network
68802024
web page
browser
router
(runs DHCP)
Link Layer 5-90
A day in the lifehellip connecting to the Internet
connecting laptop needs to get its own IP address addr of first-hop router addr of DNS server use DHCP
DHCP
UDP
IP
Eth
Phy
DHCP
DHCP
DHCP
DHCP
DHCP
DHCP
UDP
IP
Eth
Phy
DHCP
DHCP
DHCP
DHCP DHCP
DHCP request encapsulated in UDP encapsulated in IP encapsulated in 8023 Ethernet
Ethernet frame broadcast
(dest FFFFFFFFFFFF) on LAN received at router running DHCP server
Ethernet demuxed to IP demuxed UDP demuxed to DHCP
router
(runs DHCP)
Link Layer 5-91
DHCP server formulates DHCP ACK containing clientrsquos IP address IP address of first-hop router for client name amp IP address of DNS server
DHCP
UDP
IP
Eth
Phy
DHCP
DHCP
DHCP
DHCP
DHCP
UDP
IP
Eth
Phy
DHCP
DHCP
DHCP
DHCP
DHCP
encapsulation at DHCP server frame forwarded (switch learning) through LAN demultiplexing at client
Client now has IP address knows name amp addr of DNS
server IP address of its first-hop router
DHCP client receives DHCP ACK reply
A day in the lifehellip connecting to the Internet
router
(runs DHCP)
Link Layer 5-92
A day in the lifehellip ARP (before DNS before HTTP)
before sending HTTP request need IP address of wwwgooglecom DNS
DNS
UDP
IP
Eth
Phy
DNS
DNS
DNS
DNS query created encapsulated in UDP encapsulated in IP encapsulated in Eth To send frame to router need MAC address of router interface ARP
ARP query broadcast received by
router which replies with ARP reply giving MAC address of router interface
client now knows MAC address of first hop router so can now send frame containing DNS query
ARP query
Eth
Phy
ARP
ARP
ARP reply
router
(runs DHCP)
Link Layer 5-93
DNS
UDP
IP
Eth
Phy
DNS
DNS
DNS
DNS
DNS
IP datagram containing DNS query forwarded via LAN switch from client to 1st hop router
IP datagram forwarded from campus network into comcast network routed (tables created by RIP OSPF IS-IS andor BGP routing protocols) to DNS server
demuxrsquoed to DNS server
DNS server replies to client with IP address of wwwgooglecom
Comcast network
68800013
DNS server
DNS
UDP
IP
Eth
Phy
DNS
DNS
DNS
DNS
A day in the lifehellip using DNS
router
(runs DHCP)
Link Layer 5-94
A day in the lifehellipTCP connection carrying HTTP
HTTP
TCP
IP
Eth
Phy
HTTP
to send HTTP request client first opens TCP socket to web server
TCP SYN segment (step 1 in 3-way handshake) inter-domain routed to web server
TCP connection established 64233169105
web server
SYN
SYN
SYN
SYN
TCP
IP
Eth
Phy
SYN
SYN
SYN
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
web server responds with TCP SYNACK (step 2 in 3-way handshake)
router
(runs DHCP)
Link Layer 5-95
A day in the lifehellip HTTP requestreply
HTTP
TCP
IP
Eth
Phy
HTTP
HTTP request sent into TCP socket
IP datagram containing HTTP request routed to wwwgooglecom
IP datagram containing HTTP reply routed back to client
64233169105
web server
HTTP
TCP
IP
Eth
Phy web server responds with
HTTP reply (containing web page)
HTTP
HTTP
HTTP HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
web page finally () displayed
Link Layer 5-96
Chapter 5 Summary
principles behind data link layer services error detection correction
sharing a broadcast channel multiple access
link layer addressing
instantiation and implementation of various link layer technologies Ethernet
switched LANS VLANs
virtualized networks as a link layer MPLS
synthesis a day in the life of a web request
Link Layer 5-97
Chapter 5 letrsquos take a breath
journey down protocol stack complete (except PHY)
solid understanding of networking principles practice
hellip could stop here hellip but lots of interesting topics wireless
multimedia
security
network management
Link Layer 5-23
Random access protocols
when node has packet to send transmit at full channel data rate R no a priori coordination among nodes
two or more transmitting nodes ldquocollisionrdquo random access MAC protocol specifies
how to detect collisions how to recover from collisions (eg via delayed
retransmissions)
examples of random access MAC protocols slotted ALOHA ALOHA CSMA CSMACD CSMACA
Link Layer 5-24
Slotted ALOHA
assumptions all frames same size
time divided into equal size slots (time to transmit 1 frame)
nodes start to transmit only slot beginning
nodes are synchronized
if 2 or more nodes transmit in slot all nodes detect collision
operation
when node obtains fresh frame transmits in next slot
if no collision node can send new frame in next slot
if collision node retransmits frame in each subsequent slot with prob p until success
Link Layer 5-25
Pros single active node can
continuously transmit at full rate of channel
highly decentralized only slots in nodes need to be in sync
simple
Cons
collisions wasting slots
idle slots
nodes may be able to detect collision in less than time to transmit packet
clock synchronization
Slotted ALOHA
1 1 1 1
2
3
2 2
3 3
node 1
node 2
node 3
C C C S S S E E E
Link Layer 5-26
suppose N nodes with many frames to send each transmits in slot with probability p
prob that given node has success in a slot = p(1-p)N-1
prob that any node has a success = Np(1-p)N-1
max efficiency find p that maximizes Np(1-p)N-1
for many nodes take limit of Np(1-p)N-1 as N goes to infinity gives
max efficiency = 1e = 37
efficiency long-run fraction of successful slots (many nodes all with many frames to send)
at best channel used for useful transmissions 37 of time
Slotted ALOHA efficiency
Link Layer 5-27
Pure (unslotted) ALOHA
unslotted Aloha simpler no synchronization
when frame first arrives
transmit immediately
collision probability increases
frame sent at t0 collides with other frames sent in [t0-1t0+1]
Link Layer 5-28
Pure ALOHA efficiency
P(success by given node) = P(node transmits)
P(no other node transmits in [t0-1t0]
P(no other node transmits in [t0-1t0]
= p (1-p)N-1 (1-p)N-1
= p (1-p)2(N-1)
hellip choosing optimum p and then letting n
= 1(2e) = 18
even worse than slotted Aloha
Link Layer 5-29
CSMA (carrier sense multiple access)
CSMA listen before transmit if channel sensed idle transmit entire frame
if channel sensed busy defer transmission
human analogy donrsquot interrupt others
Link Layer 5-30
CSMA collisions
collisions can still occur propagation delay means two nodes may not hear each otherrsquos transmission
collision entire packet transmission time wasted distance amp propagation
delay play role in in determining collision probability
spatial layout of nodes
Link Layer 5-31
CSMACD (collision detection)
CSMACD carrier sensing deferral as in CSMA collisions detected within short time
colliding transmissions aborted reducing channel wastage
collision detection easy in wired LANs measure signal strengths compare
transmitted received signals
difficult in wireless LANs received signal strength overwhelmed by local transmission strength
human analogy the polite conversationalist
Link Layer 5-32
CSMACD (collision detection)
spatial layout of nodes
Link Layer 5-33
Ethernet CSMACD algorithm
1 NIC receives datagram from network layer creates frame
2 If NIC senses channel idle starts frame transmission If NIC senses channel busy waits until channel idle then transmits
3 If NIC transmits entire frame without detecting another transmission NIC is done with frame
4 If NIC detects another transmission while transmitting aborts and sends jam signal
5 After aborting NIC enters binary (exponential) backoff
after mth collision NIC chooses K at random from 012 hellip 2m-1 NIC waits K512 bit times returns to Step 2
longer backoff interval with more collisions
Link Layer 5-34
CSMACD efficiency
Tprop = max prop delay between 2 nodes in LAN
ttrans = time to transmit max-size frame
efficiency goes to 1
as tprop goes to 0
as ttrans goes to infinity
better performance than ALOHA and simple cheap decentralized
transpropttefficiency
51
1
Link Layer 5-35
ldquoTaking turnsrdquo MAC protocols
channel partitioning MAC protocols share channel efficiently and fairly at high load
inefficient at low load delay in channel access 1N bandwidth allocated even if only 1 active node
random access MAC protocols efficient at low load single node can fully utilize
channel
high load collision overhead
ldquotaking turnsrdquo protocols look for best of both worlds
Link Layer 5-36
polling master node ldquoinvitesrdquo
slave nodes to transmit in turn
typically used with ldquodumbrdquo slave devices
concerns
polling overhead
latency
single point of failure (master)
master
slaves
poll
data
data
ldquoTaking turnsrdquo MAC protocols
Link Layer 5-37
token passing control token passed
from one node to next sequentially
token message
concerns
token overhead
latency
single point of failure (token)
T
data
(nothing
to send)
T
ldquoTaking turnsrdquo MAC protocols
cable headend
CMTS
ISP
cable modem termination system
multiple 40Mbps downstream (broadcast) channels
single CMTS transmits into channels
multiple 30 Mbps upstream channels
multiple access all users contend for certain upstream channel time slots (others assigned)
Cable access network
cable modem
splitter
hellip
hellip
Internet framesTV channels control transmitted downstream at different frequencies
upstream Internet frames TV control transmitted upstream at different frequencies in time slots
Link Layer 5-39
DOCSIS data over cable service interface spec FDM over upstream downstream frequency channels
TDM upstream some slots assigned some have contention
downstream MAP frame assigns upstream slots
request for upstream slots (and data) transmitted random access (binary backoff) in selected slots
MAP frame for
Interval [t1 t2]
Residences with cable modems
Downstream channel i
Upstream channel j
t1 t2
Assigned minislots containing cable modem
upstream data frames
Minislots containing
minislots request frames
cable headend
CMTS
Cable access network
Link Layer 5-40
Summary of MAC protocols
channel partitioning by time frequency or code Time Division Frequency Division
random access (dynamic)
ALOHA S-ALOHA CSMA CSMACD
carrier sensing easy in some technologies (wire) hard in others (wireless)
CSMACD used in Ethernet
CSMACA used in 80211
taking turns
polling from central site token passing
bluetooth FDDI token ring
Link Layer 5-41
Link layer LANs outline
51 introduction services
52 error detection correction
53 multiple access protocols
54 LANs addressing ARP
Ethernet
switches
VLANS
55 link virtualization MPLS
56 data center networking
57 a day in the life of a web request
Link Layer 5-42
MAC addresses and ARP
32-bit IP address network-layer address for interface
used for layer 3 (network layer) forwarding
MAC (or LAN or physical or Ethernet) address function used lsquolocallyrdquo to get frame from one interface to
another physically-connected interface (same network in IP-addressing sense)
48 bit MAC address (for most LANs) burned in NIC ROM also sometimes software settable
eg 1A-2F-BB-76-09-AD
hexadecimal (base 16) notation
(each ldquonumberrdquo represents 4 bits)
Link Layer 5-43
LAN addresses and ARP
each adapter on LAN has unique LAN address
adapter
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN
(wired or
wireless)
Link Layer 5-44
LAN addresses (more)
MAC address allocation administered by IEEE
manufacturer buys portion of MAC address space (to assure uniqueness)
analogy MAC address like Social Security Number
IP address like postal address
MAC flat address portability can move LAN card from one LAN to another
IP hierarchical address not portable address depends on IP subnet to which node is
attached
Link Layer 5-45
ARP address resolution protocol
ARP table each IP node (host router) on LAN has table
IPMAC address mappings for some LAN nodes
lt IP address MAC address TTLgt
TTL (Time To Live) time after which address mapping will be forgotten (typically 20 min)
Question how to determine
interfacersquos MAC address
knowing its IP address
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN
137196723
137196778
137196714
137196788
Link Layer 5-46
ARP protocol same LAN
A wants to send datagram to B Brsquos MAC address not in
Arsquos ARP table
A broadcasts ARP query packet containing Bs IP address dest MAC address = FF-FF-
FF-FF-FF-FF
all nodes on LAN receive ARP query
B receives ARP packet replies to A with its (Bs) MAC address frame sent to Arsquos MAC
address (unicast)
A caches (saves) IP-to-MAC address pair in its ARP table until information becomes old (times out) soft state information that
times out (goes away) unless refreshed
ARP is ldquoplug-and-playrdquo nodes create their ARP
tables without intervention from net administrator
Link Layer 5-47
walkthrough send datagram from A to B via R
focus on addressing ndash at IP (datagram) and MAC layer (frame)
assume A knows Brsquos IP address
assume A knows IP address of first hop router R (how)
assume A knows Rrsquos MAC address (how)
Addressing routing to another LAN
R
1A-23-F9-CD-06-9B 222222222220
111111111110 E6-E9-00-17-BB-4B CC-49-DE-D0-AB-7D
111111111112
111111111111
74-29-9C-E8-FF-55
A
222222222222
49-BD-D2-C7-56-2A
222222222221 88-B2-2F-54-1A-0F
B
R
1A-23-F9-CD-06-9B 222222222220
111111111110 E6-E9-00-17-BB-4B CC-49-DE-D0-AB-7D
111111111112
111111111111
74-29-9C-E8-FF-55
A
222222222222
49-BD-D2-C7-56-2A
222222222221 88-B2-2F-54-1A-0F
B
Link Layer 5-48
Addressing routing to another LAN
IP
Eth
Phy
IP src 111111111111
IP dest 222222222222
A creates IP datagram with IP source A destination B
A creates link-layer frame with Rs MAC address as dest frame contains A-to-B IP datagram
MAC src 74-29-9C-E8-FF-55
MAC dest E6-E9-00-17-BB-4B
R
1A-23-F9-CD-06-9B 222222222220
111111111110 E6-E9-00-17-BB-4B CC-49-DE-D0-AB-7D
111111111112
111111111111
74-29-9C-E8-FF-55
A
222222222222
49-BD-D2-C7-56-2A
222222222221 88-B2-2F-54-1A-0F
B
Link Layer 5-49
Addressing routing to another LAN
IP
Eth
Phy
frame sent from A to R
IP
Eth
Phy
frame received at R datagram removed passed up to IP
MAC src 74-29-9C-E8-FF-55
MAC dest E6-E9-00-17-BB-4B
IP src 111111111111
IP dest 222222222222
IP src 111111111111
IP dest 222222222222
R
1A-23-F9-CD-06-9B 222222222220
111111111110 E6-E9-00-17-BB-4B CC-49-DE-D0-AB-7D
111111111112
111111111111
74-29-9C-E8-FF-55
A
222222222222
49-BD-D2-C7-56-2A
222222222221 88-B2-2F-54-1A-0F
B
Link Layer 5-50
Addressing routing to another LAN
IP src 111111111111
IP dest 222222222222
R forwards datagram with IP source A destination B
R creates link-layer frame with Bs MAC address as dest frame contains A-to-B IP datagram
MAC src 1A-23-F9-CD-06-9B
MAC dest 49-BD-D2-C7-56-2A
IP
Eth
Phy
IP
Eth
Phy
R
1A-23-F9-CD-06-9B 222222222220
111111111110 E6-E9-00-17-BB-4B CC-49-DE-D0-AB-7D
111111111112
111111111111
74-29-9C-E8-FF-55
A
222222222222
49-BD-D2-C7-56-2A
222222222221 88-B2-2F-54-1A-0F
B
Link Layer 5-51
Addressing routing to another LAN R forwards datagram with IP source A destination B
R creates link-layer frame with Bs MAC address as dest frame contains A-to-B IP datagram
IP src 111111111111
IP dest 222222222222
MAC src 1A-23-F9-CD-06-9B
MAC dest 49-BD-D2-C7-56-2A
IP
Eth
Phy
IP
Eth
Phy
R
1A-23-F9-CD-06-9B 222222222220
111111111110 E6-E9-00-17-BB-4B CC-49-DE-D0-AB-7D
111111111112
111111111111
74-29-9C-E8-FF-55
A
222222222222
49-BD-D2-C7-56-2A
222222222221 88-B2-2F-54-1A-0F
B
Link Layer 5-52
Addressing routing to another LAN R forwards datagram with IP source A destination B
R creates link-layer frame with Bs MAC address as dest frame contains A-to-B IP datagram
IP src 111111111111
IP dest 222222222222
MAC src 1A-23-F9-CD-06-9B
MAC dest 49-BD-D2-C7-56-2A
IP
Eth
Phy
Link Layer 5-53
Link layer LANs outline
51 introduction services
52 error detection correction
53 multiple access protocols
54 LANs addressing ARP
Ethernet
switches
VLANS
55 link virtualization MPLS
56 data center networking
57 a day in the life of a web request
Link Layer 5-54
Ethernet
ldquodominantrdquo wired LAN technology
cheap $20 for NIC
first widely used LAN technology
simpler cheaper than token LANs and ATM
kept up with speed race 10 Mbps ndash 10 Gbps
Metcalfersquos Ethernet sketch
Link Layer 5-55
Ethernet physical topology
bus popular through mid 90s all nodes in same collision domain (can collide with each
other)
star prevails today active switch in center
each ldquospokerdquo runs a (separate) Ethernet protocol (nodes
do not collide with each other)
switch
bus coaxial cable star
Link Layer 5-56
Ethernet frame structure
sending adapter encapsulates IP datagram (or other network layer protocol packet) in Ethernet frame
preamble
7 bytes with pattern 10101010 followed by one byte with pattern 10101011
used to synchronize receiver sender clock rates
dest address
source address
data (payload) CRC preamble
type
Link Layer 5-57
Ethernet frame structure (more)
addresses 6 byte source destination MAC addresses if adapter receives frame with matching destination
address or with broadcast address (eg ARP packet) it passes data in frame to network layer protocol
otherwise adapter discards frame
type indicates higher layer protocol (mostly IP but others possible eg Novell IPX AppleTalk)
CRC cyclic redundancy check at receiver error detected frame is dropped
dest address
source address
data (payload) CRC preamble
type
Link Layer 5-58
Ethernet unreliable connectionless
connectionless no handshaking between sending and receiving NICs
unreliable receiving NIC doesnt send acks or nacks to sending NIC
data in dropped frames recovered only if initial sender uses higher layer rdt (eg TCP) otherwise dropped data lost
Ethernetrsquos MAC protocol unslotted CSMACD wth binary backoff
Link Layer 5-59
8023 Ethernet standards link amp physical layers
many different Ethernet standards
common MAC protocol and frame format
different speeds 2 Mbps 10 Mbps 100 Mbps 1Gbps 10G bps
different physical layer media fiber cable
application
transport
network
link
physical
MAC protocol
and frame format
100BASE-TX
100BASE-T4
100BASE-FX 100BASE-T2
100BASE-SX 100BASE-BX
fiber physical layer copper (twister
pair) physical layer
Link Layer 5-60
Link layer LANs outline
51 introduction services
52 error detection correction
53 multiple access protocols
54 LANs addressing ARP
Ethernet
switches
VLANS
55 link virtualization MPLS
56 data center networking
57 a day in the life of a web request
Link Layer 5-61
Ethernet switch link-layer device takes an active role
store forward Ethernet frames
examine incoming framersquos MAC address selectively forward frame to one-or-more outgoing links when frame is to be forwarded on segment uses CSMACD to access segment
transparent
hosts are unaware of presence of switches
plug-and-play self-learning
switches do not need to be configured
Link Layer 5-62
Switch multiple simultaneous transmissions
hosts have dedicated direct connection to switch
switches buffer packets
Ethernet protocol used on each incoming link but no collisions full duplex
each link is its own collision domain
switching A-to-Arsquo and B-to-Brsquo can transmit simultaneously without collisions switch with six interfaces
(123456)
A
Arsquo
B
Brsquo C
Crsquo
1 2
3 4 5
6
Link Layer 5-63
Switch forwarding table
Q how does switch know Arsquo reachable via interface 4 Brsquo reachable via interface 5
switch with six interfaces
(123456)
A
Arsquo
B
Brsquo C
Crsquo
1 2
3 4 5
6 A each switch has a switch table each entry
(MAC address of host interface to
reach host time stamp)
looks like a routing table
Q how are entries created maintained in switch table
something like a routing protocol
A
Arsquo
B
Brsquo C
Crsquo
1 2
3 4 5
6
Link Layer 5-64
Switch self-learning
switch learns which hosts can be reached through which interfaces
when frame received switch ldquolearnsrdquo location of sender incoming LAN segment
records senderlocation pair in switch table
A Arsquo
Source A
Dest Arsquo
MAC addr interface TTL
Switch table
(initially empty) A 1 60
Link Layer 5-65
Switch frame filteringforwarding
when frame received at switch 1 record incoming link MAC address of sending host
2 index switch table using MAC destination address
3 if entry found for destination then
if destination on segment from which frame arrived then drop frame
else forward frame on interface indicated by entry
else flood forward on all interfaces except arriving
interface
A
Arsquo
B
Brsquo C
Crsquo
1 2
3 4 5
6
Link Layer 5-66
Self-learning forwarding example
A Arsquo
Source A
Dest Arsquo
MAC addr interface TTL
switch table
(initially empty) A 1 60
A Arsquo A Arsquo A Arsquo A Arsquo A Arsquo
frame destination Arsquo locaton unknown flood
Arsquo A
destination A location
known
Arsquo 4 60
selectively send
on just one link
Link Layer 5-67
Interconnecting switches
switches can be connected together
Q sending from A to G - how does S1 know to forward frame destined to F via S4 and S3
A self learning (works exactly the same as in single-switch case)
A
B
S1
C D
E
F
S2
S4
S3
H
I
G
Link Layer 5-68
Self-learning multi-switch example
Suppose C sends frame to I I responds to C
Q show switch tables and packet forwarding in S1 S2 S3 S4
A
B
S1
C D
E
F
S2
S4
S3
H
I
G
Link Layer 5-69
Institutional network
to external
network
router
IP subnet
mail server
web server
Link Layer 5-70
Switches vs routers
both are store-and-forward
routers network-layer devices (examine network-layer headers)
switches link-layer devices (examine link-layer headers)
both have forwarding tables
routers compute tables using routing algorithms IP addresses
switches learn forwarding table using flooding learning MAC addresses
application
transport
network
link
physical
network
link
physical
link
physical
switch
datagram
application
transport
network
link
physical
frame
frame
frame
datagram
Link Layer 5-71
VLANs motivation
consider CS user moves office to
EE but wants connect to CS switch
single broadcast domain
all layer-2 broadcast traffic (ARP DHCP unknown location of destination MAC address) must cross entire LAN
securityprivacy efficiency issues
Computer
Science Electrical
Engineering
Computer
Engineering
Link Layer 5-72
VLANs port-based VLAN switch ports
grouped (by switch management software) so that single physical switch helliphellip
switch(es) supporting
VLAN capabilities can
be configured to
define multiple virtual
LANS over single
physical LAN
infrastructure
Virtual Local
Area Network 1
8
9
16 10 2
7
hellip
Electrical Engineering
(VLAN ports 1-8)
Computer Science
(VLAN ports 9-15)
15
hellip
Electrical Engineering
(VLAN ports 1-8)
hellip
1
8 2
7 9
16 10
15
hellip
Computer Science
(VLAN ports 9-16)
hellip operates as multiple virtual switches
Link Layer 5-73
Port-based VLAN
1
8
9
16 10 2
7
hellip
Electrical Engineering
(VLAN ports 1-8)
Computer Science
(VLAN ports 9-15)
15
hellip
traffic isolation frames tofrom ports 1-8 can only reach ports 1-8 can also define VLAN based on
MAC addresses of endpoints rather than switch port
dynamic membership ports can be dynamically assigned among VLANs
router
forwarding between VLANS done via routing (just as with separate switches) in practice vendors sell combined
switches plus routers
Link Layer 5-74
VLANS spanning multiple switches
trunk port carries frames between VLANS defined over multiple physical switches frames forwarded within VLAN between switches canrsquot be vanilla
8021 frames (must carry VLAN ID info)
8021q protocol addsremoved additional header fields for frames forwarded between trunk ports
1
8
9
10 2
7
hellip
Electrical Engineering
(VLAN ports 1-8)
Computer Science
(VLAN ports 9-15)
15
hellip
2
7 3
Ports 235 belong to EE VLAN
Ports 4678 belong to CS VLAN
5
4 6 8 16
1
Link Layer 5-75
type
2-byte Tag Protocol Identifier
(value 81-00)
Tag Control Information (12 bit VLAN ID field
3 bit priority field like IP TOS)
Recomputed CRC
8021Q VLAN frame format
8021 frame
8021Q frame
dest address
source address
data (payload) CRC preamble
dest address
source address
preamble data (payload) CRC
type
Link Layer 5-76
Link layer LANs outline
51 introduction services
52 error detection correction
53 multiple access protocols
54 LANs addressing ARP
Ethernet
switches
VLANS
55 link virtualization MPLS
56 data center networking
57 a day in the life of a web request
Link Layer 5-77
Multiprotocol label switching (MPLS)
initial goal high-speed IP forwarding using fixed length label (instead of IP address) fast lookup using fixed length identifier (rather than
shortest prefix matching)
borrowing ideas from Virtual Circuit (VC) approach
but IP datagram still keeps IP address
PPP or Ethernet
header IP header remainder of link-layer frame MPLS header
label Exp S TTL
20 3 1 5
Link Layer 5-78
MPLS capable routers
aka label-switched router
forward packets to outgoing interface based only on label value (donrsquot inspect IP address) MPLS forwarding table distinct from IP forwarding tables
flexibility MPLS forwarding decisions can differ from those of IP use destination and source addresses to route flows to
same destination differently (traffic engineering)
re-route flows quickly if link fails pre-computed backup paths (useful for VoIP)
Link Layer 5-79
R2
D
R3 R5
A
R6
MPLS versus IP paths
IP router IP routing path to destination determined
by destination address alone
R4
Link Layer 5-80
R2
D
R3 R4
R5
A
R6
MPLS versus IP paths
IP-only router
IP routing path to destination determined by destination address alone
MPLS and IP router
MPLS routing path to destination can be based on source and dest address fast reroute precompute backup routes in
case of link failure
entry router (R4) can use different MPLS
routes to A based eg on source address
Link Layer 5-81
MPLS signaling
modify OSPF IS-IS link-state flooding protocols to carry info used by MPLS routing eg link bandwidth amount of ldquoreservedrdquo link bandwidth
D
R4
R5
A
R6
entry MPLS router uses RSVP-TE signaling protocol to set up MPLS forwarding at downstream routers
modified link state flooding
RSVP-TE
Link Layer 5-82
R1 R2
D
R3 R4 R5
0
1
0 0
A
R6
in out out
label label dest interface
6 - A 0
in out out
label label dest interface
10 6 A 1
12 9 D 0
in out out
label label dest interface
10 A 0
12 D 0
1
in out out
label label dest interface
8 6 A 0
0
8 A 1
MPLS forwarding tables
Link Layer 5-83
Link layer LANs outline
51 introduction services
52 error detection correction
53 multiple access protocols
54 LANs addressing ARP
Ethernet
switches
VLANS
55 link virtualization MPLS
56 data center networking
57 a day in the life of a web request
Link Layer 5-84
Data center networks
10rsquos to 100rsquos of thousands of hosts often closely coupled in close proximity e-business (eg Amazon)
content-servers (eg YouTube Akamai Apple Microsoft)
search engines data mining (eg Google)
challenges
multiple applications each serving massive numbers of clients
managingbalancing load avoiding processing networking data bottlenecks
Inside a 40-ft Microsoft container
Chicago data center
Link Layer 5-85
Server racks
TOR switches
Tier-1 switches
Tier-2 switches
Load balancer
Load balancer
B
1 2 3 4 5 6 7 8
A C
Border router
Access router
Internet
Data center networks
load balancer application-layer routing receives external client requests
directs workload within data center
returns results to external client (hiding data
center internals from client)
Server racks
TOR switches
Tier-1 switches
Tier-2 switches
1 2 3 4 5 6 7 8
Data center networks
rich interconnection among switches racks
increased throughput between racks (multiple routing
paths possible)
increased reliability via redundancy
Link Layer 5-87
Link layer LANs outline
51 introduction services
52 error detection correction
53 multiple access protocols
54 LANs addressing ARP
Ethernet
switches
VLANS
55 link virtualization MPLS
56 data center networking
57 a day in the life of a web request
Link Layer 5-88
Synthesis a day in the life of a web request
journey down protocol stack complete application transport network link
putting-it-all-together synthesis goal identify review understand protocols (at all
layers) involved in seemingly simple scenario requesting www page
scenario student attaches laptop to campus network requestsreceives wwwgooglecom
Link Layer 5-89
A day in the life scenario
Comcast network
68800013
Googlersquos network
64233160019 64233169105
web server
DNS server
school network
68802024
web page
browser
router
(runs DHCP)
Link Layer 5-90
A day in the lifehellip connecting to the Internet
connecting laptop needs to get its own IP address addr of first-hop router addr of DNS server use DHCP
DHCP
UDP
IP
Eth
Phy
DHCP
DHCP
DHCP
DHCP
DHCP
DHCP
UDP
IP
Eth
Phy
DHCP
DHCP
DHCP
DHCP DHCP
DHCP request encapsulated in UDP encapsulated in IP encapsulated in 8023 Ethernet
Ethernet frame broadcast
(dest FFFFFFFFFFFF) on LAN received at router running DHCP server
Ethernet demuxed to IP demuxed UDP demuxed to DHCP
router
(runs DHCP)
Link Layer 5-91
DHCP server formulates DHCP ACK containing clientrsquos IP address IP address of first-hop router for client name amp IP address of DNS server
DHCP
UDP
IP
Eth
Phy
DHCP
DHCP
DHCP
DHCP
DHCP
UDP
IP
Eth
Phy
DHCP
DHCP
DHCP
DHCP
DHCP
encapsulation at DHCP server frame forwarded (switch learning) through LAN demultiplexing at client
Client now has IP address knows name amp addr of DNS
server IP address of its first-hop router
DHCP client receives DHCP ACK reply
A day in the lifehellip connecting to the Internet
router
(runs DHCP)
Link Layer 5-92
A day in the lifehellip ARP (before DNS before HTTP)
before sending HTTP request need IP address of wwwgooglecom DNS
DNS
UDP
IP
Eth
Phy
DNS
DNS
DNS
DNS query created encapsulated in UDP encapsulated in IP encapsulated in Eth To send frame to router need MAC address of router interface ARP
ARP query broadcast received by
router which replies with ARP reply giving MAC address of router interface
client now knows MAC address of first hop router so can now send frame containing DNS query
ARP query
Eth
Phy
ARP
ARP
ARP reply
router
(runs DHCP)
Link Layer 5-93
DNS
UDP
IP
Eth
Phy
DNS
DNS
DNS
DNS
DNS
IP datagram containing DNS query forwarded via LAN switch from client to 1st hop router
IP datagram forwarded from campus network into comcast network routed (tables created by RIP OSPF IS-IS andor BGP routing protocols) to DNS server
demuxrsquoed to DNS server
DNS server replies to client with IP address of wwwgooglecom
Comcast network
68800013
DNS server
DNS
UDP
IP
Eth
Phy
DNS
DNS
DNS
DNS
A day in the lifehellip using DNS
router
(runs DHCP)
Link Layer 5-94
A day in the lifehellipTCP connection carrying HTTP
HTTP
TCP
IP
Eth
Phy
HTTP
to send HTTP request client first opens TCP socket to web server
TCP SYN segment (step 1 in 3-way handshake) inter-domain routed to web server
TCP connection established 64233169105
web server
SYN
SYN
SYN
SYN
TCP
IP
Eth
Phy
SYN
SYN
SYN
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
web server responds with TCP SYNACK (step 2 in 3-way handshake)
router
(runs DHCP)
Link Layer 5-95
A day in the lifehellip HTTP requestreply
HTTP
TCP
IP
Eth
Phy
HTTP
HTTP request sent into TCP socket
IP datagram containing HTTP request routed to wwwgooglecom
IP datagram containing HTTP reply routed back to client
64233169105
web server
HTTP
TCP
IP
Eth
Phy web server responds with
HTTP reply (containing web page)
HTTP
HTTP
HTTP HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
web page finally () displayed
Link Layer 5-96
Chapter 5 Summary
principles behind data link layer services error detection correction
sharing a broadcast channel multiple access
link layer addressing
instantiation and implementation of various link layer technologies Ethernet
switched LANS VLANs
virtualized networks as a link layer MPLS
synthesis a day in the life of a web request
Link Layer 5-97
Chapter 5 letrsquos take a breath
journey down protocol stack complete (except PHY)
solid understanding of networking principles practice
hellip could stop here hellip but lots of interesting topics wireless
multimedia
security
network management
Link Layer 5-24
Slotted ALOHA
assumptions all frames same size
time divided into equal size slots (time to transmit 1 frame)
nodes start to transmit only slot beginning
nodes are synchronized
if 2 or more nodes transmit in slot all nodes detect collision
operation
when node obtains fresh frame transmits in next slot
if no collision node can send new frame in next slot
if collision node retransmits frame in each subsequent slot with prob p until success
Link Layer 5-25
Pros single active node can
continuously transmit at full rate of channel
highly decentralized only slots in nodes need to be in sync
simple
Cons
collisions wasting slots
idle slots
nodes may be able to detect collision in less than time to transmit packet
clock synchronization
Slotted ALOHA
1 1 1 1
2
3
2 2
3 3
node 1
node 2
node 3
C C C S S S E E E
Link Layer 5-26
suppose N nodes with many frames to send each transmits in slot with probability p
prob that given node has success in a slot = p(1-p)N-1
prob that any node has a success = Np(1-p)N-1
max efficiency find p that maximizes Np(1-p)N-1
for many nodes take limit of Np(1-p)N-1 as N goes to infinity gives
max efficiency = 1e = 37
efficiency long-run fraction of successful slots (many nodes all with many frames to send)
at best channel used for useful transmissions 37 of time
Slotted ALOHA efficiency
Link Layer 5-27
Pure (unslotted) ALOHA
unslotted Aloha simpler no synchronization
when frame first arrives
transmit immediately
collision probability increases
frame sent at t0 collides with other frames sent in [t0-1t0+1]
Link Layer 5-28
Pure ALOHA efficiency
P(success by given node) = P(node transmits)
P(no other node transmits in [t0-1t0]
P(no other node transmits in [t0-1t0]
= p (1-p)N-1 (1-p)N-1
= p (1-p)2(N-1)
hellip choosing optimum p and then letting n
= 1(2e) = 18
even worse than slotted Aloha
Link Layer 5-29
CSMA (carrier sense multiple access)
CSMA listen before transmit if channel sensed idle transmit entire frame
if channel sensed busy defer transmission
human analogy donrsquot interrupt others
Link Layer 5-30
CSMA collisions
collisions can still occur propagation delay means two nodes may not hear each otherrsquos transmission
collision entire packet transmission time wasted distance amp propagation
delay play role in in determining collision probability
spatial layout of nodes
Link Layer 5-31
CSMACD (collision detection)
CSMACD carrier sensing deferral as in CSMA collisions detected within short time
colliding transmissions aborted reducing channel wastage
collision detection easy in wired LANs measure signal strengths compare
transmitted received signals
difficult in wireless LANs received signal strength overwhelmed by local transmission strength
human analogy the polite conversationalist
Link Layer 5-32
CSMACD (collision detection)
spatial layout of nodes
Link Layer 5-33
Ethernet CSMACD algorithm
1 NIC receives datagram from network layer creates frame
2 If NIC senses channel idle starts frame transmission If NIC senses channel busy waits until channel idle then transmits
3 If NIC transmits entire frame without detecting another transmission NIC is done with frame
4 If NIC detects another transmission while transmitting aborts and sends jam signal
5 After aborting NIC enters binary (exponential) backoff
after mth collision NIC chooses K at random from 012 hellip 2m-1 NIC waits K512 bit times returns to Step 2
longer backoff interval with more collisions
Link Layer 5-34
CSMACD efficiency
Tprop = max prop delay between 2 nodes in LAN
ttrans = time to transmit max-size frame
efficiency goes to 1
as tprop goes to 0
as ttrans goes to infinity
better performance than ALOHA and simple cheap decentralized
transpropttefficiency
51
1
Link Layer 5-35
ldquoTaking turnsrdquo MAC protocols
channel partitioning MAC protocols share channel efficiently and fairly at high load
inefficient at low load delay in channel access 1N bandwidth allocated even if only 1 active node
random access MAC protocols efficient at low load single node can fully utilize
channel
high load collision overhead
ldquotaking turnsrdquo protocols look for best of both worlds
Link Layer 5-36
polling master node ldquoinvitesrdquo
slave nodes to transmit in turn
typically used with ldquodumbrdquo slave devices
concerns
polling overhead
latency
single point of failure (master)
master
slaves
poll
data
data
ldquoTaking turnsrdquo MAC protocols
Link Layer 5-37
token passing control token passed
from one node to next sequentially
token message
concerns
token overhead
latency
single point of failure (token)
T
data
(nothing
to send)
T
ldquoTaking turnsrdquo MAC protocols
cable headend
CMTS
ISP
cable modem termination system
multiple 40Mbps downstream (broadcast) channels
single CMTS transmits into channels
multiple 30 Mbps upstream channels
multiple access all users contend for certain upstream channel time slots (others assigned)
Cable access network
cable modem
splitter
hellip
hellip
Internet framesTV channels control transmitted downstream at different frequencies
upstream Internet frames TV control transmitted upstream at different frequencies in time slots
Link Layer 5-39
DOCSIS data over cable service interface spec FDM over upstream downstream frequency channels
TDM upstream some slots assigned some have contention
downstream MAP frame assigns upstream slots
request for upstream slots (and data) transmitted random access (binary backoff) in selected slots
MAP frame for
Interval [t1 t2]
Residences with cable modems
Downstream channel i
Upstream channel j
t1 t2
Assigned minislots containing cable modem
upstream data frames
Minislots containing
minislots request frames
cable headend
CMTS
Cable access network
Link Layer 5-40
Summary of MAC protocols
channel partitioning by time frequency or code Time Division Frequency Division
random access (dynamic)
ALOHA S-ALOHA CSMA CSMACD
carrier sensing easy in some technologies (wire) hard in others (wireless)
CSMACD used in Ethernet
CSMACA used in 80211
taking turns
polling from central site token passing
bluetooth FDDI token ring
Link Layer 5-41
Link layer LANs outline
51 introduction services
52 error detection correction
53 multiple access protocols
54 LANs addressing ARP
Ethernet
switches
VLANS
55 link virtualization MPLS
56 data center networking
57 a day in the life of a web request
Link Layer 5-42
MAC addresses and ARP
32-bit IP address network-layer address for interface
used for layer 3 (network layer) forwarding
MAC (or LAN or physical or Ethernet) address function used lsquolocallyrdquo to get frame from one interface to
another physically-connected interface (same network in IP-addressing sense)
48 bit MAC address (for most LANs) burned in NIC ROM also sometimes software settable
eg 1A-2F-BB-76-09-AD
hexadecimal (base 16) notation
(each ldquonumberrdquo represents 4 bits)
Link Layer 5-43
LAN addresses and ARP
each adapter on LAN has unique LAN address
adapter
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN
(wired or
wireless)
Link Layer 5-44
LAN addresses (more)
MAC address allocation administered by IEEE
manufacturer buys portion of MAC address space (to assure uniqueness)
analogy MAC address like Social Security Number
IP address like postal address
MAC flat address portability can move LAN card from one LAN to another
IP hierarchical address not portable address depends on IP subnet to which node is
attached
Link Layer 5-45
ARP address resolution protocol
ARP table each IP node (host router) on LAN has table
IPMAC address mappings for some LAN nodes
lt IP address MAC address TTLgt
TTL (Time To Live) time after which address mapping will be forgotten (typically 20 min)
Question how to determine
interfacersquos MAC address
knowing its IP address
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN
137196723
137196778
137196714
137196788
Link Layer 5-46
ARP protocol same LAN
A wants to send datagram to B Brsquos MAC address not in
Arsquos ARP table
A broadcasts ARP query packet containing Bs IP address dest MAC address = FF-FF-
FF-FF-FF-FF
all nodes on LAN receive ARP query
B receives ARP packet replies to A with its (Bs) MAC address frame sent to Arsquos MAC
address (unicast)
A caches (saves) IP-to-MAC address pair in its ARP table until information becomes old (times out) soft state information that
times out (goes away) unless refreshed
ARP is ldquoplug-and-playrdquo nodes create their ARP
tables without intervention from net administrator
Link Layer 5-47
walkthrough send datagram from A to B via R
focus on addressing ndash at IP (datagram) and MAC layer (frame)
assume A knows Brsquos IP address
assume A knows IP address of first hop router R (how)
assume A knows Rrsquos MAC address (how)
Addressing routing to another LAN
R
1A-23-F9-CD-06-9B 222222222220
111111111110 E6-E9-00-17-BB-4B CC-49-DE-D0-AB-7D
111111111112
111111111111
74-29-9C-E8-FF-55
A
222222222222
49-BD-D2-C7-56-2A
222222222221 88-B2-2F-54-1A-0F
B
R
1A-23-F9-CD-06-9B 222222222220
111111111110 E6-E9-00-17-BB-4B CC-49-DE-D0-AB-7D
111111111112
111111111111
74-29-9C-E8-FF-55
A
222222222222
49-BD-D2-C7-56-2A
222222222221 88-B2-2F-54-1A-0F
B
Link Layer 5-48
Addressing routing to another LAN
IP
Eth
Phy
IP src 111111111111
IP dest 222222222222
A creates IP datagram with IP source A destination B
A creates link-layer frame with Rs MAC address as dest frame contains A-to-B IP datagram
MAC src 74-29-9C-E8-FF-55
MAC dest E6-E9-00-17-BB-4B
R
1A-23-F9-CD-06-9B 222222222220
111111111110 E6-E9-00-17-BB-4B CC-49-DE-D0-AB-7D
111111111112
111111111111
74-29-9C-E8-FF-55
A
222222222222
49-BD-D2-C7-56-2A
222222222221 88-B2-2F-54-1A-0F
B
Link Layer 5-49
Addressing routing to another LAN
IP
Eth
Phy
frame sent from A to R
IP
Eth
Phy
frame received at R datagram removed passed up to IP
MAC src 74-29-9C-E8-FF-55
MAC dest E6-E9-00-17-BB-4B
IP src 111111111111
IP dest 222222222222
IP src 111111111111
IP dest 222222222222
R
1A-23-F9-CD-06-9B 222222222220
111111111110 E6-E9-00-17-BB-4B CC-49-DE-D0-AB-7D
111111111112
111111111111
74-29-9C-E8-FF-55
A
222222222222
49-BD-D2-C7-56-2A
222222222221 88-B2-2F-54-1A-0F
B
Link Layer 5-50
Addressing routing to another LAN
IP src 111111111111
IP dest 222222222222
R forwards datagram with IP source A destination B
R creates link-layer frame with Bs MAC address as dest frame contains A-to-B IP datagram
MAC src 1A-23-F9-CD-06-9B
MAC dest 49-BD-D2-C7-56-2A
IP
Eth
Phy
IP
Eth
Phy
R
1A-23-F9-CD-06-9B 222222222220
111111111110 E6-E9-00-17-BB-4B CC-49-DE-D0-AB-7D
111111111112
111111111111
74-29-9C-E8-FF-55
A
222222222222
49-BD-D2-C7-56-2A
222222222221 88-B2-2F-54-1A-0F
B
Link Layer 5-51
Addressing routing to another LAN R forwards datagram with IP source A destination B
R creates link-layer frame with Bs MAC address as dest frame contains A-to-B IP datagram
IP src 111111111111
IP dest 222222222222
MAC src 1A-23-F9-CD-06-9B
MAC dest 49-BD-D2-C7-56-2A
IP
Eth
Phy
IP
Eth
Phy
R
1A-23-F9-CD-06-9B 222222222220
111111111110 E6-E9-00-17-BB-4B CC-49-DE-D0-AB-7D
111111111112
111111111111
74-29-9C-E8-FF-55
A
222222222222
49-BD-D2-C7-56-2A
222222222221 88-B2-2F-54-1A-0F
B
Link Layer 5-52
Addressing routing to another LAN R forwards datagram with IP source A destination B
R creates link-layer frame with Bs MAC address as dest frame contains A-to-B IP datagram
IP src 111111111111
IP dest 222222222222
MAC src 1A-23-F9-CD-06-9B
MAC dest 49-BD-D2-C7-56-2A
IP
Eth
Phy
Link Layer 5-53
Link layer LANs outline
51 introduction services
52 error detection correction
53 multiple access protocols
54 LANs addressing ARP
Ethernet
switches
VLANS
55 link virtualization MPLS
56 data center networking
57 a day in the life of a web request
Link Layer 5-54
Ethernet
ldquodominantrdquo wired LAN technology
cheap $20 for NIC
first widely used LAN technology
simpler cheaper than token LANs and ATM
kept up with speed race 10 Mbps ndash 10 Gbps
Metcalfersquos Ethernet sketch
Link Layer 5-55
Ethernet physical topology
bus popular through mid 90s all nodes in same collision domain (can collide with each
other)
star prevails today active switch in center
each ldquospokerdquo runs a (separate) Ethernet protocol (nodes
do not collide with each other)
switch
bus coaxial cable star
Link Layer 5-56
Ethernet frame structure
sending adapter encapsulates IP datagram (or other network layer protocol packet) in Ethernet frame
preamble
7 bytes with pattern 10101010 followed by one byte with pattern 10101011
used to synchronize receiver sender clock rates
dest address
source address
data (payload) CRC preamble
type
Link Layer 5-57
Ethernet frame structure (more)
addresses 6 byte source destination MAC addresses if adapter receives frame with matching destination
address or with broadcast address (eg ARP packet) it passes data in frame to network layer protocol
otherwise adapter discards frame
type indicates higher layer protocol (mostly IP but others possible eg Novell IPX AppleTalk)
CRC cyclic redundancy check at receiver error detected frame is dropped
dest address
source address
data (payload) CRC preamble
type
Link Layer 5-58
Ethernet unreliable connectionless
connectionless no handshaking between sending and receiving NICs
unreliable receiving NIC doesnt send acks or nacks to sending NIC
data in dropped frames recovered only if initial sender uses higher layer rdt (eg TCP) otherwise dropped data lost
Ethernetrsquos MAC protocol unslotted CSMACD wth binary backoff
Link Layer 5-59
8023 Ethernet standards link amp physical layers
many different Ethernet standards
common MAC protocol and frame format
different speeds 2 Mbps 10 Mbps 100 Mbps 1Gbps 10G bps
different physical layer media fiber cable
application
transport
network
link
physical
MAC protocol
and frame format
100BASE-TX
100BASE-T4
100BASE-FX 100BASE-T2
100BASE-SX 100BASE-BX
fiber physical layer copper (twister
pair) physical layer
Link Layer 5-60
Link layer LANs outline
51 introduction services
52 error detection correction
53 multiple access protocols
54 LANs addressing ARP
Ethernet
switches
VLANS
55 link virtualization MPLS
56 data center networking
57 a day in the life of a web request
Link Layer 5-61
Ethernet switch link-layer device takes an active role
store forward Ethernet frames
examine incoming framersquos MAC address selectively forward frame to one-or-more outgoing links when frame is to be forwarded on segment uses CSMACD to access segment
transparent
hosts are unaware of presence of switches
plug-and-play self-learning
switches do not need to be configured
Link Layer 5-62
Switch multiple simultaneous transmissions
hosts have dedicated direct connection to switch
switches buffer packets
Ethernet protocol used on each incoming link but no collisions full duplex
each link is its own collision domain
switching A-to-Arsquo and B-to-Brsquo can transmit simultaneously without collisions switch with six interfaces
(123456)
A
Arsquo
B
Brsquo C
Crsquo
1 2
3 4 5
6
Link Layer 5-63
Switch forwarding table
Q how does switch know Arsquo reachable via interface 4 Brsquo reachable via interface 5
switch with six interfaces
(123456)
A
Arsquo
B
Brsquo C
Crsquo
1 2
3 4 5
6 A each switch has a switch table each entry
(MAC address of host interface to
reach host time stamp)
looks like a routing table
Q how are entries created maintained in switch table
something like a routing protocol
A
Arsquo
B
Brsquo C
Crsquo
1 2
3 4 5
6
Link Layer 5-64
Switch self-learning
switch learns which hosts can be reached through which interfaces
when frame received switch ldquolearnsrdquo location of sender incoming LAN segment
records senderlocation pair in switch table
A Arsquo
Source A
Dest Arsquo
MAC addr interface TTL
Switch table
(initially empty) A 1 60
Link Layer 5-65
Switch frame filteringforwarding
when frame received at switch 1 record incoming link MAC address of sending host
2 index switch table using MAC destination address
3 if entry found for destination then
if destination on segment from which frame arrived then drop frame
else forward frame on interface indicated by entry
else flood forward on all interfaces except arriving
interface
A
Arsquo
B
Brsquo C
Crsquo
1 2
3 4 5
6
Link Layer 5-66
Self-learning forwarding example
A Arsquo
Source A
Dest Arsquo
MAC addr interface TTL
switch table
(initially empty) A 1 60
A Arsquo A Arsquo A Arsquo A Arsquo A Arsquo
frame destination Arsquo locaton unknown flood
Arsquo A
destination A location
known
Arsquo 4 60
selectively send
on just one link
Link Layer 5-67
Interconnecting switches
switches can be connected together
Q sending from A to G - how does S1 know to forward frame destined to F via S4 and S3
A self learning (works exactly the same as in single-switch case)
A
B
S1
C D
E
F
S2
S4
S3
H
I
G
Link Layer 5-68
Self-learning multi-switch example
Suppose C sends frame to I I responds to C
Q show switch tables and packet forwarding in S1 S2 S3 S4
A
B
S1
C D
E
F
S2
S4
S3
H
I
G
Link Layer 5-69
Institutional network
to external
network
router
IP subnet
mail server
web server
Link Layer 5-70
Switches vs routers
both are store-and-forward
routers network-layer devices (examine network-layer headers)
switches link-layer devices (examine link-layer headers)
both have forwarding tables
routers compute tables using routing algorithms IP addresses
switches learn forwarding table using flooding learning MAC addresses
application
transport
network
link
physical
network
link
physical
link
physical
switch
datagram
application
transport
network
link
physical
frame
frame
frame
datagram
Link Layer 5-71
VLANs motivation
consider CS user moves office to
EE but wants connect to CS switch
single broadcast domain
all layer-2 broadcast traffic (ARP DHCP unknown location of destination MAC address) must cross entire LAN
securityprivacy efficiency issues
Computer
Science Electrical
Engineering
Computer
Engineering
Link Layer 5-72
VLANs port-based VLAN switch ports
grouped (by switch management software) so that single physical switch helliphellip
switch(es) supporting
VLAN capabilities can
be configured to
define multiple virtual
LANS over single
physical LAN
infrastructure
Virtual Local
Area Network 1
8
9
16 10 2
7
hellip
Electrical Engineering
(VLAN ports 1-8)
Computer Science
(VLAN ports 9-15)
15
hellip
Electrical Engineering
(VLAN ports 1-8)
hellip
1
8 2
7 9
16 10
15
hellip
Computer Science
(VLAN ports 9-16)
hellip operates as multiple virtual switches
Link Layer 5-73
Port-based VLAN
1
8
9
16 10 2
7
hellip
Electrical Engineering
(VLAN ports 1-8)
Computer Science
(VLAN ports 9-15)
15
hellip
traffic isolation frames tofrom ports 1-8 can only reach ports 1-8 can also define VLAN based on
MAC addresses of endpoints rather than switch port
dynamic membership ports can be dynamically assigned among VLANs
router
forwarding between VLANS done via routing (just as with separate switches) in practice vendors sell combined
switches plus routers
Link Layer 5-74
VLANS spanning multiple switches
trunk port carries frames between VLANS defined over multiple physical switches frames forwarded within VLAN between switches canrsquot be vanilla
8021 frames (must carry VLAN ID info)
8021q protocol addsremoved additional header fields for frames forwarded between trunk ports
1
8
9
10 2
7
hellip
Electrical Engineering
(VLAN ports 1-8)
Computer Science
(VLAN ports 9-15)
15
hellip
2
7 3
Ports 235 belong to EE VLAN
Ports 4678 belong to CS VLAN
5
4 6 8 16
1
Link Layer 5-75
type
2-byte Tag Protocol Identifier
(value 81-00)
Tag Control Information (12 bit VLAN ID field
3 bit priority field like IP TOS)
Recomputed CRC
8021Q VLAN frame format
8021 frame
8021Q frame
dest address
source address
data (payload) CRC preamble
dest address
source address
preamble data (payload) CRC
type
Link Layer 5-76
Link layer LANs outline
51 introduction services
52 error detection correction
53 multiple access protocols
54 LANs addressing ARP
Ethernet
switches
VLANS
55 link virtualization MPLS
56 data center networking
57 a day in the life of a web request
Link Layer 5-77
Multiprotocol label switching (MPLS)
initial goal high-speed IP forwarding using fixed length label (instead of IP address) fast lookup using fixed length identifier (rather than
shortest prefix matching)
borrowing ideas from Virtual Circuit (VC) approach
but IP datagram still keeps IP address
PPP or Ethernet
header IP header remainder of link-layer frame MPLS header
label Exp S TTL
20 3 1 5
Link Layer 5-78
MPLS capable routers
aka label-switched router
forward packets to outgoing interface based only on label value (donrsquot inspect IP address) MPLS forwarding table distinct from IP forwarding tables
flexibility MPLS forwarding decisions can differ from those of IP use destination and source addresses to route flows to
same destination differently (traffic engineering)
re-route flows quickly if link fails pre-computed backup paths (useful for VoIP)
Link Layer 5-79
R2
D
R3 R5
A
R6
MPLS versus IP paths
IP router IP routing path to destination determined
by destination address alone
R4
Link Layer 5-80
R2
D
R3 R4
R5
A
R6
MPLS versus IP paths
IP-only router
IP routing path to destination determined by destination address alone
MPLS and IP router
MPLS routing path to destination can be based on source and dest address fast reroute precompute backup routes in
case of link failure
entry router (R4) can use different MPLS
routes to A based eg on source address
Link Layer 5-81
MPLS signaling
modify OSPF IS-IS link-state flooding protocols to carry info used by MPLS routing eg link bandwidth amount of ldquoreservedrdquo link bandwidth
D
R4
R5
A
R6
entry MPLS router uses RSVP-TE signaling protocol to set up MPLS forwarding at downstream routers
modified link state flooding
RSVP-TE
Link Layer 5-82
R1 R2
D
R3 R4 R5
0
1
0 0
A
R6
in out out
label label dest interface
6 - A 0
in out out
label label dest interface
10 6 A 1
12 9 D 0
in out out
label label dest interface
10 A 0
12 D 0
1
in out out
label label dest interface
8 6 A 0
0
8 A 1
MPLS forwarding tables
Link Layer 5-83
Link layer LANs outline
51 introduction services
52 error detection correction
53 multiple access protocols
54 LANs addressing ARP
Ethernet
switches
VLANS
55 link virtualization MPLS
56 data center networking
57 a day in the life of a web request
Link Layer 5-84
Data center networks
10rsquos to 100rsquos of thousands of hosts often closely coupled in close proximity e-business (eg Amazon)
content-servers (eg YouTube Akamai Apple Microsoft)
search engines data mining (eg Google)
challenges
multiple applications each serving massive numbers of clients
managingbalancing load avoiding processing networking data bottlenecks
Inside a 40-ft Microsoft container
Chicago data center
Link Layer 5-85
Server racks
TOR switches
Tier-1 switches
Tier-2 switches
Load balancer
Load balancer
B
1 2 3 4 5 6 7 8
A C
Border router
Access router
Internet
Data center networks
load balancer application-layer routing receives external client requests
directs workload within data center
returns results to external client (hiding data
center internals from client)
Server racks
TOR switches
Tier-1 switches
Tier-2 switches
1 2 3 4 5 6 7 8
Data center networks
rich interconnection among switches racks
increased throughput between racks (multiple routing
paths possible)
increased reliability via redundancy
Link Layer 5-87
Link layer LANs outline
51 introduction services
52 error detection correction
53 multiple access protocols
54 LANs addressing ARP
Ethernet
switches
VLANS
55 link virtualization MPLS
56 data center networking
57 a day in the life of a web request
Link Layer 5-88
Synthesis a day in the life of a web request
journey down protocol stack complete application transport network link
putting-it-all-together synthesis goal identify review understand protocols (at all
layers) involved in seemingly simple scenario requesting www page
scenario student attaches laptop to campus network requestsreceives wwwgooglecom
Link Layer 5-89
A day in the life scenario
Comcast network
68800013
Googlersquos network
64233160019 64233169105
web server
DNS server
school network
68802024
web page
browser
router
(runs DHCP)
Link Layer 5-90
A day in the lifehellip connecting to the Internet
connecting laptop needs to get its own IP address addr of first-hop router addr of DNS server use DHCP
DHCP
UDP
IP
Eth
Phy
DHCP
DHCP
DHCP
DHCP
DHCP
DHCP
UDP
IP
Eth
Phy
DHCP
DHCP
DHCP
DHCP DHCP
DHCP request encapsulated in UDP encapsulated in IP encapsulated in 8023 Ethernet
Ethernet frame broadcast
(dest FFFFFFFFFFFF) on LAN received at router running DHCP server
Ethernet demuxed to IP demuxed UDP demuxed to DHCP
router
(runs DHCP)
Link Layer 5-91
DHCP server formulates DHCP ACK containing clientrsquos IP address IP address of first-hop router for client name amp IP address of DNS server
DHCP
UDP
IP
Eth
Phy
DHCP
DHCP
DHCP
DHCP
DHCP
UDP
IP
Eth
Phy
DHCP
DHCP
DHCP
DHCP
DHCP
encapsulation at DHCP server frame forwarded (switch learning) through LAN demultiplexing at client
Client now has IP address knows name amp addr of DNS
server IP address of its first-hop router
DHCP client receives DHCP ACK reply
A day in the lifehellip connecting to the Internet
router
(runs DHCP)
Link Layer 5-92
A day in the lifehellip ARP (before DNS before HTTP)
before sending HTTP request need IP address of wwwgooglecom DNS
DNS
UDP
IP
Eth
Phy
DNS
DNS
DNS
DNS query created encapsulated in UDP encapsulated in IP encapsulated in Eth To send frame to router need MAC address of router interface ARP
ARP query broadcast received by
router which replies with ARP reply giving MAC address of router interface
client now knows MAC address of first hop router so can now send frame containing DNS query
ARP query
Eth
Phy
ARP
ARP
ARP reply
router
(runs DHCP)
Link Layer 5-93
DNS
UDP
IP
Eth
Phy
DNS
DNS
DNS
DNS
DNS
IP datagram containing DNS query forwarded via LAN switch from client to 1st hop router
IP datagram forwarded from campus network into comcast network routed (tables created by RIP OSPF IS-IS andor BGP routing protocols) to DNS server
demuxrsquoed to DNS server
DNS server replies to client with IP address of wwwgooglecom
Comcast network
68800013
DNS server
DNS
UDP
IP
Eth
Phy
DNS
DNS
DNS
DNS
A day in the lifehellip using DNS
router
(runs DHCP)
Link Layer 5-94
A day in the lifehellipTCP connection carrying HTTP
HTTP
TCP
IP
Eth
Phy
HTTP
to send HTTP request client first opens TCP socket to web server
TCP SYN segment (step 1 in 3-way handshake) inter-domain routed to web server
TCP connection established 64233169105
web server
SYN
SYN
SYN
SYN
TCP
IP
Eth
Phy
SYN
SYN
SYN
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
web server responds with TCP SYNACK (step 2 in 3-way handshake)
router
(runs DHCP)
Link Layer 5-95
A day in the lifehellip HTTP requestreply
HTTP
TCP
IP
Eth
Phy
HTTP
HTTP request sent into TCP socket
IP datagram containing HTTP request routed to wwwgooglecom
IP datagram containing HTTP reply routed back to client
64233169105
web server
HTTP
TCP
IP
Eth
Phy web server responds with
HTTP reply (containing web page)
HTTP
HTTP
HTTP HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
web page finally () displayed
Link Layer 5-96
Chapter 5 Summary
principles behind data link layer services error detection correction
sharing a broadcast channel multiple access
link layer addressing
instantiation and implementation of various link layer technologies Ethernet
switched LANS VLANs
virtualized networks as a link layer MPLS
synthesis a day in the life of a web request
Link Layer 5-97
Chapter 5 letrsquos take a breath
journey down protocol stack complete (except PHY)
solid understanding of networking principles practice
hellip could stop here hellip but lots of interesting topics wireless
multimedia
security
network management
Link Layer 5-25
Pros single active node can
continuously transmit at full rate of channel
highly decentralized only slots in nodes need to be in sync
simple
Cons
collisions wasting slots
idle slots
nodes may be able to detect collision in less than time to transmit packet
clock synchronization
Slotted ALOHA
1 1 1 1
2
3
2 2
3 3
node 1
node 2
node 3
C C C S S S E E E
Link Layer 5-26
suppose N nodes with many frames to send each transmits in slot with probability p
prob that given node has success in a slot = p(1-p)N-1
prob that any node has a success = Np(1-p)N-1
max efficiency find p that maximizes Np(1-p)N-1
for many nodes take limit of Np(1-p)N-1 as N goes to infinity gives
max efficiency = 1e = 37
efficiency long-run fraction of successful slots (many nodes all with many frames to send)
at best channel used for useful transmissions 37 of time
Slotted ALOHA efficiency
Link Layer 5-27
Pure (unslotted) ALOHA
unslotted Aloha simpler no synchronization
when frame first arrives
transmit immediately
collision probability increases
frame sent at t0 collides with other frames sent in [t0-1t0+1]
Link Layer 5-28
Pure ALOHA efficiency
P(success by given node) = P(node transmits)
P(no other node transmits in [t0-1t0]
P(no other node transmits in [t0-1t0]
= p (1-p)N-1 (1-p)N-1
= p (1-p)2(N-1)
hellip choosing optimum p and then letting n
= 1(2e) = 18
even worse than slotted Aloha
Link Layer 5-29
CSMA (carrier sense multiple access)
CSMA listen before transmit if channel sensed idle transmit entire frame
if channel sensed busy defer transmission
human analogy donrsquot interrupt others
Link Layer 5-30
CSMA collisions
collisions can still occur propagation delay means two nodes may not hear each otherrsquos transmission
collision entire packet transmission time wasted distance amp propagation
delay play role in in determining collision probability
spatial layout of nodes
Link Layer 5-31
CSMACD (collision detection)
CSMACD carrier sensing deferral as in CSMA collisions detected within short time
colliding transmissions aborted reducing channel wastage
collision detection easy in wired LANs measure signal strengths compare
transmitted received signals
difficult in wireless LANs received signal strength overwhelmed by local transmission strength
human analogy the polite conversationalist
Link Layer 5-32
CSMACD (collision detection)
spatial layout of nodes
Link Layer 5-33
Ethernet CSMACD algorithm
1 NIC receives datagram from network layer creates frame
2 If NIC senses channel idle starts frame transmission If NIC senses channel busy waits until channel idle then transmits
3 If NIC transmits entire frame without detecting another transmission NIC is done with frame
4 If NIC detects another transmission while transmitting aborts and sends jam signal
5 After aborting NIC enters binary (exponential) backoff
after mth collision NIC chooses K at random from 012 hellip 2m-1 NIC waits K512 bit times returns to Step 2
longer backoff interval with more collisions
Link Layer 5-34
CSMACD efficiency
Tprop = max prop delay between 2 nodes in LAN
ttrans = time to transmit max-size frame
efficiency goes to 1
as tprop goes to 0
as ttrans goes to infinity
better performance than ALOHA and simple cheap decentralized
transpropttefficiency
51
1
Link Layer 5-35
ldquoTaking turnsrdquo MAC protocols
channel partitioning MAC protocols share channel efficiently and fairly at high load
inefficient at low load delay in channel access 1N bandwidth allocated even if only 1 active node
random access MAC protocols efficient at low load single node can fully utilize
channel
high load collision overhead
ldquotaking turnsrdquo protocols look for best of both worlds
Link Layer 5-36
polling master node ldquoinvitesrdquo
slave nodes to transmit in turn
typically used with ldquodumbrdquo slave devices
concerns
polling overhead
latency
single point of failure (master)
master
slaves
poll
data
data
ldquoTaking turnsrdquo MAC protocols
Link Layer 5-37
token passing control token passed
from one node to next sequentially
token message
concerns
token overhead
latency
single point of failure (token)
T
data
(nothing
to send)
T
ldquoTaking turnsrdquo MAC protocols
cable headend
CMTS
ISP
cable modem termination system
multiple 40Mbps downstream (broadcast) channels
single CMTS transmits into channels
multiple 30 Mbps upstream channels
multiple access all users contend for certain upstream channel time slots (others assigned)
Cable access network
cable modem
splitter
hellip
hellip
Internet framesTV channels control transmitted downstream at different frequencies
upstream Internet frames TV control transmitted upstream at different frequencies in time slots
Link Layer 5-39
DOCSIS data over cable service interface spec FDM over upstream downstream frequency channels
TDM upstream some slots assigned some have contention
downstream MAP frame assigns upstream slots
request for upstream slots (and data) transmitted random access (binary backoff) in selected slots
MAP frame for
Interval [t1 t2]
Residences with cable modems
Downstream channel i
Upstream channel j
t1 t2
Assigned minislots containing cable modem
upstream data frames
Minislots containing
minislots request frames
cable headend
CMTS
Cable access network
Link Layer 5-40
Summary of MAC protocols
channel partitioning by time frequency or code Time Division Frequency Division
random access (dynamic)
ALOHA S-ALOHA CSMA CSMACD
carrier sensing easy in some technologies (wire) hard in others (wireless)
CSMACD used in Ethernet
CSMACA used in 80211
taking turns
polling from central site token passing
bluetooth FDDI token ring
Link Layer 5-41
Link layer LANs outline
51 introduction services
52 error detection correction
53 multiple access protocols
54 LANs addressing ARP
Ethernet
switches
VLANS
55 link virtualization MPLS
56 data center networking
57 a day in the life of a web request
Link Layer 5-42
MAC addresses and ARP
32-bit IP address network-layer address for interface
used for layer 3 (network layer) forwarding
MAC (or LAN or physical or Ethernet) address function used lsquolocallyrdquo to get frame from one interface to
another physically-connected interface (same network in IP-addressing sense)
48 bit MAC address (for most LANs) burned in NIC ROM also sometimes software settable
eg 1A-2F-BB-76-09-AD
hexadecimal (base 16) notation
(each ldquonumberrdquo represents 4 bits)
Link Layer 5-43
LAN addresses and ARP
each adapter on LAN has unique LAN address
adapter
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN
(wired or
wireless)
Link Layer 5-44
LAN addresses (more)
MAC address allocation administered by IEEE
manufacturer buys portion of MAC address space (to assure uniqueness)
analogy MAC address like Social Security Number
IP address like postal address
MAC flat address portability can move LAN card from one LAN to another
IP hierarchical address not portable address depends on IP subnet to which node is
attached
Link Layer 5-45
ARP address resolution protocol
ARP table each IP node (host router) on LAN has table
IPMAC address mappings for some LAN nodes
lt IP address MAC address TTLgt
TTL (Time To Live) time after which address mapping will be forgotten (typically 20 min)
Question how to determine
interfacersquos MAC address
knowing its IP address
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN
137196723
137196778
137196714
137196788
Link Layer 5-46
ARP protocol same LAN
A wants to send datagram to B Brsquos MAC address not in
Arsquos ARP table
A broadcasts ARP query packet containing Bs IP address dest MAC address = FF-FF-
FF-FF-FF-FF
all nodes on LAN receive ARP query
B receives ARP packet replies to A with its (Bs) MAC address frame sent to Arsquos MAC
address (unicast)
A caches (saves) IP-to-MAC address pair in its ARP table until information becomes old (times out) soft state information that
times out (goes away) unless refreshed
ARP is ldquoplug-and-playrdquo nodes create their ARP
tables without intervention from net administrator
Link Layer 5-47
walkthrough send datagram from A to B via R
focus on addressing ndash at IP (datagram) and MAC layer (frame)
assume A knows Brsquos IP address
assume A knows IP address of first hop router R (how)
assume A knows Rrsquos MAC address (how)
Addressing routing to another LAN
R
1A-23-F9-CD-06-9B 222222222220
111111111110 E6-E9-00-17-BB-4B CC-49-DE-D0-AB-7D
111111111112
111111111111
74-29-9C-E8-FF-55
A
222222222222
49-BD-D2-C7-56-2A
222222222221 88-B2-2F-54-1A-0F
B
R
1A-23-F9-CD-06-9B 222222222220
111111111110 E6-E9-00-17-BB-4B CC-49-DE-D0-AB-7D
111111111112
111111111111
74-29-9C-E8-FF-55
A
222222222222
49-BD-D2-C7-56-2A
222222222221 88-B2-2F-54-1A-0F
B
Link Layer 5-48
Addressing routing to another LAN
IP
Eth
Phy
IP src 111111111111
IP dest 222222222222
A creates IP datagram with IP source A destination B
A creates link-layer frame with Rs MAC address as dest frame contains A-to-B IP datagram
MAC src 74-29-9C-E8-FF-55
MAC dest E6-E9-00-17-BB-4B
R
1A-23-F9-CD-06-9B 222222222220
111111111110 E6-E9-00-17-BB-4B CC-49-DE-D0-AB-7D
111111111112
111111111111
74-29-9C-E8-FF-55
A
222222222222
49-BD-D2-C7-56-2A
222222222221 88-B2-2F-54-1A-0F
B
Link Layer 5-49
Addressing routing to another LAN
IP
Eth
Phy
frame sent from A to R
IP
Eth
Phy
frame received at R datagram removed passed up to IP
MAC src 74-29-9C-E8-FF-55
MAC dest E6-E9-00-17-BB-4B
IP src 111111111111
IP dest 222222222222
IP src 111111111111
IP dest 222222222222
R
1A-23-F9-CD-06-9B 222222222220
111111111110 E6-E9-00-17-BB-4B CC-49-DE-D0-AB-7D
111111111112
111111111111
74-29-9C-E8-FF-55
A
222222222222
49-BD-D2-C7-56-2A
222222222221 88-B2-2F-54-1A-0F
B
Link Layer 5-50
Addressing routing to another LAN
IP src 111111111111
IP dest 222222222222
R forwards datagram with IP source A destination B
R creates link-layer frame with Bs MAC address as dest frame contains A-to-B IP datagram
MAC src 1A-23-F9-CD-06-9B
MAC dest 49-BD-D2-C7-56-2A
IP
Eth
Phy
IP
Eth
Phy
R
1A-23-F9-CD-06-9B 222222222220
111111111110 E6-E9-00-17-BB-4B CC-49-DE-D0-AB-7D
111111111112
111111111111
74-29-9C-E8-FF-55
A
222222222222
49-BD-D2-C7-56-2A
222222222221 88-B2-2F-54-1A-0F
B
Link Layer 5-51
Addressing routing to another LAN R forwards datagram with IP source A destination B
R creates link-layer frame with Bs MAC address as dest frame contains A-to-B IP datagram
IP src 111111111111
IP dest 222222222222
MAC src 1A-23-F9-CD-06-9B
MAC dest 49-BD-D2-C7-56-2A
IP
Eth
Phy
IP
Eth
Phy
R
1A-23-F9-CD-06-9B 222222222220
111111111110 E6-E9-00-17-BB-4B CC-49-DE-D0-AB-7D
111111111112
111111111111
74-29-9C-E8-FF-55
A
222222222222
49-BD-D2-C7-56-2A
222222222221 88-B2-2F-54-1A-0F
B
Link Layer 5-52
Addressing routing to another LAN R forwards datagram with IP source A destination B
R creates link-layer frame with Bs MAC address as dest frame contains A-to-B IP datagram
IP src 111111111111
IP dest 222222222222
MAC src 1A-23-F9-CD-06-9B
MAC dest 49-BD-D2-C7-56-2A
IP
Eth
Phy
Link Layer 5-53
Link layer LANs outline
51 introduction services
52 error detection correction
53 multiple access protocols
54 LANs addressing ARP
Ethernet
switches
VLANS
55 link virtualization MPLS
56 data center networking
57 a day in the life of a web request
Link Layer 5-54
Ethernet
ldquodominantrdquo wired LAN technology
cheap $20 for NIC
first widely used LAN technology
simpler cheaper than token LANs and ATM
kept up with speed race 10 Mbps ndash 10 Gbps
Metcalfersquos Ethernet sketch
Link Layer 5-55
Ethernet physical topology
bus popular through mid 90s all nodes in same collision domain (can collide with each
other)
star prevails today active switch in center
each ldquospokerdquo runs a (separate) Ethernet protocol (nodes
do not collide with each other)
switch
bus coaxial cable star
Link Layer 5-56
Ethernet frame structure
sending adapter encapsulates IP datagram (or other network layer protocol packet) in Ethernet frame
preamble
7 bytes with pattern 10101010 followed by one byte with pattern 10101011
used to synchronize receiver sender clock rates
dest address
source address
data (payload) CRC preamble
type
Link Layer 5-57
Ethernet frame structure (more)
addresses 6 byte source destination MAC addresses if adapter receives frame with matching destination
address or with broadcast address (eg ARP packet) it passes data in frame to network layer protocol
otherwise adapter discards frame
type indicates higher layer protocol (mostly IP but others possible eg Novell IPX AppleTalk)
CRC cyclic redundancy check at receiver error detected frame is dropped
dest address
source address
data (payload) CRC preamble
type
Link Layer 5-58
Ethernet unreliable connectionless
connectionless no handshaking between sending and receiving NICs
unreliable receiving NIC doesnt send acks or nacks to sending NIC
data in dropped frames recovered only if initial sender uses higher layer rdt (eg TCP) otherwise dropped data lost
Ethernetrsquos MAC protocol unslotted CSMACD wth binary backoff
Link Layer 5-59
8023 Ethernet standards link amp physical layers
many different Ethernet standards
common MAC protocol and frame format
different speeds 2 Mbps 10 Mbps 100 Mbps 1Gbps 10G bps
different physical layer media fiber cable
application
transport
network
link
physical
MAC protocol
and frame format
100BASE-TX
100BASE-T4
100BASE-FX 100BASE-T2
100BASE-SX 100BASE-BX
fiber physical layer copper (twister
pair) physical layer
Link Layer 5-60
Link layer LANs outline
51 introduction services
52 error detection correction
53 multiple access protocols
54 LANs addressing ARP
Ethernet
switches
VLANS
55 link virtualization MPLS
56 data center networking
57 a day in the life of a web request
Link Layer 5-61
Ethernet switch link-layer device takes an active role
store forward Ethernet frames
examine incoming framersquos MAC address selectively forward frame to one-or-more outgoing links when frame is to be forwarded on segment uses CSMACD to access segment
transparent
hosts are unaware of presence of switches
plug-and-play self-learning
switches do not need to be configured
Link Layer 5-62
Switch multiple simultaneous transmissions
hosts have dedicated direct connection to switch
switches buffer packets
Ethernet protocol used on each incoming link but no collisions full duplex
each link is its own collision domain
switching A-to-Arsquo and B-to-Brsquo can transmit simultaneously without collisions switch with six interfaces
(123456)
A
Arsquo
B
Brsquo C
Crsquo
1 2
3 4 5
6
Link Layer 5-63
Switch forwarding table
Q how does switch know Arsquo reachable via interface 4 Brsquo reachable via interface 5
switch with six interfaces
(123456)
A
Arsquo
B
Brsquo C
Crsquo
1 2
3 4 5
6 A each switch has a switch table each entry
(MAC address of host interface to
reach host time stamp)
looks like a routing table
Q how are entries created maintained in switch table
something like a routing protocol
A
Arsquo
B
Brsquo C
Crsquo
1 2
3 4 5
6
Link Layer 5-64
Switch self-learning
switch learns which hosts can be reached through which interfaces
when frame received switch ldquolearnsrdquo location of sender incoming LAN segment
records senderlocation pair in switch table
A Arsquo
Source A
Dest Arsquo
MAC addr interface TTL
Switch table
(initially empty) A 1 60
Link Layer 5-65
Switch frame filteringforwarding
when frame received at switch 1 record incoming link MAC address of sending host
2 index switch table using MAC destination address
3 if entry found for destination then
if destination on segment from which frame arrived then drop frame
else forward frame on interface indicated by entry
else flood forward on all interfaces except arriving
interface
A
Arsquo
B
Brsquo C
Crsquo
1 2
3 4 5
6
Link Layer 5-66
Self-learning forwarding example
A Arsquo
Source A
Dest Arsquo
MAC addr interface TTL
switch table
(initially empty) A 1 60
A Arsquo A Arsquo A Arsquo A Arsquo A Arsquo
frame destination Arsquo locaton unknown flood
Arsquo A
destination A location
known
Arsquo 4 60
selectively send
on just one link
Link Layer 5-67
Interconnecting switches
switches can be connected together
Q sending from A to G - how does S1 know to forward frame destined to F via S4 and S3
A self learning (works exactly the same as in single-switch case)
A
B
S1
C D
E
F
S2
S4
S3
H
I
G
Link Layer 5-68
Self-learning multi-switch example
Suppose C sends frame to I I responds to C
Q show switch tables and packet forwarding in S1 S2 S3 S4
A
B
S1
C D
E
F
S2
S4
S3
H
I
G
Link Layer 5-69
Institutional network
to external
network
router
IP subnet
mail server
web server
Link Layer 5-70
Switches vs routers
both are store-and-forward
routers network-layer devices (examine network-layer headers)
switches link-layer devices (examine link-layer headers)
both have forwarding tables
routers compute tables using routing algorithms IP addresses
switches learn forwarding table using flooding learning MAC addresses
application
transport
network
link
physical
network
link
physical
link
physical
switch
datagram
application
transport
network
link
physical
frame
frame
frame
datagram
Link Layer 5-71
VLANs motivation
consider CS user moves office to
EE but wants connect to CS switch
single broadcast domain
all layer-2 broadcast traffic (ARP DHCP unknown location of destination MAC address) must cross entire LAN
securityprivacy efficiency issues
Computer
Science Electrical
Engineering
Computer
Engineering
Link Layer 5-72
VLANs port-based VLAN switch ports
grouped (by switch management software) so that single physical switch helliphellip
switch(es) supporting
VLAN capabilities can
be configured to
define multiple virtual
LANS over single
physical LAN
infrastructure
Virtual Local
Area Network 1
8
9
16 10 2
7
hellip
Electrical Engineering
(VLAN ports 1-8)
Computer Science
(VLAN ports 9-15)
15
hellip
Electrical Engineering
(VLAN ports 1-8)
hellip
1
8 2
7 9
16 10
15
hellip
Computer Science
(VLAN ports 9-16)
hellip operates as multiple virtual switches
Link Layer 5-73
Port-based VLAN
1
8
9
16 10 2
7
hellip
Electrical Engineering
(VLAN ports 1-8)
Computer Science
(VLAN ports 9-15)
15
hellip
traffic isolation frames tofrom ports 1-8 can only reach ports 1-8 can also define VLAN based on
MAC addresses of endpoints rather than switch port
dynamic membership ports can be dynamically assigned among VLANs
router
forwarding between VLANS done via routing (just as with separate switches) in practice vendors sell combined
switches plus routers
Link Layer 5-74
VLANS spanning multiple switches
trunk port carries frames between VLANS defined over multiple physical switches frames forwarded within VLAN between switches canrsquot be vanilla
8021 frames (must carry VLAN ID info)
8021q protocol addsremoved additional header fields for frames forwarded between trunk ports
1
8
9
10 2
7
hellip
Electrical Engineering
(VLAN ports 1-8)
Computer Science
(VLAN ports 9-15)
15
hellip
2
7 3
Ports 235 belong to EE VLAN
Ports 4678 belong to CS VLAN
5
4 6 8 16
1
Link Layer 5-75
type
2-byte Tag Protocol Identifier
(value 81-00)
Tag Control Information (12 bit VLAN ID field
3 bit priority field like IP TOS)
Recomputed CRC
8021Q VLAN frame format
8021 frame
8021Q frame
dest address
source address
data (payload) CRC preamble
dest address
source address
preamble data (payload) CRC
type
Link Layer 5-76
Link layer LANs outline
51 introduction services
52 error detection correction
53 multiple access protocols
54 LANs addressing ARP
Ethernet
switches
VLANS
55 link virtualization MPLS
56 data center networking
57 a day in the life of a web request
Link Layer 5-77
Multiprotocol label switching (MPLS)
initial goal high-speed IP forwarding using fixed length label (instead of IP address) fast lookup using fixed length identifier (rather than
shortest prefix matching)
borrowing ideas from Virtual Circuit (VC) approach
but IP datagram still keeps IP address
PPP or Ethernet
header IP header remainder of link-layer frame MPLS header
label Exp S TTL
20 3 1 5
Link Layer 5-78
MPLS capable routers
aka label-switched router
forward packets to outgoing interface based only on label value (donrsquot inspect IP address) MPLS forwarding table distinct from IP forwarding tables
flexibility MPLS forwarding decisions can differ from those of IP use destination and source addresses to route flows to
same destination differently (traffic engineering)
re-route flows quickly if link fails pre-computed backup paths (useful for VoIP)
Link Layer 5-79
R2
D
R3 R5
A
R6
MPLS versus IP paths
IP router IP routing path to destination determined
by destination address alone
R4
Link Layer 5-80
R2
D
R3 R4
R5
A
R6
MPLS versus IP paths
IP-only router
IP routing path to destination determined by destination address alone
MPLS and IP router
MPLS routing path to destination can be based on source and dest address fast reroute precompute backup routes in
case of link failure
entry router (R4) can use different MPLS
routes to A based eg on source address
Link Layer 5-81
MPLS signaling
modify OSPF IS-IS link-state flooding protocols to carry info used by MPLS routing eg link bandwidth amount of ldquoreservedrdquo link bandwidth
D
R4
R5
A
R6
entry MPLS router uses RSVP-TE signaling protocol to set up MPLS forwarding at downstream routers
modified link state flooding
RSVP-TE
Link Layer 5-82
R1 R2
D
R3 R4 R5
0
1
0 0
A
R6
in out out
label label dest interface
6 - A 0
in out out
label label dest interface
10 6 A 1
12 9 D 0
in out out
label label dest interface
10 A 0
12 D 0
1
in out out
label label dest interface
8 6 A 0
0
8 A 1
MPLS forwarding tables
Link Layer 5-83
Link layer LANs outline
51 introduction services
52 error detection correction
53 multiple access protocols
54 LANs addressing ARP
Ethernet
switches
VLANS
55 link virtualization MPLS
56 data center networking
57 a day in the life of a web request
Link Layer 5-84
Data center networks
10rsquos to 100rsquos of thousands of hosts often closely coupled in close proximity e-business (eg Amazon)
content-servers (eg YouTube Akamai Apple Microsoft)
search engines data mining (eg Google)
challenges
multiple applications each serving massive numbers of clients
managingbalancing load avoiding processing networking data bottlenecks
Inside a 40-ft Microsoft container
Chicago data center
Link Layer 5-85
Server racks
TOR switches
Tier-1 switches
Tier-2 switches
Load balancer
Load balancer
B
1 2 3 4 5 6 7 8
A C
Border router
Access router
Internet
Data center networks
load balancer application-layer routing receives external client requests
directs workload within data center
returns results to external client (hiding data
center internals from client)
Server racks
TOR switches
Tier-1 switches
Tier-2 switches
1 2 3 4 5 6 7 8
Data center networks
rich interconnection among switches racks
increased throughput between racks (multiple routing
paths possible)
increased reliability via redundancy
Link Layer 5-87
Link layer LANs outline
51 introduction services
52 error detection correction
53 multiple access protocols
54 LANs addressing ARP
Ethernet
switches
VLANS
55 link virtualization MPLS
56 data center networking
57 a day in the life of a web request
Link Layer 5-88
Synthesis a day in the life of a web request
journey down protocol stack complete application transport network link
putting-it-all-together synthesis goal identify review understand protocols (at all
layers) involved in seemingly simple scenario requesting www page
scenario student attaches laptop to campus network requestsreceives wwwgooglecom
Link Layer 5-89
A day in the life scenario
Comcast network
68800013
Googlersquos network
64233160019 64233169105
web server
DNS server
school network
68802024
web page
browser
router
(runs DHCP)
Link Layer 5-90
A day in the lifehellip connecting to the Internet
connecting laptop needs to get its own IP address addr of first-hop router addr of DNS server use DHCP
DHCP
UDP
IP
Eth
Phy
DHCP
DHCP
DHCP
DHCP
DHCP
DHCP
UDP
IP
Eth
Phy
DHCP
DHCP
DHCP
DHCP DHCP
DHCP request encapsulated in UDP encapsulated in IP encapsulated in 8023 Ethernet
Ethernet frame broadcast
(dest FFFFFFFFFFFF) on LAN received at router running DHCP server
Ethernet demuxed to IP demuxed UDP demuxed to DHCP
router
(runs DHCP)
Link Layer 5-91
DHCP server formulates DHCP ACK containing clientrsquos IP address IP address of first-hop router for client name amp IP address of DNS server
DHCP
UDP
IP
Eth
Phy
DHCP
DHCP
DHCP
DHCP
DHCP
UDP
IP
Eth
Phy
DHCP
DHCP
DHCP
DHCP
DHCP
encapsulation at DHCP server frame forwarded (switch learning) through LAN demultiplexing at client
Client now has IP address knows name amp addr of DNS
server IP address of its first-hop router
DHCP client receives DHCP ACK reply
A day in the lifehellip connecting to the Internet
router
(runs DHCP)
Link Layer 5-92
A day in the lifehellip ARP (before DNS before HTTP)
before sending HTTP request need IP address of wwwgooglecom DNS
DNS
UDP
IP
Eth
Phy
DNS
DNS
DNS
DNS query created encapsulated in UDP encapsulated in IP encapsulated in Eth To send frame to router need MAC address of router interface ARP
ARP query broadcast received by
router which replies with ARP reply giving MAC address of router interface
client now knows MAC address of first hop router so can now send frame containing DNS query
ARP query
Eth
Phy
ARP
ARP
ARP reply
router
(runs DHCP)
Link Layer 5-93
DNS
UDP
IP
Eth
Phy
DNS
DNS
DNS
DNS
DNS
IP datagram containing DNS query forwarded via LAN switch from client to 1st hop router
IP datagram forwarded from campus network into comcast network routed (tables created by RIP OSPF IS-IS andor BGP routing protocols) to DNS server
demuxrsquoed to DNS server
DNS server replies to client with IP address of wwwgooglecom
Comcast network
68800013
DNS server
DNS
UDP
IP
Eth
Phy
DNS
DNS
DNS
DNS
A day in the lifehellip using DNS
router
(runs DHCP)
Link Layer 5-94
A day in the lifehellipTCP connection carrying HTTP
HTTP
TCP
IP
Eth
Phy
HTTP
to send HTTP request client first opens TCP socket to web server
TCP SYN segment (step 1 in 3-way handshake) inter-domain routed to web server
TCP connection established 64233169105
web server
SYN
SYN
SYN
SYN
TCP
IP
Eth
Phy
SYN
SYN
SYN
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
web server responds with TCP SYNACK (step 2 in 3-way handshake)
router
(runs DHCP)
Link Layer 5-95
A day in the lifehellip HTTP requestreply
HTTP
TCP
IP
Eth
Phy
HTTP
HTTP request sent into TCP socket
IP datagram containing HTTP request routed to wwwgooglecom
IP datagram containing HTTP reply routed back to client
64233169105
web server
HTTP
TCP
IP
Eth
Phy web server responds with
HTTP reply (containing web page)
HTTP
HTTP
HTTP HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
web page finally () displayed
Link Layer 5-96
Chapter 5 Summary
principles behind data link layer services error detection correction
sharing a broadcast channel multiple access
link layer addressing
instantiation and implementation of various link layer technologies Ethernet
switched LANS VLANs
virtualized networks as a link layer MPLS
synthesis a day in the life of a web request
Link Layer 5-97
Chapter 5 letrsquos take a breath
journey down protocol stack complete (except PHY)
solid understanding of networking principles practice
hellip could stop here hellip but lots of interesting topics wireless
multimedia
security
network management
Link Layer 5-26
suppose N nodes with many frames to send each transmits in slot with probability p
prob that given node has success in a slot = p(1-p)N-1
prob that any node has a success = Np(1-p)N-1
max efficiency find p that maximizes Np(1-p)N-1
for many nodes take limit of Np(1-p)N-1 as N goes to infinity gives
max efficiency = 1e = 37
efficiency long-run fraction of successful slots (many nodes all with many frames to send)
at best channel used for useful transmissions 37 of time
Slotted ALOHA efficiency
Link Layer 5-27
Pure (unslotted) ALOHA
unslotted Aloha simpler no synchronization
when frame first arrives
transmit immediately
collision probability increases
frame sent at t0 collides with other frames sent in [t0-1t0+1]
Link Layer 5-28
Pure ALOHA efficiency
P(success by given node) = P(node transmits)
P(no other node transmits in [t0-1t0]
P(no other node transmits in [t0-1t0]
= p (1-p)N-1 (1-p)N-1
= p (1-p)2(N-1)
hellip choosing optimum p and then letting n
= 1(2e) = 18
even worse than slotted Aloha
Link Layer 5-29
CSMA (carrier sense multiple access)
CSMA listen before transmit if channel sensed idle transmit entire frame
if channel sensed busy defer transmission
human analogy donrsquot interrupt others
Link Layer 5-30
CSMA collisions
collisions can still occur propagation delay means two nodes may not hear each otherrsquos transmission
collision entire packet transmission time wasted distance amp propagation
delay play role in in determining collision probability
spatial layout of nodes
Link Layer 5-31
CSMACD (collision detection)
CSMACD carrier sensing deferral as in CSMA collisions detected within short time
colliding transmissions aborted reducing channel wastage
collision detection easy in wired LANs measure signal strengths compare
transmitted received signals
difficult in wireless LANs received signal strength overwhelmed by local transmission strength
human analogy the polite conversationalist
Link Layer 5-32
CSMACD (collision detection)
spatial layout of nodes
Link Layer 5-33
Ethernet CSMACD algorithm
1 NIC receives datagram from network layer creates frame
2 If NIC senses channel idle starts frame transmission If NIC senses channel busy waits until channel idle then transmits
3 If NIC transmits entire frame without detecting another transmission NIC is done with frame
4 If NIC detects another transmission while transmitting aborts and sends jam signal
5 After aborting NIC enters binary (exponential) backoff
after mth collision NIC chooses K at random from 012 hellip 2m-1 NIC waits K512 bit times returns to Step 2
longer backoff interval with more collisions
Link Layer 5-34
CSMACD efficiency
Tprop = max prop delay between 2 nodes in LAN
ttrans = time to transmit max-size frame
efficiency goes to 1
as tprop goes to 0
as ttrans goes to infinity
better performance than ALOHA and simple cheap decentralized
transpropttefficiency
51
1
Link Layer 5-35
ldquoTaking turnsrdquo MAC protocols
channel partitioning MAC protocols share channel efficiently and fairly at high load
inefficient at low load delay in channel access 1N bandwidth allocated even if only 1 active node
random access MAC protocols efficient at low load single node can fully utilize
channel
high load collision overhead
ldquotaking turnsrdquo protocols look for best of both worlds
Link Layer 5-36
polling master node ldquoinvitesrdquo
slave nodes to transmit in turn
typically used with ldquodumbrdquo slave devices
concerns
polling overhead
latency
single point of failure (master)
master
slaves
poll
data
data
ldquoTaking turnsrdquo MAC protocols
Link Layer 5-37
token passing control token passed
from one node to next sequentially
token message
concerns
token overhead
latency
single point of failure (token)
T
data
(nothing
to send)
T
ldquoTaking turnsrdquo MAC protocols
cable headend
CMTS
ISP
cable modem termination system
multiple 40Mbps downstream (broadcast) channels
single CMTS transmits into channels
multiple 30 Mbps upstream channels
multiple access all users contend for certain upstream channel time slots (others assigned)
Cable access network
cable modem
splitter
hellip
hellip
Internet framesTV channels control transmitted downstream at different frequencies
upstream Internet frames TV control transmitted upstream at different frequencies in time slots
Link Layer 5-39
DOCSIS data over cable service interface spec FDM over upstream downstream frequency channels
TDM upstream some slots assigned some have contention
downstream MAP frame assigns upstream slots
request for upstream slots (and data) transmitted random access (binary backoff) in selected slots
MAP frame for
Interval [t1 t2]
Residences with cable modems
Downstream channel i
Upstream channel j
t1 t2
Assigned minislots containing cable modem
upstream data frames
Minislots containing
minislots request frames
cable headend
CMTS
Cable access network
Link Layer 5-40
Summary of MAC protocols
channel partitioning by time frequency or code Time Division Frequency Division
random access (dynamic)
ALOHA S-ALOHA CSMA CSMACD
carrier sensing easy in some technologies (wire) hard in others (wireless)
CSMACD used in Ethernet
CSMACA used in 80211
taking turns
polling from central site token passing
bluetooth FDDI token ring
Link Layer 5-41
Link layer LANs outline
51 introduction services
52 error detection correction
53 multiple access protocols
54 LANs addressing ARP
Ethernet
switches
VLANS
55 link virtualization MPLS
56 data center networking
57 a day in the life of a web request
Link Layer 5-42
MAC addresses and ARP
32-bit IP address network-layer address for interface
used for layer 3 (network layer) forwarding
MAC (or LAN or physical or Ethernet) address function used lsquolocallyrdquo to get frame from one interface to
another physically-connected interface (same network in IP-addressing sense)
48 bit MAC address (for most LANs) burned in NIC ROM also sometimes software settable
eg 1A-2F-BB-76-09-AD
hexadecimal (base 16) notation
(each ldquonumberrdquo represents 4 bits)
Link Layer 5-43
LAN addresses and ARP
each adapter on LAN has unique LAN address
adapter
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN
(wired or
wireless)
Link Layer 5-44
LAN addresses (more)
MAC address allocation administered by IEEE
manufacturer buys portion of MAC address space (to assure uniqueness)
analogy MAC address like Social Security Number
IP address like postal address
MAC flat address portability can move LAN card from one LAN to another
IP hierarchical address not portable address depends on IP subnet to which node is
attached
Link Layer 5-45
ARP address resolution protocol
ARP table each IP node (host router) on LAN has table
IPMAC address mappings for some LAN nodes
lt IP address MAC address TTLgt
TTL (Time To Live) time after which address mapping will be forgotten (typically 20 min)
Question how to determine
interfacersquos MAC address
knowing its IP address
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN
137196723
137196778
137196714
137196788
Link Layer 5-46
ARP protocol same LAN
A wants to send datagram to B Brsquos MAC address not in
Arsquos ARP table
A broadcasts ARP query packet containing Bs IP address dest MAC address = FF-FF-
FF-FF-FF-FF
all nodes on LAN receive ARP query
B receives ARP packet replies to A with its (Bs) MAC address frame sent to Arsquos MAC
address (unicast)
A caches (saves) IP-to-MAC address pair in its ARP table until information becomes old (times out) soft state information that
times out (goes away) unless refreshed
ARP is ldquoplug-and-playrdquo nodes create their ARP
tables without intervention from net administrator
Link Layer 5-47
walkthrough send datagram from A to B via R
focus on addressing ndash at IP (datagram) and MAC layer (frame)
assume A knows Brsquos IP address
assume A knows IP address of first hop router R (how)
assume A knows Rrsquos MAC address (how)
Addressing routing to another LAN
R
1A-23-F9-CD-06-9B 222222222220
111111111110 E6-E9-00-17-BB-4B CC-49-DE-D0-AB-7D
111111111112
111111111111
74-29-9C-E8-FF-55
A
222222222222
49-BD-D2-C7-56-2A
222222222221 88-B2-2F-54-1A-0F
B
R
1A-23-F9-CD-06-9B 222222222220
111111111110 E6-E9-00-17-BB-4B CC-49-DE-D0-AB-7D
111111111112
111111111111
74-29-9C-E8-FF-55
A
222222222222
49-BD-D2-C7-56-2A
222222222221 88-B2-2F-54-1A-0F
B
Link Layer 5-48
Addressing routing to another LAN
IP
Eth
Phy
IP src 111111111111
IP dest 222222222222
A creates IP datagram with IP source A destination B
A creates link-layer frame with Rs MAC address as dest frame contains A-to-B IP datagram
MAC src 74-29-9C-E8-FF-55
MAC dest E6-E9-00-17-BB-4B
R
1A-23-F9-CD-06-9B 222222222220
111111111110 E6-E9-00-17-BB-4B CC-49-DE-D0-AB-7D
111111111112
111111111111
74-29-9C-E8-FF-55
A
222222222222
49-BD-D2-C7-56-2A
222222222221 88-B2-2F-54-1A-0F
B
Link Layer 5-49
Addressing routing to another LAN
IP
Eth
Phy
frame sent from A to R
IP
Eth
Phy
frame received at R datagram removed passed up to IP
MAC src 74-29-9C-E8-FF-55
MAC dest E6-E9-00-17-BB-4B
IP src 111111111111
IP dest 222222222222
IP src 111111111111
IP dest 222222222222
R
1A-23-F9-CD-06-9B 222222222220
111111111110 E6-E9-00-17-BB-4B CC-49-DE-D0-AB-7D
111111111112
111111111111
74-29-9C-E8-FF-55
A
222222222222
49-BD-D2-C7-56-2A
222222222221 88-B2-2F-54-1A-0F
B
Link Layer 5-50
Addressing routing to another LAN
IP src 111111111111
IP dest 222222222222
R forwards datagram with IP source A destination B
R creates link-layer frame with Bs MAC address as dest frame contains A-to-B IP datagram
MAC src 1A-23-F9-CD-06-9B
MAC dest 49-BD-D2-C7-56-2A
IP
Eth
Phy
IP
Eth
Phy
R
1A-23-F9-CD-06-9B 222222222220
111111111110 E6-E9-00-17-BB-4B CC-49-DE-D0-AB-7D
111111111112
111111111111
74-29-9C-E8-FF-55
A
222222222222
49-BD-D2-C7-56-2A
222222222221 88-B2-2F-54-1A-0F
B
Link Layer 5-51
Addressing routing to another LAN R forwards datagram with IP source A destination B
R creates link-layer frame with Bs MAC address as dest frame contains A-to-B IP datagram
IP src 111111111111
IP dest 222222222222
MAC src 1A-23-F9-CD-06-9B
MAC dest 49-BD-D2-C7-56-2A
IP
Eth
Phy
IP
Eth
Phy
R
1A-23-F9-CD-06-9B 222222222220
111111111110 E6-E9-00-17-BB-4B CC-49-DE-D0-AB-7D
111111111112
111111111111
74-29-9C-E8-FF-55
A
222222222222
49-BD-D2-C7-56-2A
222222222221 88-B2-2F-54-1A-0F
B
Link Layer 5-52
Addressing routing to another LAN R forwards datagram with IP source A destination B
R creates link-layer frame with Bs MAC address as dest frame contains A-to-B IP datagram
IP src 111111111111
IP dest 222222222222
MAC src 1A-23-F9-CD-06-9B
MAC dest 49-BD-D2-C7-56-2A
IP
Eth
Phy
Link Layer 5-53
Link layer LANs outline
51 introduction services
52 error detection correction
53 multiple access protocols
54 LANs addressing ARP
Ethernet
switches
VLANS
55 link virtualization MPLS
56 data center networking
57 a day in the life of a web request
Link Layer 5-54
Ethernet
ldquodominantrdquo wired LAN technology
cheap $20 for NIC
first widely used LAN technology
simpler cheaper than token LANs and ATM
kept up with speed race 10 Mbps ndash 10 Gbps
Metcalfersquos Ethernet sketch
Link Layer 5-55
Ethernet physical topology
bus popular through mid 90s all nodes in same collision domain (can collide with each
other)
star prevails today active switch in center
each ldquospokerdquo runs a (separate) Ethernet protocol (nodes
do not collide with each other)
switch
bus coaxial cable star
Link Layer 5-56
Ethernet frame structure
sending adapter encapsulates IP datagram (or other network layer protocol packet) in Ethernet frame
preamble
7 bytes with pattern 10101010 followed by one byte with pattern 10101011
used to synchronize receiver sender clock rates
dest address
source address
data (payload) CRC preamble
type
Link Layer 5-57
Ethernet frame structure (more)
addresses 6 byte source destination MAC addresses if adapter receives frame with matching destination
address or with broadcast address (eg ARP packet) it passes data in frame to network layer protocol
otherwise adapter discards frame
type indicates higher layer protocol (mostly IP but others possible eg Novell IPX AppleTalk)
CRC cyclic redundancy check at receiver error detected frame is dropped
dest address
source address
data (payload) CRC preamble
type
Link Layer 5-58
Ethernet unreliable connectionless
connectionless no handshaking between sending and receiving NICs
unreliable receiving NIC doesnt send acks or nacks to sending NIC
data in dropped frames recovered only if initial sender uses higher layer rdt (eg TCP) otherwise dropped data lost
Ethernetrsquos MAC protocol unslotted CSMACD wth binary backoff
Link Layer 5-59
8023 Ethernet standards link amp physical layers
many different Ethernet standards
common MAC protocol and frame format
different speeds 2 Mbps 10 Mbps 100 Mbps 1Gbps 10G bps
different physical layer media fiber cable
application
transport
network
link
physical
MAC protocol
and frame format
100BASE-TX
100BASE-T4
100BASE-FX 100BASE-T2
100BASE-SX 100BASE-BX
fiber physical layer copper (twister
pair) physical layer
Link Layer 5-60
Link layer LANs outline
51 introduction services
52 error detection correction
53 multiple access protocols
54 LANs addressing ARP
Ethernet
switches
VLANS
55 link virtualization MPLS
56 data center networking
57 a day in the life of a web request
Link Layer 5-61
Ethernet switch link-layer device takes an active role
store forward Ethernet frames
examine incoming framersquos MAC address selectively forward frame to one-or-more outgoing links when frame is to be forwarded on segment uses CSMACD to access segment
transparent
hosts are unaware of presence of switches
plug-and-play self-learning
switches do not need to be configured
Link Layer 5-62
Switch multiple simultaneous transmissions
hosts have dedicated direct connection to switch
switches buffer packets
Ethernet protocol used on each incoming link but no collisions full duplex
each link is its own collision domain
switching A-to-Arsquo and B-to-Brsquo can transmit simultaneously without collisions switch with six interfaces
(123456)
A
Arsquo
B
Brsquo C
Crsquo
1 2
3 4 5
6
Link Layer 5-63
Switch forwarding table
Q how does switch know Arsquo reachable via interface 4 Brsquo reachable via interface 5
switch with six interfaces
(123456)
A
Arsquo
B
Brsquo C
Crsquo
1 2
3 4 5
6 A each switch has a switch table each entry
(MAC address of host interface to
reach host time stamp)
looks like a routing table
Q how are entries created maintained in switch table
something like a routing protocol
A
Arsquo
B
Brsquo C
Crsquo
1 2
3 4 5
6
Link Layer 5-64
Switch self-learning
switch learns which hosts can be reached through which interfaces
when frame received switch ldquolearnsrdquo location of sender incoming LAN segment
records senderlocation pair in switch table
A Arsquo
Source A
Dest Arsquo
MAC addr interface TTL
Switch table
(initially empty) A 1 60
Link Layer 5-65
Switch frame filteringforwarding
when frame received at switch 1 record incoming link MAC address of sending host
2 index switch table using MAC destination address
3 if entry found for destination then
if destination on segment from which frame arrived then drop frame
else forward frame on interface indicated by entry
else flood forward on all interfaces except arriving
interface
A
Arsquo
B
Brsquo C
Crsquo
1 2
3 4 5
6
Link Layer 5-66
Self-learning forwarding example
A Arsquo
Source A
Dest Arsquo
MAC addr interface TTL
switch table
(initially empty) A 1 60
A Arsquo A Arsquo A Arsquo A Arsquo A Arsquo
frame destination Arsquo locaton unknown flood
Arsquo A
destination A location
known
Arsquo 4 60
selectively send
on just one link
Link Layer 5-67
Interconnecting switches
switches can be connected together
Q sending from A to G - how does S1 know to forward frame destined to F via S4 and S3
A self learning (works exactly the same as in single-switch case)
A
B
S1
C D
E
F
S2
S4
S3
H
I
G
Link Layer 5-68
Self-learning multi-switch example
Suppose C sends frame to I I responds to C
Q show switch tables and packet forwarding in S1 S2 S3 S4
A
B
S1
C D
E
F
S2
S4
S3
H
I
G
Link Layer 5-69
Institutional network
to external
network
router
IP subnet
mail server
web server
Link Layer 5-70
Switches vs routers
both are store-and-forward
routers network-layer devices (examine network-layer headers)
switches link-layer devices (examine link-layer headers)
both have forwarding tables
routers compute tables using routing algorithms IP addresses
switches learn forwarding table using flooding learning MAC addresses
application
transport
network
link
physical
network
link
physical
link
physical
switch
datagram
application
transport
network
link
physical
frame
frame
frame
datagram
Link Layer 5-71
VLANs motivation
consider CS user moves office to
EE but wants connect to CS switch
single broadcast domain
all layer-2 broadcast traffic (ARP DHCP unknown location of destination MAC address) must cross entire LAN
securityprivacy efficiency issues
Computer
Science Electrical
Engineering
Computer
Engineering
Link Layer 5-72
VLANs port-based VLAN switch ports
grouped (by switch management software) so that single physical switch helliphellip
switch(es) supporting
VLAN capabilities can
be configured to
define multiple virtual
LANS over single
physical LAN
infrastructure
Virtual Local
Area Network 1
8
9
16 10 2
7
hellip
Electrical Engineering
(VLAN ports 1-8)
Computer Science
(VLAN ports 9-15)
15
hellip
Electrical Engineering
(VLAN ports 1-8)
hellip
1
8 2
7 9
16 10
15
hellip
Computer Science
(VLAN ports 9-16)
hellip operates as multiple virtual switches
Link Layer 5-73
Port-based VLAN
1
8
9
16 10 2
7
hellip
Electrical Engineering
(VLAN ports 1-8)
Computer Science
(VLAN ports 9-15)
15
hellip
traffic isolation frames tofrom ports 1-8 can only reach ports 1-8 can also define VLAN based on
MAC addresses of endpoints rather than switch port
dynamic membership ports can be dynamically assigned among VLANs
router
forwarding between VLANS done via routing (just as with separate switches) in practice vendors sell combined
switches plus routers
Link Layer 5-74
VLANS spanning multiple switches
trunk port carries frames between VLANS defined over multiple physical switches frames forwarded within VLAN between switches canrsquot be vanilla
8021 frames (must carry VLAN ID info)
8021q protocol addsremoved additional header fields for frames forwarded between trunk ports
1
8
9
10 2
7
hellip
Electrical Engineering
(VLAN ports 1-8)
Computer Science
(VLAN ports 9-15)
15
hellip
2
7 3
Ports 235 belong to EE VLAN
Ports 4678 belong to CS VLAN
5
4 6 8 16
1
Link Layer 5-75
type
2-byte Tag Protocol Identifier
(value 81-00)
Tag Control Information (12 bit VLAN ID field
3 bit priority field like IP TOS)
Recomputed CRC
8021Q VLAN frame format
8021 frame
8021Q frame
dest address
source address
data (payload) CRC preamble
dest address
source address
preamble data (payload) CRC
type
Link Layer 5-76
Link layer LANs outline
51 introduction services
52 error detection correction
53 multiple access protocols
54 LANs addressing ARP
Ethernet
switches
VLANS
55 link virtualization MPLS
56 data center networking
57 a day in the life of a web request
Link Layer 5-77
Multiprotocol label switching (MPLS)
initial goal high-speed IP forwarding using fixed length label (instead of IP address) fast lookup using fixed length identifier (rather than
shortest prefix matching)
borrowing ideas from Virtual Circuit (VC) approach
but IP datagram still keeps IP address
PPP or Ethernet
header IP header remainder of link-layer frame MPLS header
label Exp S TTL
20 3 1 5
Link Layer 5-78
MPLS capable routers
aka label-switched router
forward packets to outgoing interface based only on label value (donrsquot inspect IP address) MPLS forwarding table distinct from IP forwarding tables
flexibility MPLS forwarding decisions can differ from those of IP use destination and source addresses to route flows to
same destination differently (traffic engineering)
re-route flows quickly if link fails pre-computed backup paths (useful for VoIP)
Link Layer 5-79
R2
D
R3 R5
A
R6
MPLS versus IP paths
IP router IP routing path to destination determined
by destination address alone
R4
Link Layer 5-80
R2
D
R3 R4
R5
A
R6
MPLS versus IP paths
IP-only router
IP routing path to destination determined by destination address alone
MPLS and IP router
MPLS routing path to destination can be based on source and dest address fast reroute precompute backup routes in
case of link failure
entry router (R4) can use different MPLS
routes to A based eg on source address
Link Layer 5-81
MPLS signaling
modify OSPF IS-IS link-state flooding protocols to carry info used by MPLS routing eg link bandwidth amount of ldquoreservedrdquo link bandwidth
D
R4
R5
A
R6
entry MPLS router uses RSVP-TE signaling protocol to set up MPLS forwarding at downstream routers
modified link state flooding
RSVP-TE
Link Layer 5-82
R1 R2
D
R3 R4 R5
0
1
0 0
A
R6
in out out
label label dest interface
6 - A 0
in out out
label label dest interface
10 6 A 1
12 9 D 0
in out out
label label dest interface
10 A 0
12 D 0
1
in out out
label label dest interface
8 6 A 0
0
8 A 1
MPLS forwarding tables
Link Layer 5-83
Link layer LANs outline
51 introduction services
52 error detection correction
53 multiple access protocols
54 LANs addressing ARP
Ethernet
switches
VLANS
55 link virtualization MPLS
56 data center networking
57 a day in the life of a web request
Link Layer 5-84
Data center networks
10rsquos to 100rsquos of thousands of hosts often closely coupled in close proximity e-business (eg Amazon)
content-servers (eg YouTube Akamai Apple Microsoft)
search engines data mining (eg Google)
challenges
multiple applications each serving massive numbers of clients
managingbalancing load avoiding processing networking data bottlenecks
Inside a 40-ft Microsoft container
Chicago data center
Link Layer 5-85
Server racks
TOR switches
Tier-1 switches
Tier-2 switches
Load balancer
Load balancer
B
1 2 3 4 5 6 7 8
A C
Border router
Access router
Internet
Data center networks
load balancer application-layer routing receives external client requests
directs workload within data center
returns results to external client (hiding data
center internals from client)
Server racks
TOR switches
Tier-1 switches
Tier-2 switches
1 2 3 4 5 6 7 8
Data center networks
rich interconnection among switches racks
increased throughput between racks (multiple routing
paths possible)
increased reliability via redundancy
Link Layer 5-87
Link layer LANs outline
51 introduction services
52 error detection correction
53 multiple access protocols
54 LANs addressing ARP
Ethernet
switches
VLANS
55 link virtualization MPLS
56 data center networking
57 a day in the life of a web request
Link Layer 5-88
Synthesis a day in the life of a web request
journey down protocol stack complete application transport network link
putting-it-all-together synthesis goal identify review understand protocols (at all
layers) involved in seemingly simple scenario requesting www page
scenario student attaches laptop to campus network requestsreceives wwwgooglecom
Link Layer 5-89
A day in the life scenario
Comcast network
68800013
Googlersquos network
64233160019 64233169105
web server
DNS server
school network
68802024
web page
browser
router
(runs DHCP)
Link Layer 5-90
A day in the lifehellip connecting to the Internet
connecting laptop needs to get its own IP address addr of first-hop router addr of DNS server use DHCP
DHCP
UDP
IP
Eth
Phy
DHCP
DHCP
DHCP
DHCP
DHCP
DHCP
UDP
IP
Eth
Phy
DHCP
DHCP
DHCP
DHCP DHCP
DHCP request encapsulated in UDP encapsulated in IP encapsulated in 8023 Ethernet
Ethernet frame broadcast
(dest FFFFFFFFFFFF) on LAN received at router running DHCP server
Ethernet demuxed to IP demuxed UDP demuxed to DHCP
router
(runs DHCP)
Link Layer 5-91
DHCP server formulates DHCP ACK containing clientrsquos IP address IP address of first-hop router for client name amp IP address of DNS server
DHCP
UDP
IP
Eth
Phy
DHCP
DHCP
DHCP
DHCP
DHCP
UDP
IP
Eth
Phy
DHCP
DHCP
DHCP
DHCP
DHCP
encapsulation at DHCP server frame forwarded (switch learning) through LAN demultiplexing at client
Client now has IP address knows name amp addr of DNS
server IP address of its first-hop router
DHCP client receives DHCP ACK reply
A day in the lifehellip connecting to the Internet
router
(runs DHCP)
Link Layer 5-92
A day in the lifehellip ARP (before DNS before HTTP)
before sending HTTP request need IP address of wwwgooglecom DNS
DNS
UDP
IP
Eth
Phy
DNS
DNS
DNS
DNS query created encapsulated in UDP encapsulated in IP encapsulated in Eth To send frame to router need MAC address of router interface ARP
ARP query broadcast received by
router which replies with ARP reply giving MAC address of router interface
client now knows MAC address of first hop router so can now send frame containing DNS query
ARP query
Eth
Phy
ARP
ARP
ARP reply
router
(runs DHCP)
Link Layer 5-93
DNS
UDP
IP
Eth
Phy
DNS
DNS
DNS
DNS
DNS
IP datagram containing DNS query forwarded via LAN switch from client to 1st hop router
IP datagram forwarded from campus network into comcast network routed (tables created by RIP OSPF IS-IS andor BGP routing protocols) to DNS server
demuxrsquoed to DNS server
DNS server replies to client with IP address of wwwgooglecom
Comcast network
68800013
DNS server
DNS
UDP
IP
Eth
Phy
DNS
DNS
DNS
DNS
A day in the lifehellip using DNS
router
(runs DHCP)
Link Layer 5-94
A day in the lifehellipTCP connection carrying HTTP
HTTP
TCP
IP
Eth
Phy
HTTP
to send HTTP request client first opens TCP socket to web server
TCP SYN segment (step 1 in 3-way handshake) inter-domain routed to web server
TCP connection established 64233169105
web server
SYN
SYN
SYN
SYN
TCP
IP
Eth
Phy
SYN
SYN
SYN
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
web server responds with TCP SYNACK (step 2 in 3-way handshake)
router
(runs DHCP)
Link Layer 5-95
A day in the lifehellip HTTP requestreply
HTTP
TCP
IP
Eth
Phy
HTTP
HTTP request sent into TCP socket
IP datagram containing HTTP request routed to wwwgooglecom
IP datagram containing HTTP reply routed back to client
64233169105
web server
HTTP
TCP
IP
Eth
Phy web server responds with
HTTP reply (containing web page)
HTTP
HTTP
HTTP HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
web page finally () displayed
Link Layer 5-96
Chapter 5 Summary
principles behind data link layer services error detection correction
sharing a broadcast channel multiple access
link layer addressing
instantiation and implementation of various link layer technologies Ethernet
switched LANS VLANs
virtualized networks as a link layer MPLS
synthesis a day in the life of a web request
Link Layer 5-97
Chapter 5 letrsquos take a breath
journey down protocol stack complete (except PHY)
solid understanding of networking principles practice
hellip could stop here hellip but lots of interesting topics wireless
multimedia
security
network management
Link Layer 5-27
Pure (unslotted) ALOHA
unslotted Aloha simpler no synchronization
when frame first arrives
transmit immediately
collision probability increases
frame sent at t0 collides with other frames sent in [t0-1t0+1]
Link Layer 5-28
Pure ALOHA efficiency
P(success by given node) = P(node transmits)
P(no other node transmits in [t0-1t0]
P(no other node transmits in [t0-1t0]
= p (1-p)N-1 (1-p)N-1
= p (1-p)2(N-1)
hellip choosing optimum p and then letting n
= 1(2e) = 18
even worse than slotted Aloha
Link Layer 5-29
CSMA (carrier sense multiple access)
CSMA listen before transmit if channel sensed idle transmit entire frame
if channel sensed busy defer transmission
human analogy donrsquot interrupt others
Link Layer 5-30
CSMA collisions
collisions can still occur propagation delay means two nodes may not hear each otherrsquos transmission
collision entire packet transmission time wasted distance amp propagation
delay play role in in determining collision probability
spatial layout of nodes
Link Layer 5-31
CSMACD (collision detection)
CSMACD carrier sensing deferral as in CSMA collisions detected within short time
colliding transmissions aborted reducing channel wastage
collision detection easy in wired LANs measure signal strengths compare
transmitted received signals
difficult in wireless LANs received signal strength overwhelmed by local transmission strength
human analogy the polite conversationalist
Link Layer 5-32
CSMACD (collision detection)
spatial layout of nodes
Link Layer 5-33
Ethernet CSMACD algorithm
1 NIC receives datagram from network layer creates frame
2 If NIC senses channel idle starts frame transmission If NIC senses channel busy waits until channel idle then transmits
3 If NIC transmits entire frame without detecting another transmission NIC is done with frame
4 If NIC detects another transmission while transmitting aborts and sends jam signal
5 After aborting NIC enters binary (exponential) backoff
after mth collision NIC chooses K at random from 012 hellip 2m-1 NIC waits K512 bit times returns to Step 2
longer backoff interval with more collisions
Link Layer 5-34
CSMACD efficiency
Tprop = max prop delay between 2 nodes in LAN
ttrans = time to transmit max-size frame
efficiency goes to 1
as tprop goes to 0
as ttrans goes to infinity
better performance than ALOHA and simple cheap decentralized
transpropttefficiency
51
1
Link Layer 5-35
ldquoTaking turnsrdquo MAC protocols
channel partitioning MAC protocols share channel efficiently and fairly at high load
inefficient at low load delay in channel access 1N bandwidth allocated even if only 1 active node
random access MAC protocols efficient at low load single node can fully utilize
channel
high load collision overhead
ldquotaking turnsrdquo protocols look for best of both worlds
Link Layer 5-36
polling master node ldquoinvitesrdquo
slave nodes to transmit in turn
typically used with ldquodumbrdquo slave devices
concerns
polling overhead
latency
single point of failure (master)
master
slaves
poll
data
data
ldquoTaking turnsrdquo MAC protocols
Link Layer 5-37
token passing control token passed
from one node to next sequentially
token message
concerns
token overhead
latency
single point of failure (token)
T
data
(nothing
to send)
T
ldquoTaking turnsrdquo MAC protocols
cable headend
CMTS
ISP
cable modem termination system
multiple 40Mbps downstream (broadcast) channels
single CMTS transmits into channels
multiple 30 Mbps upstream channels
multiple access all users contend for certain upstream channel time slots (others assigned)
Cable access network
cable modem
splitter
hellip
hellip
Internet framesTV channels control transmitted downstream at different frequencies
upstream Internet frames TV control transmitted upstream at different frequencies in time slots
Link Layer 5-39
DOCSIS data over cable service interface spec FDM over upstream downstream frequency channels
TDM upstream some slots assigned some have contention
downstream MAP frame assigns upstream slots
request for upstream slots (and data) transmitted random access (binary backoff) in selected slots
MAP frame for
Interval [t1 t2]
Residences with cable modems
Downstream channel i
Upstream channel j
t1 t2
Assigned minislots containing cable modem
upstream data frames
Minislots containing
minislots request frames
cable headend
CMTS
Cable access network
Link Layer 5-40
Summary of MAC protocols
channel partitioning by time frequency or code Time Division Frequency Division
random access (dynamic)
ALOHA S-ALOHA CSMA CSMACD
carrier sensing easy in some technologies (wire) hard in others (wireless)
CSMACD used in Ethernet
CSMACA used in 80211
taking turns
polling from central site token passing
bluetooth FDDI token ring
Link Layer 5-41
Link layer LANs outline
51 introduction services
52 error detection correction
53 multiple access protocols
54 LANs addressing ARP
Ethernet
switches
VLANS
55 link virtualization MPLS
56 data center networking
57 a day in the life of a web request
Link Layer 5-42
MAC addresses and ARP
32-bit IP address network-layer address for interface
used for layer 3 (network layer) forwarding
MAC (or LAN or physical or Ethernet) address function used lsquolocallyrdquo to get frame from one interface to
another physically-connected interface (same network in IP-addressing sense)
48 bit MAC address (for most LANs) burned in NIC ROM also sometimes software settable
eg 1A-2F-BB-76-09-AD
hexadecimal (base 16) notation
(each ldquonumberrdquo represents 4 bits)
Link Layer 5-43
LAN addresses and ARP
each adapter on LAN has unique LAN address
adapter
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN
(wired or
wireless)
Link Layer 5-44
LAN addresses (more)
MAC address allocation administered by IEEE
manufacturer buys portion of MAC address space (to assure uniqueness)
analogy MAC address like Social Security Number
IP address like postal address
MAC flat address portability can move LAN card from one LAN to another
IP hierarchical address not portable address depends on IP subnet to which node is
attached
Link Layer 5-45
ARP address resolution protocol
ARP table each IP node (host router) on LAN has table
IPMAC address mappings for some LAN nodes
lt IP address MAC address TTLgt
TTL (Time To Live) time after which address mapping will be forgotten (typically 20 min)
Question how to determine
interfacersquos MAC address
knowing its IP address
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN
137196723
137196778
137196714
137196788
Link Layer 5-46
ARP protocol same LAN
A wants to send datagram to B Brsquos MAC address not in
Arsquos ARP table
A broadcasts ARP query packet containing Bs IP address dest MAC address = FF-FF-
FF-FF-FF-FF
all nodes on LAN receive ARP query
B receives ARP packet replies to A with its (Bs) MAC address frame sent to Arsquos MAC
address (unicast)
A caches (saves) IP-to-MAC address pair in its ARP table until information becomes old (times out) soft state information that
times out (goes away) unless refreshed
ARP is ldquoplug-and-playrdquo nodes create their ARP
tables without intervention from net administrator
Link Layer 5-47
walkthrough send datagram from A to B via R
focus on addressing ndash at IP (datagram) and MAC layer (frame)
assume A knows Brsquos IP address
assume A knows IP address of first hop router R (how)
assume A knows Rrsquos MAC address (how)
Addressing routing to another LAN
R
1A-23-F9-CD-06-9B 222222222220
111111111110 E6-E9-00-17-BB-4B CC-49-DE-D0-AB-7D
111111111112
111111111111
74-29-9C-E8-FF-55
A
222222222222
49-BD-D2-C7-56-2A
222222222221 88-B2-2F-54-1A-0F
B
R
1A-23-F9-CD-06-9B 222222222220
111111111110 E6-E9-00-17-BB-4B CC-49-DE-D0-AB-7D
111111111112
111111111111
74-29-9C-E8-FF-55
A
222222222222
49-BD-D2-C7-56-2A
222222222221 88-B2-2F-54-1A-0F
B
Link Layer 5-48
Addressing routing to another LAN
IP
Eth
Phy
IP src 111111111111
IP dest 222222222222
A creates IP datagram with IP source A destination B
A creates link-layer frame with Rs MAC address as dest frame contains A-to-B IP datagram
MAC src 74-29-9C-E8-FF-55
MAC dest E6-E9-00-17-BB-4B
R
1A-23-F9-CD-06-9B 222222222220
111111111110 E6-E9-00-17-BB-4B CC-49-DE-D0-AB-7D
111111111112
111111111111
74-29-9C-E8-FF-55
A
222222222222
49-BD-D2-C7-56-2A
222222222221 88-B2-2F-54-1A-0F
B
Link Layer 5-49
Addressing routing to another LAN
IP
Eth
Phy
frame sent from A to R
IP
Eth
Phy
frame received at R datagram removed passed up to IP
MAC src 74-29-9C-E8-FF-55
MAC dest E6-E9-00-17-BB-4B
IP src 111111111111
IP dest 222222222222
IP src 111111111111
IP dest 222222222222
R
1A-23-F9-CD-06-9B 222222222220
111111111110 E6-E9-00-17-BB-4B CC-49-DE-D0-AB-7D
111111111112
111111111111
74-29-9C-E8-FF-55
A
222222222222
49-BD-D2-C7-56-2A
222222222221 88-B2-2F-54-1A-0F
B
Link Layer 5-50
Addressing routing to another LAN
IP src 111111111111
IP dest 222222222222
R forwards datagram with IP source A destination B
R creates link-layer frame with Bs MAC address as dest frame contains A-to-B IP datagram
MAC src 1A-23-F9-CD-06-9B
MAC dest 49-BD-D2-C7-56-2A
IP
Eth
Phy
IP
Eth
Phy
R
1A-23-F9-CD-06-9B 222222222220
111111111110 E6-E9-00-17-BB-4B CC-49-DE-D0-AB-7D
111111111112
111111111111
74-29-9C-E8-FF-55
A
222222222222
49-BD-D2-C7-56-2A
222222222221 88-B2-2F-54-1A-0F
B
Link Layer 5-51
Addressing routing to another LAN R forwards datagram with IP source A destination B
R creates link-layer frame with Bs MAC address as dest frame contains A-to-B IP datagram
IP src 111111111111
IP dest 222222222222
MAC src 1A-23-F9-CD-06-9B
MAC dest 49-BD-D2-C7-56-2A
IP
Eth
Phy
IP
Eth
Phy
R
1A-23-F9-CD-06-9B 222222222220
111111111110 E6-E9-00-17-BB-4B CC-49-DE-D0-AB-7D
111111111112
111111111111
74-29-9C-E8-FF-55
A
222222222222
49-BD-D2-C7-56-2A
222222222221 88-B2-2F-54-1A-0F
B
Link Layer 5-52
Addressing routing to another LAN R forwards datagram with IP source A destination B
R creates link-layer frame with Bs MAC address as dest frame contains A-to-B IP datagram
IP src 111111111111
IP dest 222222222222
MAC src 1A-23-F9-CD-06-9B
MAC dest 49-BD-D2-C7-56-2A
IP
Eth
Phy
Link Layer 5-53
Link layer LANs outline
51 introduction services
52 error detection correction
53 multiple access protocols
54 LANs addressing ARP
Ethernet
switches
VLANS
55 link virtualization MPLS
56 data center networking
57 a day in the life of a web request
Link Layer 5-54
Ethernet
ldquodominantrdquo wired LAN technology
cheap $20 for NIC
first widely used LAN technology
simpler cheaper than token LANs and ATM
kept up with speed race 10 Mbps ndash 10 Gbps
Metcalfersquos Ethernet sketch
Link Layer 5-55
Ethernet physical topology
bus popular through mid 90s all nodes in same collision domain (can collide with each
other)
star prevails today active switch in center
each ldquospokerdquo runs a (separate) Ethernet protocol (nodes
do not collide with each other)
switch
bus coaxial cable star
Link Layer 5-56
Ethernet frame structure
sending adapter encapsulates IP datagram (or other network layer protocol packet) in Ethernet frame
preamble
7 bytes with pattern 10101010 followed by one byte with pattern 10101011
used to synchronize receiver sender clock rates
dest address
source address
data (payload) CRC preamble
type
Link Layer 5-57
Ethernet frame structure (more)
addresses 6 byte source destination MAC addresses if adapter receives frame with matching destination
address or with broadcast address (eg ARP packet) it passes data in frame to network layer protocol
otherwise adapter discards frame
type indicates higher layer protocol (mostly IP but others possible eg Novell IPX AppleTalk)
CRC cyclic redundancy check at receiver error detected frame is dropped
dest address
source address
data (payload) CRC preamble
type
Link Layer 5-58
Ethernet unreliable connectionless
connectionless no handshaking between sending and receiving NICs
unreliable receiving NIC doesnt send acks or nacks to sending NIC
data in dropped frames recovered only if initial sender uses higher layer rdt (eg TCP) otherwise dropped data lost
Ethernetrsquos MAC protocol unslotted CSMACD wth binary backoff
Link Layer 5-59
8023 Ethernet standards link amp physical layers
many different Ethernet standards
common MAC protocol and frame format
different speeds 2 Mbps 10 Mbps 100 Mbps 1Gbps 10G bps
different physical layer media fiber cable
application
transport
network
link
physical
MAC protocol
and frame format
100BASE-TX
100BASE-T4
100BASE-FX 100BASE-T2
100BASE-SX 100BASE-BX
fiber physical layer copper (twister
pair) physical layer
Link Layer 5-60
Link layer LANs outline
51 introduction services
52 error detection correction
53 multiple access protocols
54 LANs addressing ARP
Ethernet
switches
VLANS
55 link virtualization MPLS
56 data center networking
57 a day in the life of a web request
Link Layer 5-61
Ethernet switch link-layer device takes an active role
store forward Ethernet frames
examine incoming framersquos MAC address selectively forward frame to one-or-more outgoing links when frame is to be forwarded on segment uses CSMACD to access segment
transparent
hosts are unaware of presence of switches
plug-and-play self-learning
switches do not need to be configured
Link Layer 5-62
Switch multiple simultaneous transmissions
hosts have dedicated direct connection to switch
switches buffer packets
Ethernet protocol used on each incoming link but no collisions full duplex
each link is its own collision domain
switching A-to-Arsquo and B-to-Brsquo can transmit simultaneously without collisions switch with six interfaces
(123456)
A
Arsquo
B
Brsquo C
Crsquo
1 2
3 4 5
6
Link Layer 5-63
Switch forwarding table
Q how does switch know Arsquo reachable via interface 4 Brsquo reachable via interface 5
switch with six interfaces
(123456)
A
Arsquo
B
Brsquo C
Crsquo
1 2
3 4 5
6 A each switch has a switch table each entry
(MAC address of host interface to
reach host time stamp)
looks like a routing table
Q how are entries created maintained in switch table
something like a routing protocol
A
Arsquo
B
Brsquo C
Crsquo
1 2
3 4 5
6
Link Layer 5-64
Switch self-learning
switch learns which hosts can be reached through which interfaces
when frame received switch ldquolearnsrdquo location of sender incoming LAN segment
records senderlocation pair in switch table
A Arsquo
Source A
Dest Arsquo
MAC addr interface TTL
Switch table
(initially empty) A 1 60
Link Layer 5-65
Switch frame filteringforwarding
when frame received at switch 1 record incoming link MAC address of sending host
2 index switch table using MAC destination address
3 if entry found for destination then
if destination on segment from which frame arrived then drop frame
else forward frame on interface indicated by entry
else flood forward on all interfaces except arriving
interface
A
Arsquo
B
Brsquo C
Crsquo
1 2
3 4 5
6
Link Layer 5-66
Self-learning forwarding example
A Arsquo
Source A
Dest Arsquo
MAC addr interface TTL
switch table
(initially empty) A 1 60
A Arsquo A Arsquo A Arsquo A Arsquo A Arsquo
frame destination Arsquo locaton unknown flood
Arsquo A
destination A location
known
Arsquo 4 60
selectively send
on just one link
Link Layer 5-67
Interconnecting switches
switches can be connected together
Q sending from A to G - how does S1 know to forward frame destined to F via S4 and S3
A self learning (works exactly the same as in single-switch case)
A
B
S1
C D
E
F
S2
S4
S3
H
I
G
Link Layer 5-68
Self-learning multi-switch example
Suppose C sends frame to I I responds to C
Q show switch tables and packet forwarding in S1 S2 S3 S4
A
B
S1
C D
E
F
S2
S4
S3
H
I
G
Link Layer 5-69
Institutional network
to external
network
router
IP subnet
mail server
web server
Link Layer 5-70
Switches vs routers
both are store-and-forward
routers network-layer devices (examine network-layer headers)
switches link-layer devices (examine link-layer headers)
both have forwarding tables
routers compute tables using routing algorithms IP addresses
switches learn forwarding table using flooding learning MAC addresses
application
transport
network
link
physical
network
link
physical
link
physical
switch
datagram
application
transport
network
link
physical
frame
frame
frame
datagram
Link Layer 5-71
VLANs motivation
consider CS user moves office to
EE but wants connect to CS switch
single broadcast domain
all layer-2 broadcast traffic (ARP DHCP unknown location of destination MAC address) must cross entire LAN
securityprivacy efficiency issues
Computer
Science Electrical
Engineering
Computer
Engineering
Link Layer 5-72
VLANs port-based VLAN switch ports
grouped (by switch management software) so that single physical switch helliphellip
switch(es) supporting
VLAN capabilities can
be configured to
define multiple virtual
LANS over single
physical LAN
infrastructure
Virtual Local
Area Network 1
8
9
16 10 2
7
hellip
Electrical Engineering
(VLAN ports 1-8)
Computer Science
(VLAN ports 9-15)
15
hellip
Electrical Engineering
(VLAN ports 1-8)
hellip
1
8 2
7 9
16 10
15
hellip
Computer Science
(VLAN ports 9-16)
hellip operates as multiple virtual switches
Link Layer 5-73
Port-based VLAN
1
8
9
16 10 2
7
hellip
Electrical Engineering
(VLAN ports 1-8)
Computer Science
(VLAN ports 9-15)
15
hellip
traffic isolation frames tofrom ports 1-8 can only reach ports 1-8 can also define VLAN based on
MAC addresses of endpoints rather than switch port
dynamic membership ports can be dynamically assigned among VLANs
router
forwarding between VLANS done via routing (just as with separate switches) in practice vendors sell combined
switches plus routers
Link Layer 5-74
VLANS spanning multiple switches
trunk port carries frames between VLANS defined over multiple physical switches frames forwarded within VLAN between switches canrsquot be vanilla
8021 frames (must carry VLAN ID info)
8021q protocol addsremoved additional header fields for frames forwarded between trunk ports
1
8
9
10 2
7
hellip
Electrical Engineering
(VLAN ports 1-8)
Computer Science
(VLAN ports 9-15)
15
hellip
2
7 3
Ports 235 belong to EE VLAN
Ports 4678 belong to CS VLAN
5
4 6 8 16
1
Link Layer 5-75
type
2-byte Tag Protocol Identifier
(value 81-00)
Tag Control Information (12 bit VLAN ID field
3 bit priority field like IP TOS)
Recomputed CRC
8021Q VLAN frame format
8021 frame
8021Q frame
dest address
source address
data (payload) CRC preamble
dest address
source address
preamble data (payload) CRC
type
Link Layer 5-76
Link layer LANs outline
51 introduction services
52 error detection correction
53 multiple access protocols
54 LANs addressing ARP
Ethernet
switches
VLANS
55 link virtualization MPLS
56 data center networking
57 a day in the life of a web request
Link Layer 5-77
Multiprotocol label switching (MPLS)
initial goal high-speed IP forwarding using fixed length label (instead of IP address) fast lookup using fixed length identifier (rather than
shortest prefix matching)
borrowing ideas from Virtual Circuit (VC) approach
but IP datagram still keeps IP address
PPP or Ethernet
header IP header remainder of link-layer frame MPLS header
label Exp S TTL
20 3 1 5
Link Layer 5-78
MPLS capable routers
aka label-switched router
forward packets to outgoing interface based only on label value (donrsquot inspect IP address) MPLS forwarding table distinct from IP forwarding tables
flexibility MPLS forwarding decisions can differ from those of IP use destination and source addresses to route flows to
same destination differently (traffic engineering)
re-route flows quickly if link fails pre-computed backup paths (useful for VoIP)
Link Layer 5-79
R2
D
R3 R5
A
R6
MPLS versus IP paths
IP router IP routing path to destination determined
by destination address alone
R4
Link Layer 5-80
R2
D
R3 R4
R5
A
R6
MPLS versus IP paths
IP-only router
IP routing path to destination determined by destination address alone
MPLS and IP router
MPLS routing path to destination can be based on source and dest address fast reroute precompute backup routes in
case of link failure
entry router (R4) can use different MPLS
routes to A based eg on source address
Link Layer 5-81
MPLS signaling
modify OSPF IS-IS link-state flooding protocols to carry info used by MPLS routing eg link bandwidth amount of ldquoreservedrdquo link bandwidth
D
R4
R5
A
R6
entry MPLS router uses RSVP-TE signaling protocol to set up MPLS forwarding at downstream routers
modified link state flooding
RSVP-TE
Link Layer 5-82
R1 R2
D
R3 R4 R5
0
1
0 0
A
R6
in out out
label label dest interface
6 - A 0
in out out
label label dest interface
10 6 A 1
12 9 D 0
in out out
label label dest interface
10 A 0
12 D 0
1
in out out
label label dest interface
8 6 A 0
0
8 A 1
MPLS forwarding tables
Link Layer 5-83
Link layer LANs outline
51 introduction services
52 error detection correction
53 multiple access protocols
54 LANs addressing ARP
Ethernet
switches
VLANS
55 link virtualization MPLS
56 data center networking
57 a day in the life of a web request
Link Layer 5-84
Data center networks
10rsquos to 100rsquos of thousands of hosts often closely coupled in close proximity e-business (eg Amazon)
content-servers (eg YouTube Akamai Apple Microsoft)
search engines data mining (eg Google)
challenges
multiple applications each serving massive numbers of clients
managingbalancing load avoiding processing networking data bottlenecks
Inside a 40-ft Microsoft container
Chicago data center
Link Layer 5-85
Server racks
TOR switches
Tier-1 switches
Tier-2 switches
Load balancer
Load balancer
B
1 2 3 4 5 6 7 8
A C
Border router
Access router
Internet
Data center networks
load balancer application-layer routing receives external client requests
directs workload within data center
returns results to external client (hiding data
center internals from client)
Server racks
TOR switches
Tier-1 switches
Tier-2 switches
1 2 3 4 5 6 7 8
Data center networks
rich interconnection among switches racks
increased throughput between racks (multiple routing
paths possible)
increased reliability via redundancy
Link Layer 5-87
Link layer LANs outline
51 introduction services
52 error detection correction
53 multiple access protocols
54 LANs addressing ARP
Ethernet
switches
VLANS
55 link virtualization MPLS
56 data center networking
57 a day in the life of a web request
Link Layer 5-88
Synthesis a day in the life of a web request
journey down protocol stack complete application transport network link
putting-it-all-together synthesis goal identify review understand protocols (at all
layers) involved in seemingly simple scenario requesting www page
scenario student attaches laptop to campus network requestsreceives wwwgooglecom
Link Layer 5-89
A day in the life scenario
Comcast network
68800013
Googlersquos network
64233160019 64233169105
web server
DNS server
school network
68802024
web page
browser
router
(runs DHCP)
Link Layer 5-90
A day in the lifehellip connecting to the Internet
connecting laptop needs to get its own IP address addr of first-hop router addr of DNS server use DHCP
DHCP
UDP
IP
Eth
Phy
DHCP
DHCP
DHCP
DHCP
DHCP
DHCP
UDP
IP
Eth
Phy
DHCP
DHCP
DHCP
DHCP DHCP
DHCP request encapsulated in UDP encapsulated in IP encapsulated in 8023 Ethernet
Ethernet frame broadcast
(dest FFFFFFFFFFFF) on LAN received at router running DHCP server
Ethernet demuxed to IP demuxed UDP demuxed to DHCP
router
(runs DHCP)
Link Layer 5-91
DHCP server formulates DHCP ACK containing clientrsquos IP address IP address of first-hop router for client name amp IP address of DNS server
DHCP
UDP
IP
Eth
Phy
DHCP
DHCP
DHCP
DHCP
DHCP
UDP
IP
Eth
Phy
DHCP
DHCP
DHCP
DHCP
DHCP
encapsulation at DHCP server frame forwarded (switch learning) through LAN demultiplexing at client
Client now has IP address knows name amp addr of DNS
server IP address of its first-hop router
DHCP client receives DHCP ACK reply
A day in the lifehellip connecting to the Internet
router
(runs DHCP)
Link Layer 5-92
A day in the lifehellip ARP (before DNS before HTTP)
before sending HTTP request need IP address of wwwgooglecom DNS
DNS
UDP
IP
Eth
Phy
DNS
DNS
DNS
DNS query created encapsulated in UDP encapsulated in IP encapsulated in Eth To send frame to router need MAC address of router interface ARP
ARP query broadcast received by
router which replies with ARP reply giving MAC address of router interface
client now knows MAC address of first hop router so can now send frame containing DNS query
ARP query
Eth
Phy
ARP
ARP
ARP reply
router
(runs DHCP)
Link Layer 5-93
DNS
UDP
IP
Eth
Phy
DNS
DNS
DNS
DNS
DNS
IP datagram containing DNS query forwarded via LAN switch from client to 1st hop router
IP datagram forwarded from campus network into comcast network routed (tables created by RIP OSPF IS-IS andor BGP routing protocols) to DNS server
demuxrsquoed to DNS server
DNS server replies to client with IP address of wwwgooglecom
Comcast network
68800013
DNS server
DNS
UDP
IP
Eth
Phy
DNS
DNS
DNS
DNS
A day in the lifehellip using DNS
router
(runs DHCP)
Link Layer 5-94
A day in the lifehellipTCP connection carrying HTTP
HTTP
TCP
IP
Eth
Phy
HTTP
to send HTTP request client first opens TCP socket to web server
TCP SYN segment (step 1 in 3-way handshake) inter-domain routed to web server
TCP connection established 64233169105
web server
SYN
SYN
SYN
SYN
TCP
IP
Eth
Phy
SYN
SYN
SYN
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
web server responds with TCP SYNACK (step 2 in 3-way handshake)
router
(runs DHCP)
Link Layer 5-95
A day in the lifehellip HTTP requestreply
HTTP
TCP
IP
Eth
Phy
HTTP
HTTP request sent into TCP socket
IP datagram containing HTTP request routed to wwwgooglecom
IP datagram containing HTTP reply routed back to client
64233169105
web server
HTTP
TCP
IP
Eth
Phy web server responds with
HTTP reply (containing web page)
HTTP
HTTP
HTTP HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
web page finally () displayed
Link Layer 5-96
Chapter 5 Summary
principles behind data link layer services error detection correction
sharing a broadcast channel multiple access
link layer addressing
instantiation and implementation of various link layer technologies Ethernet
switched LANS VLANs
virtualized networks as a link layer MPLS
synthesis a day in the life of a web request
Link Layer 5-97
Chapter 5 letrsquos take a breath
journey down protocol stack complete (except PHY)
solid understanding of networking principles practice
hellip could stop here hellip but lots of interesting topics wireless
multimedia
security
network management
Link Layer 5-28
Pure ALOHA efficiency
P(success by given node) = P(node transmits)
P(no other node transmits in [t0-1t0]
P(no other node transmits in [t0-1t0]
= p (1-p)N-1 (1-p)N-1
= p (1-p)2(N-1)
hellip choosing optimum p and then letting n
= 1(2e) = 18
even worse than slotted Aloha
Link Layer 5-29
CSMA (carrier sense multiple access)
CSMA listen before transmit if channel sensed idle transmit entire frame
if channel sensed busy defer transmission
human analogy donrsquot interrupt others
Link Layer 5-30
CSMA collisions
collisions can still occur propagation delay means two nodes may not hear each otherrsquos transmission
collision entire packet transmission time wasted distance amp propagation
delay play role in in determining collision probability
spatial layout of nodes
Link Layer 5-31
CSMACD (collision detection)
CSMACD carrier sensing deferral as in CSMA collisions detected within short time
colliding transmissions aborted reducing channel wastage
collision detection easy in wired LANs measure signal strengths compare
transmitted received signals
difficult in wireless LANs received signal strength overwhelmed by local transmission strength
human analogy the polite conversationalist
Link Layer 5-32
CSMACD (collision detection)
spatial layout of nodes
Link Layer 5-33
Ethernet CSMACD algorithm
1 NIC receives datagram from network layer creates frame
2 If NIC senses channel idle starts frame transmission If NIC senses channel busy waits until channel idle then transmits
3 If NIC transmits entire frame without detecting another transmission NIC is done with frame
4 If NIC detects another transmission while transmitting aborts and sends jam signal
5 After aborting NIC enters binary (exponential) backoff
after mth collision NIC chooses K at random from 012 hellip 2m-1 NIC waits K512 bit times returns to Step 2
longer backoff interval with more collisions
Link Layer 5-34
CSMACD efficiency
Tprop = max prop delay between 2 nodes in LAN
ttrans = time to transmit max-size frame
efficiency goes to 1
as tprop goes to 0
as ttrans goes to infinity
better performance than ALOHA and simple cheap decentralized
transpropttefficiency
51
1
Link Layer 5-35
ldquoTaking turnsrdquo MAC protocols
channel partitioning MAC protocols share channel efficiently and fairly at high load
inefficient at low load delay in channel access 1N bandwidth allocated even if only 1 active node
random access MAC protocols efficient at low load single node can fully utilize
channel
high load collision overhead
ldquotaking turnsrdquo protocols look for best of both worlds
Link Layer 5-36
polling master node ldquoinvitesrdquo
slave nodes to transmit in turn
typically used with ldquodumbrdquo slave devices
concerns
polling overhead
latency
single point of failure (master)
master
slaves
poll
data
data
ldquoTaking turnsrdquo MAC protocols
Link Layer 5-37
token passing control token passed
from one node to next sequentially
token message
concerns
token overhead
latency
single point of failure (token)
T
data
(nothing
to send)
T
ldquoTaking turnsrdquo MAC protocols
cable headend
CMTS
ISP
cable modem termination system
multiple 40Mbps downstream (broadcast) channels
single CMTS transmits into channels
multiple 30 Mbps upstream channels
multiple access all users contend for certain upstream channel time slots (others assigned)
Cable access network
cable modem
splitter
hellip
hellip
Internet framesTV channels control transmitted downstream at different frequencies
upstream Internet frames TV control transmitted upstream at different frequencies in time slots
Link Layer 5-39
DOCSIS data over cable service interface spec FDM over upstream downstream frequency channels
TDM upstream some slots assigned some have contention
downstream MAP frame assigns upstream slots
request for upstream slots (and data) transmitted random access (binary backoff) in selected slots
MAP frame for
Interval [t1 t2]
Residences with cable modems
Downstream channel i
Upstream channel j
t1 t2
Assigned minislots containing cable modem
upstream data frames
Minislots containing
minislots request frames
cable headend
CMTS
Cable access network
Link Layer 5-40
Summary of MAC protocols
channel partitioning by time frequency or code Time Division Frequency Division
random access (dynamic)
ALOHA S-ALOHA CSMA CSMACD
carrier sensing easy in some technologies (wire) hard in others (wireless)
CSMACD used in Ethernet
CSMACA used in 80211
taking turns
polling from central site token passing
bluetooth FDDI token ring
Link Layer 5-41
Link layer LANs outline
51 introduction services
52 error detection correction
53 multiple access protocols
54 LANs addressing ARP
Ethernet
switches
VLANS
55 link virtualization MPLS
56 data center networking
57 a day in the life of a web request
Link Layer 5-42
MAC addresses and ARP
32-bit IP address network-layer address for interface
used for layer 3 (network layer) forwarding
MAC (or LAN or physical or Ethernet) address function used lsquolocallyrdquo to get frame from one interface to
another physically-connected interface (same network in IP-addressing sense)
48 bit MAC address (for most LANs) burned in NIC ROM also sometimes software settable
eg 1A-2F-BB-76-09-AD
hexadecimal (base 16) notation
(each ldquonumberrdquo represents 4 bits)
Link Layer 5-43
LAN addresses and ARP
each adapter on LAN has unique LAN address
adapter
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN
(wired or
wireless)
Link Layer 5-44
LAN addresses (more)
MAC address allocation administered by IEEE
manufacturer buys portion of MAC address space (to assure uniqueness)
analogy MAC address like Social Security Number
IP address like postal address
MAC flat address portability can move LAN card from one LAN to another
IP hierarchical address not portable address depends on IP subnet to which node is
attached
Link Layer 5-45
ARP address resolution protocol
ARP table each IP node (host router) on LAN has table
IPMAC address mappings for some LAN nodes
lt IP address MAC address TTLgt
TTL (Time To Live) time after which address mapping will be forgotten (typically 20 min)
Question how to determine
interfacersquos MAC address
knowing its IP address
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN
137196723
137196778
137196714
137196788
Link Layer 5-46
ARP protocol same LAN
A wants to send datagram to B Brsquos MAC address not in
Arsquos ARP table
A broadcasts ARP query packet containing Bs IP address dest MAC address = FF-FF-
FF-FF-FF-FF
all nodes on LAN receive ARP query
B receives ARP packet replies to A with its (Bs) MAC address frame sent to Arsquos MAC
address (unicast)
A caches (saves) IP-to-MAC address pair in its ARP table until information becomes old (times out) soft state information that
times out (goes away) unless refreshed
ARP is ldquoplug-and-playrdquo nodes create their ARP
tables without intervention from net administrator
Link Layer 5-47
walkthrough send datagram from A to B via R
focus on addressing ndash at IP (datagram) and MAC layer (frame)
assume A knows Brsquos IP address
assume A knows IP address of first hop router R (how)
assume A knows Rrsquos MAC address (how)
Addressing routing to another LAN
R
1A-23-F9-CD-06-9B 222222222220
111111111110 E6-E9-00-17-BB-4B CC-49-DE-D0-AB-7D
111111111112
111111111111
74-29-9C-E8-FF-55
A
222222222222
49-BD-D2-C7-56-2A
222222222221 88-B2-2F-54-1A-0F
B
R
1A-23-F9-CD-06-9B 222222222220
111111111110 E6-E9-00-17-BB-4B CC-49-DE-D0-AB-7D
111111111112
111111111111
74-29-9C-E8-FF-55
A
222222222222
49-BD-D2-C7-56-2A
222222222221 88-B2-2F-54-1A-0F
B
Link Layer 5-48
Addressing routing to another LAN
IP
Eth
Phy
IP src 111111111111
IP dest 222222222222
A creates IP datagram with IP source A destination B
A creates link-layer frame with Rs MAC address as dest frame contains A-to-B IP datagram
MAC src 74-29-9C-E8-FF-55
MAC dest E6-E9-00-17-BB-4B
R
1A-23-F9-CD-06-9B 222222222220
111111111110 E6-E9-00-17-BB-4B CC-49-DE-D0-AB-7D
111111111112
111111111111
74-29-9C-E8-FF-55
A
222222222222
49-BD-D2-C7-56-2A
222222222221 88-B2-2F-54-1A-0F
B
Link Layer 5-49
Addressing routing to another LAN
IP
Eth
Phy
frame sent from A to R
IP
Eth
Phy
frame received at R datagram removed passed up to IP
MAC src 74-29-9C-E8-FF-55
MAC dest E6-E9-00-17-BB-4B
IP src 111111111111
IP dest 222222222222
IP src 111111111111
IP dest 222222222222
R
1A-23-F9-CD-06-9B 222222222220
111111111110 E6-E9-00-17-BB-4B CC-49-DE-D0-AB-7D
111111111112
111111111111
74-29-9C-E8-FF-55
A
222222222222
49-BD-D2-C7-56-2A
222222222221 88-B2-2F-54-1A-0F
B
Link Layer 5-50
Addressing routing to another LAN
IP src 111111111111
IP dest 222222222222
R forwards datagram with IP source A destination B
R creates link-layer frame with Bs MAC address as dest frame contains A-to-B IP datagram
MAC src 1A-23-F9-CD-06-9B
MAC dest 49-BD-D2-C7-56-2A
IP
Eth
Phy
IP
Eth
Phy
R
1A-23-F9-CD-06-9B 222222222220
111111111110 E6-E9-00-17-BB-4B CC-49-DE-D0-AB-7D
111111111112
111111111111
74-29-9C-E8-FF-55
A
222222222222
49-BD-D2-C7-56-2A
222222222221 88-B2-2F-54-1A-0F
B
Link Layer 5-51
Addressing routing to another LAN R forwards datagram with IP source A destination B
R creates link-layer frame with Bs MAC address as dest frame contains A-to-B IP datagram
IP src 111111111111
IP dest 222222222222
MAC src 1A-23-F9-CD-06-9B
MAC dest 49-BD-D2-C7-56-2A
IP
Eth
Phy
IP
Eth
Phy
R
1A-23-F9-CD-06-9B 222222222220
111111111110 E6-E9-00-17-BB-4B CC-49-DE-D0-AB-7D
111111111112
111111111111
74-29-9C-E8-FF-55
A
222222222222
49-BD-D2-C7-56-2A
222222222221 88-B2-2F-54-1A-0F
B
Link Layer 5-52
Addressing routing to another LAN R forwards datagram with IP source A destination B
R creates link-layer frame with Bs MAC address as dest frame contains A-to-B IP datagram
IP src 111111111111
IP dest 222222222222
MAC src 1A-23-F9-CD-06-9B
MAC dest 49-BD-D2-C7-56-2A
IP
Eth
Phy
Link Layer 5-53
Link layer LANs outline
51 introduction services
52 error detection correction
53 multiple access protocols
54 LANs addressing ARP
Ethernet
switches
VLANS
55 link virtualization MPLS
56 data center networking
57 a day in the life of a web request
Link Layer 5-54
Ethernet
ldquodominantrdquo wired LAN technology
cheap $20 for NIC
first widely used LAN technology
simpler cheaper than token LANs and ATM
kept up with speed race 10 Mbps ndash 10 Gbps
Metcalfersquos Ethernet sketch
Link Layer 5-55
Ethernet physical topology
bus popular through mid 90s all nodes in same collision domain (can collide with each
other)
star prevails today active switch in center
each ldquospokerdquo runs a (separate) Ethernet protocol (nodes
do not collide with each other)
switch
bus coaxial cable star
Link Layer 5-56
Ethernet frame structure
sending adapter encapsulates IP datagram (or other network layer protocol packet) in Ethernet frame
preamble
7 bytes with pattern 10101010 followed by one byte with pattern 10101011
used to synchronize receiver sender clock rates
dest address
source address
data (payload) CRC preamble
type
Link Layer 5-57
Ethernet frame structure (more)
addresses 6 byte source destination MAC addresses if adapter receives frame with matching destination
address or with broadcast address (eg ARP packet) it passes data in frame to network layer protocol
otherwise adapter discards frame
type indicates higher layer protocol (mostly IP but others possible eg Novell IPX AppleTalk)
CRC cyclic redundancy check at receiver error detected frame is dropped
dest address
source address
data (payload) CRC preamble
type
Link Layer 5-58
Ethernet unreliable connectionless
connectionless no handshaking between sending and receiving NICs
unreliable receiving NIC doesnt send acks or nacks to sending NIC
data in dropped frames recovered only if initial sender uses higher layer rdt (eg TCP) otherwise dropped data lost
Ethernetrsquos MAC protocol unslotted CSMACD wth binary backoff
Link Layer 5-59
8023 Ethernet standards link amp physical layers
many different Ethernet standards
common MAC protocol and frame format
different speeds 2 Mbps 10 Mbps 100 Mbps 1Gbps 10G bps
different physical layer media fiber cable
application
transport
network
link
physical
MAC protocol
and frame format
100BASE-TX
100BASE-T4
100BASE-FX 100BASE-T2
100BASE-SX 100BASE-BX
fiber physical layer copper (twister
pair) physical layer
Link Layer 5-60
Link layer LANs outline
51 introduction services
52 error detection correction
53 multiple access protocols
54 LANs addressing ARP
Ethernet
switches
VLANS
55 link virtualization MPLS
56 data center networking
57 a day in the life of a web request
Link Layer 5-61
Ethernet switch link-layer device takes an active role
store forward Ethernet frames
examine incoming framersquos MAC address selectively forward frame to one-or-more outgoing links when frame is to be forwarded on segment uses CSMACD to access segment
transparent
hosts are unaware of presence of switches
plug-and-play self-learning
switches do not need to be configured
Link Layer 5-62
Switch multiple simultaneous transmissions
hosts have dedicated direct connection to switch
switches buffer packets
Ethernet protocol used on each incoming link but no collisions full duplex
each link is its own collision domain
switching A-to-Arsquo and B-to-Brsquo can transmit simultaneously without collisions switch with six interfaces
(123456)
A
Arsquo
B
Brsquo C
Crsquo
1 2
3 4 5
6
Link Layer 5-63
Switch forwarding table
Q how does switch know Arsquo reachable via interface 4 Brsquo reachable via interface 5
switch with six interfaces
(123456)
A
Arsquo
B
Brsquo C
Crsquo
1 2
3 4 5
6 A each switch has a switch table each entry
(MAC address of host interface to
reach host time stamp)
looks like a routing table
Q how are entries created maintained in switch table
something like a routing protocol
A
Arsquo
B
Brsquo C
Crsquo
1 2
3 4 5
6
Link Layer 5-64
Switch self-learning
switch learns which hosts can be reached through which interfaces
when frame received switch ldquolearnsrdquo location of sender incoming LAN segment
records senderlocation pair in switch table
A Arsquo
Source A
Dest Arsquo
MAC addr interface TTL
Switch table
(initially empty) A 1 60
Link Layer 5-65
Switch frame filteringforwarding
when frame received at switch 1 record incoming link MAC address of sending host
2 index switch table using MAC destination address
3 if entry found for destination then
if destination on segment from which frame arrived then drop frame
else forward frame on interface indicated by entry
else flood forward on all interfaces except arriving
interface
A
Arsquo
B
Brsquo C
Crsquo
1 2
3 4 5
6
Link Layer 5-66
Self-learning forwarding example
A Arsquo
Source A
Dest Arsquo
MAC addr interface TTL
switch table
(initially empty) A 1 60
A Arsquo A Arsquo A Arsquo A Arsquo A Arsquo
frame destination Arsquo locaton unknown flood
Arsquo A
destination A location
known
Arsquo 4 60
selectively send
on just one link
Link Layer 5-67
Interconnecting switches
switches can be connected together
Q sending from A to G - how does S1 know to forward frame destined to F via S4 and S3
A self learning (works exactly the same as in single-switch case)
A
B
S1
C D
E
F
S2
S4
S3
H
I
G
Link Layer 5-68
Self-learning multi-switch example
Suppose C sends frame to I I responds to C
Q show switch tables and packet forwarding in S1 S2 S3 S4
A
B
S1
C D
E
F
S2
S4
S3
H
I
G
Link Layer 5-69
Institutional network
to external
network
router
IP subnet
mail server
web server
Link Layer 5-70
Switches vs routers
both are store-and-forward
routers network-layer devices (examine network-layer headers)
switches link-layer devices (examine link-layer headers)
both have forwarding tables
routers compute tables using routing algorithms IP addresses
switches learn forwarding table using flooding learning MAC addresses
application
transport
network
link
physical
network
link
physical
link
physical
switch
datagram
application
transport
network
link
physical
frame
frame
frame
datagram
Link Layer 5-71
VLANs motivation
consider CS user moves office to
EE but wants connect to CS switch
single broadcast domain
all layer-2 broadcast traffic (ARP DHCP unknown location of destination MAC address) must cross entire LAN
securityprivacy efficiency issues
Computer
Science Electrical
Engineering
Computer
Engineering
Link Layer 5-72
VLANs port-based VLAN switch ports
grouped (by switch management software) so that single physical switch helliphellip
switch(es) supporting
VLAN capabilities can
be configured to
define multiple virtual
LANS over single
physical LAN
infrastructure
Virtual Local
Area Network 1
8
9
16 10 2
7
hellip
Electrical Engineering
(VLAN ports 1-8)
Computer Science
(VLAN ports 9-15)
15
hellip
Electrical Engineering
(VLAN ports 1-8)
hellip
1
8 2
7 9
16 10
15
hellip
Computer Science
(VLAN ports 9-16)
hellip operates as multiple virtual switches
Link Layer 5-73
Port-based VLAN
1
8
9
16 10 2
7
hellip
Electrical Engineering
(VLAN ports 1-8)
Computer Science
(VLAN ports 9-15)
15
hellip
traffic isolation frames tofrom ports 1-8 can only reach ports 1-8 can also define VLAN based on
MAC addresses of endpoints rather than switch port
dynamic membership ports can be dynamically assigned among VLANs
router
forwarding between VLANS done via routing (just as with separate switches) in practice vendors sell combined
switches plus routers
Link Layer 5-74
VLANS spanning multiple switches
trunk port carries frames between VLANS defined over multiple physical switches frames forwarded within VLAN between switches canrsquot be vanilla
8021 frames (must carry VLAN ID info)
8021q protocol addsremoved additional header fields for frames forwarded between trunk ports
1
8
9
10 2
7
hellip
Electrical Engineering
(VLAN ports 1-8)
Computer Science
(VLAN ports 9-15)
15
hellip
2
7 3
Ports 235 belong to EE VLAN
Ports 4678 belong to CS VLAN
5
4 6 8 16
1
Link Layer 5-75
type
2-byte Tag Protocol Identifier
(value 81-00)
Tag Control Information (12 bit VLAN ID field
3 bit priority field like IP TOS)
Recomputed CRC
8021Q VLAN frame format
8021 frame
8021Q frame
dest address
source address
data (payload) CRC preamble
dest address
source address
preamble data (payload) CRC
type
Link Layer 5-76
Link layer LANs outline
51 introduction services
52 error detection correction
53 multiple access protocols
54 LANs addressing ARP
Ethernet
switches
VLANS
55 link virtualization MPLS
56 data center networking
57 a day in the life of a web request
Link Layer 5-77
Multiprotocol label switching (MPLS)
initial goal high-speed IP forwarding using fixed length label (instead of IP address) fast lookup using fixed length identifier (rather than
shortest prefix matching)
borrowing ideas from Virtual Circuit (VC) approach
but IP datagram still keeps IP address
PPP or Ethernet
header IP header remainder of link-layer frame MPLS header
label Exp S TTL
20 3 1 5
Link Layer 5-78
MPLS capable routers
aka label-switched router
forward packets to outgoing interface based only on label value (donrsquot inspect IP address) MPLS forwarding table distinct from IP forwarding tables
flexibility MPLS forwarding decisions can differ from those of IP use destination and source addresses to route flows to
same destination differently (traffic engineering)
re-route flows quickly if link fails pre-computed backup paths (useful for VoIP)
Link Layer 5-79
R2
D
R3 R5
A
R6
MPLS versus IP paths
IP router IP routing path to destination determined
by destination address alone
R4
Link Layer 5-80
R2
D
R3 R4
R5
A
R6
MPLS versus IP paths
IP-only router
IP routing path to destination determined by destination address alone
MPLS and IP router
MPLS routing path to destination can be based on source and dest address fast reroute precompute backup routes in
case of link failure
entry router (R4) can use different MPLS
routes to A based eg on source address
Link Layer 5-81
MPLS signaling
modify OSPF IS-IS link-state flooding protocols to carry info used by MPLS routing eg link bandwidth amount of ldquoreservedrdquo link bandwidth
D
R4
R5
A
R6
entry MPLS router uses RSVP-TE signaling protocol to set up MPLS forwarding at downstream routers
modified link state flooding
RSVP-TE
Link Layer 5-82
R1 R2
D
R3 R4 R5
0
1
0 0
A
R6
in out out
label label dest interface
6 - A 0
in out out
label label dest interface
10 6 A 1
12 9 D 0
in out out
label label dest interface
10 A 0
12 D 0
1
in out out
label label dest interface
8 6 A 0
0
8 A 1
MPLS forwarding tables
Link Layer 5-83
Link layer LANs outline
51 introduction services
52 error detection correction
53 multiple access protocols
54 LANs addressing ARP
Ethernet
switches
VLANS
55 link virtualization MPLS
56 data center networking
57 a day in the life of a web request
Link Layer 5-84
Data center networks
10rsquos to 100rsquos of thousands of hosts often closely coupled in close proximity e-business (eg Amazon)
content-servers (eg YouTube Akamai Apple Microsoft)
search engines data mining (eg Google)
challenges
multiple applications each serving massive numbers of clients
managingbalancing load avoiding processing networking data bottlenecks
Inside a 40-ft Microsoft container
Chicago data center
Link Layer 5-85
Server racks
TOR switches
Tier-1 switches
Tier-2 switches
Load balancer
Load balancer
B
1 2 3 4 5 6 7 8
A C
Border router
Access router
Internet
Data center networks
load balancer application-layer routing receives external client requests
directs workload within data center
returns results to external client (hiding data
center internals from client)
Server racks
TOR switches
Tier-1 switches
Tier-2 switches
1 2 3 4 5 6 7 8
Data center networks
rich interconnection among switches racks
increased throughput between racks (multiple routing
paths possible)
increased reliability via redundancy
Link Layer 5-87
Link layer LANs outline
51 introduction services
52 error detection correction
53 multiple access protocols
54 LANs addressing ARP
Ethernet
switches
VLANS
55 link virtualization MPLS
56 data center networking
57 a day in the life of a web request
Link Layer 5-88
Synthesis a day in the life of a web request
journey down protocol stack complete application transport network link
putting-it-all-together synthesis goal identify review understand protocols (at all
layers) involved in seemingly simple scenario requesting www page
scenario student attaches laptop to campus network requestsreceives wwwgooglecom
Link Layer 5-89
A day in the life scenario
Comcast network
68800013
Googlersquos network
64233160019 64233169105
web server
DNS server
school network
68802024
web page
browser
router
(runs DHCP)
Link Layer 5-90
A day in the lifehellip connecting to the Internet
connecting laptop needs to get its own IP address addr of first-hop router addr of DNS server use DHCP
DHCP
UDP
IP
Eth
Phy
DHCP
DHCP
DHCP
DHCP
DHCP
DHCP
UDP
IP
Eth
Phy
DHCP
DHCP
DHCP
DHCP DHCP
DHCP request encapsulated in UDP encapsulated in IP encapsulated in 8023 Ethernet
Ethernet frame broadcast
(dest FFFFFFFFFFFF) on LAN received at router running DHCP server
Ethernet demuxed to IP demuxed UDP demuxed to DHCP
router
(runs DHCP)
Link Layer 5-91
DHCP server formulates DHCP ACK containing clientrsquos IP address IP address of first-hop router for client name amp IP address of DNS server
DHCP
UDP
IP
Eth
Phy
DHCP
DHCP
DHCP
DHCP
DHCP
UDP
IP
Eth
Phy
DHCP
DHCP
DHCP
DHCP
DHCP
encapsulation at DHCP server frame forwarded (switch learning) through LAN demultiplexing at client
Client now has IP address knows name amp addr of DNS
server IP address of its first-hop router
DHCP client receives DHCP ACK reply
A day in the lifehellip connecting to the Internet
router
(runs DHCP)
Link Layer 5-92
A day in the lifehellip ARP (before DNS before HTTP)
before sending HTTP request need IP address of wwwgooglecom DNS
DNS
UDP
IP
Eth
Phy
DNS
DNS
DNS
DNS query created encapsulated in UDP encapsulated in IP encapsulated in Eth To send frame to router need MAC address of router interface ARP
ARP query broadcast received by
router which replies with ARP reply giving MAC address of router interface
client now knows MAC address of first hop router so can now send frame containing DNS query
ARP query
Eth
Phy
ARP
ARP
ARP reply
router
(runs DHCP)
Link Layer 5-93
DNS
UDP
IP
Eth
Phy
DNS
DNS
DNS
DNS
DNS
IP datagram containing DNS query forwarded via LAN switch from client to 1st hop router
IP datagram forwarded from campus network into comcast network routed (tables created by RIP OSPF IS-IS andor BGP routing protocols) to DNS server
demuxrsquoed to DNS server
DNS server replies to client with IP address of wwwgooglecom
Comcast network
68800013
DNS server
DNS
UDP
IP
Eth
Phy
DNS
DNS
DNS
DNS
A day in the lifehellip using DNS
router
(runs DHCP)
Link Layer 5-94
A day in the lifehellipTCP connection carrying HTTP
HTTP
TCP
IP
Eth
Phy
HTTP
to send HTTP request client first opens TCP socket to web server
TCP SYN segment (step 1 in 3-way handshake) inter-domain routed to web server
TCP connection established 64233169105
web server
SYN
SYN
SYN
SYN
TCP
IP
Eth
Phy
SYN
SYN
SYN
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
web server responds with TCP SYNACK (step 2 in 3-way handshake)
router
(runs DHCP)
Link Layer 5-95
A day in the lifehellip HTTP requestreply
HTTP
TCP
IP
Eth
Phy
HTTP
HTTP request sent into TCP socket
IP datagram containing HTTP request routed to wwwgooglecom
IP datagram containing HTTP reply routed back to client
64233169105
web server
HTTP
TCP
IP
Eth
Phy web server responds with
HTTP reply (containing web page)
HTTP
HTTP
HTTP HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
web page finally () displayed
Link Layer 5-96
Chapter 5 Summary
principles behind data link layer services error detection correction
sharing a broadcast channel multiple access
link layer addressing
instantiation and implementation of various link layer technologies Ethernet
switched LANS VLANs
virtualized networks as a link layer MPLS
synthesis a day in the life of a web request
Link Layer 5-97
Chapter 5 letrsquos take a breath
journey down protocol stack complete (except PHY)
solid understanding of networking principles practice
hellip could stop here hellip but lots of interesting topics wireless
multimedia
security
network management
Link Layer 5-29
CSMA (carrier sense multiple access)
CSMA listen before transmit if channel sensed idle transmit entire frame
if channel sensed busy defer transmission
human analogy donrsquot interrupt others
Link Layer 5-30
CSMA collisions
collisions can still occur propagation delay means two nodes may not hear each otherrsquos transmission
collision entire packet transmission time wasted distance amp propagation
delay play role in in determining collision probability
spatial layout of nodes
Link Layer 5-31
CSMACD (collision detection)
CSMACD carrier sensing deferral as in CSMA collisions detected within short time
colliding transmissions aborted reducing channel wastage
collision detection easy in wired LANs measure signal strengths compare
transmitted received signals
difficult in wireless LANs received signal strength overwhelmed by local transmission strength
human analogy the polite conversationalist
Link Layer 5-32
CSMACD (collision detection)
spatial layout of nodes
Link Layer 5-33
Ethernet CSMACD algorithm
1 NIC receives datagram from network layer creates frame
2 If NIC senses channel idle starts frame transmission If NIC senses channel busy waits until channel idle then transmits
3 If NIC transmits entire frame without detecting another transmission NIC is done with frame
4 If NIC detects another transmission while transmitting aborts and sends jam signal
5 After aborting NIC enters binary (exponential) backoff
after mth collision NIC chooses K at random from 012 hellip 2m-1 NIC waits K512 bit times returns to Step 2
longer backoff interval with more collisions
Link Layer 5-34
CSMACD efficiency
Tprop = max prop delay between 2 nodes in LAN
ttrans = time to transmit max-size frame
efficiency goes to 1
as tprop goes to 0
as ttrans goes to infinity
better performance than ALOHA and simple cheap decentralized
transpropttefficiency
51
1
Link Layer 5-35
ldquoTaking turnsrdquo MAC protocols
channel partitioning MAC protocols share channel efficiently and fairly at high load
inefficient at low load delay in channel access 1N bandwidth allocated even if only 1 active node
random access MAC protocols efficient at low load single node can fully utilize
channel
high load collision overhead
ldquotaking turnsrdquo protocols look for best of both worlds
Link Layer 5-36
polling master node ldquoinvitesrdquo
slave nodes to transmit in turn
typically used with ldquodumbrdquo slave devices
concerns
polling overhead
latency
single point of failure (master)
master
slaves
poll
data
data
ldquoTaking turnsrdquo MAC protocols
Link Layer 5-37
token passing control token passed
from one node to next sequentially
token message
concerns
token overhead
latency
single point of failure (token)
T
data
(nothing
to send)
T
ldquoTaking turnsrdquo MAC protocols
cable headend
CMTS
ISP
cable modem termination system
multiple 40Mbps downstream (broadcast) channels
single CMTS transmits into channels
multiple 30 Mbps upstream channels
multiple access all users contend for certain upstream channel time slots (others assigned)
Cable access network
cable modem
splitter
hellip
hellip
Internet framesTV channels control transmitted downstream at different frequencies
upstream Internet frames TV control transmitted upstream at different frequencies in time slots
Link Layer 5-39
DOCSIS data over cable service interface spec FDM over upstream downstream frequency channels
TDM upstream some slots assigned some have contention
downstream MAP frame assigns upstream slots
request for upstream slots (and data) transmitted random access (binary backoff) in selected slots
MAP frame for
Interval [t1 t2]
Residences with cable modems
Downstream channel i
Upstream channel j
t1 t2
Assigned minislots containing cable modem
upstream data frames
Minislots containing
minislots request frames
cable headend
CMTS
Cable access network
Link Layer 5-40
Summary of MAC protocols
channel partitioning by time frequency or code Time Division Frequency Division
random access (dynamic)
ALOHA S-ALOHA CSMA CSMACD
carrier sensing easy in some technologies (wire) hard in others (wireless)
CSMACD used in Ethernet
CSMACA used in 80211
taking turns
polling from central site token passing
bluetooth FDDI token ring
Link Layer 5-41
Link layer LANs outline
51 introduction services
52 error detection correction
53 multiple access protocols
54 LANs addressing ARP
Ethernet
switches
VLANS
55 link virtualization MPLS
56 data center networking
57 a day in the life of a web request
Link Layer 5-42
MAC addresses and ARP
32-bit IP address network-layer address for interface
used for layer 3 (network layer) forwarding
MAC (or LAN or physical or Ethernet) address function used lsquolocallyrdquo to get frame from one interface to
another physically-connected interface (same network in IP-addressing sense)
48 bit MAC address (for most LANs) burned in NIC ROM also sometimes software settable
eg 1A-2F-BB-76-09-AD
hexadecimal (base 16) notation
(each ldquonumberrdquo represents 4 bits)
Link Layer 5-43
LAN addresses and ARP
each adapter on LAN has unique LAN address
adapter
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN
(wired or
wireless)
Link Layer 5-44
LAN addresses (more)
MAC address allocation administered by IEEE
manufacturer buys portion of MAC address space (to assure uniqueness)
analogy MAC address like Social Security Number
IP address like postal address
MAC flat address portability can move LAN card from one LAN to another
IP hierarchical address not portable address depends on IP subnet to which node is
attached
Link Layer 5-45
ARP address resolution protocol
ARP table each IP node (host router) on LAN has table
IPMAC address mappings for some LAN nodes
lt IP address MAC address TTLgt
TTL (Time To Live) time after which address mapping will be forgotten (typically 20 min)
Question how to determine
interfacersquos MAC address
knowing its IP address
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN
137196723
137196778
137196714
137196788
Link Layer 5-46
ARP protocol same LAN
A wants to send datagram to B Brsquos MAC address not in
Arsquos ARP table
A broadcasts ARP query packet containing Bs IP address dest MAC address = FF-FF-
FF-FF-FF-FF
all nodes on LAN receive ARP query
B receives ARP packet replies to A with its (Bs) MAC address frame sent to Arsquos MAC
address (unicast)
A caches (saves) IP-to-MAC address pair in its ARP table until information becomes old (times out) soft state information that
times out (goes away) unless refreshed
ARP is ldquoplug-and-playrdquo nodes create their ARP
tables without intervention from net administrator
Link Layer 5-47
walkthrough send datagram from A to B via R
focus on addressing ndash at IP (datagram) and MAC layer (frame)
assume A knows Brsquos IP address
assume A knows IP address of first hop router R (how)
assume A knows Rrsquos MAC address (how)
Addressing routing to another LAN
R
1A-23-F9-CD-06-9B 222222222220
111111111110 E6-E9-00-17-BB-4B CC-49-DE-D0-AB-7D
111111111112
111111111111
74-29-9C-E8-FF-55
A
222222222222
49-BD-D2-C7-56-2A
222222222221 88-B2-2F-54-1A-0F
B
R
1A-23-F9-CD-06-9B 222222222220
111111111110 E6-E9-00-17-BB-4B CC-49-DE-D0-AB-7D
111111111112
111111111111
74-29-9C-E8-FF-55
A
222222222222
49-BD-D2-C7-56-2A
222222222221 88-B2-2F-54-1A-0F
B
Link Layer 5-48
Addressing routing to another LAN
IP
Eth
Phy
IP src 111111111111
IP dest 222222222222
A creates IP datagram with IP source A destination B
A creates link-layer frame with Rs MAC address as dest frame contains A-to-B IP datagram
MAC src 74-29-9C-E8-FF-55
MAC dest E6-E9-00-17-BB-4B
R
1A-23-F9-CD-06-9B 222222222220
111111111110 E6-E9-00-17-BB-4B CC-49-DE-D0-AB-7D
111111111112
111111111111
74-29-9C-E8-FF-55
A
222222222222
49-BD-D2-C7-56-2A
222222222221 88-B2-2F-54-1A-0F
B
Link Layer 5-49
Addressing routing to another LAN
IP
Eth
Phy
frame sent from A to R
IP
Eth
Phy
frame received at R datagram removed passed up to IP
MAC src 74-29-9C-E8-FF-55
MAC dest E6-E9-00-17-BB-4B
IP src 111111111111
IP dest 222222222222
IP src 111111111111
IP dest 222222222222
R
1A-23-F9-CD-06-9B 222222222220
111111111110 E6-E9-00-17-BB-4B CC-49-DE-D0-AB-7D
111111111112
111111111111
74-29-9C-E8-FF-55
A
222222222222
49-BD-D2-C7-56-2A
222222222221 88-B2-2F-54-1A-0F
B
Link Layer 5-50
Addressing routing to another LAN
IP src 111111111111
IP dest 222222222222
R forwards datagram with IP source A destination B
R creates link-layer frame with Bs MAC address as dest frame contains A-to-B IP datagram
MAC src 1A-23-F9-CD-06-9B
MAC dest 49-BD-D2-C7-56-2A
IP
Eth
Phy
IP
Eth
Phy
R
1A-23-F9-CD-06-9B 222222222220
111111111110 E6-E9-00-17-BB-4B CC-49-DE-D0-AB-7D
111111111112
111111111111
74-29-9C-E8-FF-55
A
222222222222
49-BD-D2-C7-56-2A
222222222221 88-B2-2F-54-1A-0F
B
Link Layer 5-51
Addressing routing to another LAN R forwards datagram with IP source A destination B
R creates link-layer frame with Bs MAC address as dest frame contains A-to-B IP datagram
IP src 111111111111
IP dest 222222222222
MAC src 1A-23-F9-CD-06-9B
MAC dest 49-BD-D2-C7-56-2A
IP
Eth
Phy
IP
Eth
Phy
R
1A-23-F9-CD-06-9B 222222222220
111111111110 E6-E9-00-17-BB-4B CC-49-DE-D0-AB-7D
111111111112
111111111111
74-29-9C-E8-FF-55
A
222222222222
49-BD-D2-C7-56-2A
222222222221 88-B2-2F-54-1A-0F
B
Link Layer 5-52
Addressing routing to another LAN R forwards datagram with IP source A destination B
R creates link-layer frame with Bs MAC address as dest frame contains A-to-B IP datagram
IP src 111111111111
IP dest 222222222222
MAC src 1A-23-F9-CD-06-9B
MAC dest 49-BD-D2-C7-56-2A
IP
Eth
Phy
Link Layer 5-53
Link layer LANs outline
51 introduction services
52 error detection correction
53 multiple access protocols
54 LANs addressing ARP
Ethernet
switches
VLANS
55 link virtualization MPLS
56 data center networking
57 a day in the life of a web request
Link Layer 5-54
Ethernet
ldquodominantrdquo wired LAN technology
cheap $20 for NIC
first widely used LAN technology
simpler cheaper than token LANs and ATM
kept up with speed race 10 Mbps ndash 10 Gbps
Metcalfersquos Ethernet sketch
Link Layer 5-55
Ethernet physical topology
bus popular through mid 90s all nodes in same collision domain (can collide with each
other)
star prevails today active switch in center
each ldquospokerdquo runs a (separate) Ethernet protocol (nodes
do not collide with each other)
switch
bus coaxial cable star
Link Layer 5-56
Ethernet frame structure
sending adapter encapsulates IP datagram (or other network layer protocol packet) in Ethernet frame
preamble
7 bytes with pattern 10101010 followed by one byte with pattern 10101011
used to synchronize receiver sender clock rates
dest address
source address
data (payload) CRC preamble
type
Link Layer 5-57
Ethernet frame structure (more)
addresses 6 byte source destination MAC addresses if adapter receives frame with matching destination
address or with broadcast address (eg ARP packet) it passes data in frame to network layer protocol
otherwise adapter discards frame
type indicates higher layer protocol (mostly IP but others possible eg Novell IPX AppleTalk)
CRC cyclic redundancy check at receiver error detected frame is dropped
dest address
source address
data (payload) CRC preamble
type
Link Layer 5-58
Ethernet unreliable connectionless
connectionless no handshaking between sending and receiving NICs
unreliable receiving NIC doesnt send acks or nacks to sending NIC
data in dropped frames recovered only if initial sender uses higher layer rdt (eg TCP) otherwise dropped data lost
Ethernetrsquos MAC protocol unslotted CSMACD wth binary backoff
Link Layer 5-59
8023 Ethernet standards link amp physical layers
many different Ethernet standards
common MAC protocol and frame format
different speeds 2 Mbps 10 Mbps 100 Mbps 1Gbps 10G bps
different physical layer media fiber cable
application
transport
network
link
physical
MAC protocol
and frame format
100BASE-TX
100BASE-T4
100BASE-FX 100BASE-T2
100BASE-SX 100BASE-BX
fiber physical layer copper (twister
pair) physical layer
Link Layer 5-60
Link layer LANs outline
51 introduction services
52 error detection correction
53 multiple access protocols
54 LANs addressing ARP
Ethernet
switches
VLANS
55 link virtualization MPLS
56 data center networking
57 a day in the life of a web request
Link Layer 5-61
Ethernet switch link-layer device takes an active role
store forward Ethernet frames
examine incoming framersquos MAC address selectively forward frame to one-or-more outgoing links when frame is to be forwarded on segment uses CSMACD to access segment
transparent
hosts are unaware of presence of switches
plug-and-play self-learning
switches do not need to be configured
Link Layer 5-62
Switch multiple simultaneous transmissions
hosts have dedicated direct connection to switch
switches buffer packets
Ethernet protocol used on each incoming link but no collisions full duplex
each link is its own collision domain
switching A-to-Arsquo and B-to-Brsquo can transmit simultaneously without collisions switch with six interfaces
(123456)
A
Arsquo
B
Brsquo C
Crsquo
1 2
3 4 5
6
Link Layer 5-63
Switch forwarding table
Q how does switch know Arsquo reachable via interface 4 Brsquo reachable via interface 5
switch with six interfaces
(123456)
A
Arsquo
B
Brsquo C
Crsquo
1 2
3 4 5
6 A each switch has a switch table each entry
(MAC address of host interface to
reach host time stamp)
looks like a routing table
Q how are entries created maintained in switch table
something like a routing protocol
A
Arsquo
B
Brsquo C
Crsquo
1 2
3 4 5
6
Link Layer 5-64
Switch self-learning
switch learns which hosts can be reached through which interfaces
when frame received switch ldquolearnsrdquo location of sender incoming LAN segment
records senderlocation pair in switch table
A Arsquo
Source A
Dest Arsquo
MAC addr interface TTL
Switch table
(initially empty) A 1 60
Link Layer 5-65
Switch frame filteringforwarding
when frame received at switch 1 record incoming link MAC address of sending host
2 index switch table using MAC destination address
3 if entry found for destination then
if destination on segment from which frame arrived then drop frame
else forward frame on interface indicated by entry
else flood forward on all interfaces except arriving
interface
A
Arsquo
B
Brsquo C
Crsquo
1 2
3 4 5
6
Link Layer 5-66
Self-learning forwarding example
A Arsquo
Source A
Dest Arsquo
MAC addr interface TTL
switch table
(initially empty) A 1 60
A Arsquo A Arsquo A Arsquo A Arsquo A Arsquo
frame destination Arsquo locaton unknown flood
Arsquo A
destination A location
known
Arsquo 4 60
selectively send
on just one link
Link Layer 5-67
Interconnecting switches
switches can be connected together
Q sending from A to G - how does S1 know to forward frame destined to F via S4 and S3
A self learning (works exactly the same as in single-switch case)
A
B
S1
C D
E
F
S2
S4
S3
H
I
G
Link Layer 5-68
Self-learning multi-switch example
Suppose C sends frame to I I responds to C
Q show switch tables and packet forwarding in S1 S2 S3 S4
A
B
S1
C D
E
F
S2
S4
S3
H
I
G
Link Layer 5-69
Institutional network
to external
network
router
IP subnet
mail server
web server
Link Layer 5-70
Switches vs routers
both are store-and-forward
routers network-layer devices (examine network-layer headers)
switches link-layer devices (examine link-layer headers)
both have forwarding tables
routers compute tables using routing algorithms IP addresses
switches learn forwarding table using flooding learning MAC addresses
application
transport
network
link
physical
network
link
physical
link
physical
switch
datagram
application
transport
network
link
physical
frame
frame
frame
datagram
Link Layer 5-71
VLANs motivation
consider CS user moves office to
EE but wants connect to CS switch
single broadcast domain
all layer-2 broadcast traffic (ARP DHCP unknown location of destination MAC address) must cross entire LAN
securityprivacy efficiency issues
Computer
Science Electrical
Engineering
Computer
Engineering
Link Layer 5-72
VLANs port-based VLAN switch ports
grouped (by switch management software) so that single physical switch helliphellip
switch(es) supporting
VLAN capabilities can
be configured to
define multiple virtual
LANS over single
physical LAN
infrastructure
Virtual Local
Area Network 1
8
9
16 10 2
7
hellip
Electrical Engineering
(VLAN ports 1-8)
Computer Science
(VLAN ports 9-15)
15
hellip
Electrical Engineering
(VLAN ports 1-8)
hellip
1
8 2
7 9
16 10
15
hellip
Computer Science
(VLAN ports 9-16)
hellip operates as multiple virtual switches
Link Layer 5-73
Port-based VLAN
1
8
9
16 10 2
7
hellip
Electrical Engineering
(VLAN ports 1-8)
Computer Science
(VLAN ports 9-15)
15
hellip
traffic isolation frames tofrom ports 1-8 can only reach ports 1-8 can also define VLAN based on
MAC addresses of endpoints rather than switch port
dynamic membership ports can be dynamically assigned among VLANs
router
forwarding between VLANS done via routing (just as with separate switches) in practice vendors sell combined
switches plus routers
Link Layer 5-74
VLANS spanning multiple switches
trunk port carries frames between VLANS defined over multiple physical switches frames forwarded within VLAN between switches canrsquot be vanilla
8021 frames (must carry VLAN ID info)
8021q protocol addsremoved additional header fields for frames forwarded between trunk ports
1
8
9
10 2
7
hellip
Electrical Engineering
(VLAN ports 1-8)
Computer Science
(VLAN ports 9-15)
15
hellip
2
7 3
Ports 235 belong to EE VLAN
Ports 4678 belong to CS VLAN
5
4 6 8 16
1
Link Layer 5-75
type
2-byte Tag Protocol Identifier
(value 81-00)
Tag Control Information (12 bit VLAN ID field
3 bit priority field like IP TOS)
Recomputed CRC
8021Q VLAN frame format
8021 frame
8021Q frame
dest address
source address
data (payload) CRC preamble
dest address
source address
preamble data (payload) CRC
type
Link Layer 5-76
Link layer LANs outline
51 introduction services
52 error detection correction
53 multiple access protocols
54 LANs addressing ARP
Ethernet
switches
VLANS
55 link virtualization MPLS
56 data center networking
57 a day in the life of a web request
Link Layer 5-77
Multiprotocol label switching (MPLS)
initial goal high-speed IP forwarding using fixed length label (instead of IP address) fast lookup using fixed length identifier (rather than
shortest prefix matching)
borrowing ideas from Virtual Circuit (VC) approach
but IP datagram still keeps IP address
PPP or Ethernet
header IP header remainder of link-layer frame MPLS header
label Exp S TTL
20 3 1 5
Link Layer 5-78
MPLS capable routers
aka label-switched router
forward packets to outgoing interface based only on label value (donrsquot inspect IP address) MPLS forwarding table distinct from IP forwarding tables
flexibility MPLS forwarding decisions can differ from those of IP use destination and source addresses to route flows to
same destination differently (traffic engineering)
re-route flows quickly if link fails pre-computed backup paths (useful for VoIP)
Link Layer 5-79
R2
D
R3 R5
A
R6
MPLS versus IP paths
IP router IP routing path to destination determined
by destination address alone
R4
Link Layer 5-80
R2
D
R3 R4
R5
A
R6
MPLS versus IP paths
IP-only router
IP routing path to destination determined by destination address alone
MPLS and IP router
MPLS routing path to destination can be based on source and dest address fast reroute precompute backup routes in
case of link failure
entry router (R4) can use different MPLS
routes to A based eg on source address
Link Layer 5-81
MPLS signaling
modify OSPF IS-IS link-state flooding protocols to carry info used by MPLS routing eg link bandwidth amount of ldquoreservedrdquo link bandwidth
D
R4
R5
A
R6
entry MPLS router uses RSVP-TE signaling protocol to set up MPLS forwarding at downstream routers
modified link state flooding
RSVP-TE
Link Layer 5-82
R1 R2
D
R3 R4 R5
0
1
0 0
A
R6
in out out
label label dest interface
6 - A 0
in out out
label label dest interface
10 6 A 1
12 9 D 0
in out out
label label dest interface
10 A 0
12 D 0
1
in out out
label label dest interface
8 6 A 0
0
8 A 1
MPLS forwarding tables
Link Layer 5-83
Link layer LANs outline
51 introduction services
52 error detection correction
53 multiple access protocols
54 LANs addressing ARP
Ethernet
switches
VLANS
55 link virtualization MPLS
56 data center networking
57 a day in the life of a web request
Link Layer 5-84
Data center networks
10rsquos to 100rsquos of thousands of hosts often closely coupled in close proximity e-business (eg Amazon)
content-servers (eg YouTube Akamai Apple Microsoft)
search engines data mining (eg Google)
challenges
multiple applications each serving massive numbers of clients
managingbalancing load avoiding processing networking data bottlenecks
Inside a 40-ft Microsoft container
Chicago data center
Link Layer 5-85
Server racks
TOR switches
Tier-1 switches
Tier-2 switches
Load balancer
Load balancer
B
1 2 3 4 5 6 7 8
A C
Border router
Access router
Internet
Data center networks
load balancer application-layer routing receives external client requests
directs workload within data center
returns results to external client (hiding data
center internals from client)
Server racks
TOR switches
Tier-1 switches
Tier-2 switches
1 2 3 4 5 6 7 8
Data center networks
rich interconnection among switches racks
increased throughput between racks (multiple routing
paths possible)
increased reliability via redundancy
Link Layer 5-87
Link layer LANs outline
51 introduction services
52 error detection correction
53 multiple access protocols
54 LANs addressing ARP
Ethernet
switches
VLANS
55 link virtualization MPLS
56 data center networking
57 a day in the life of a web request
Link Layer 5-88
Synthesis a day in the life of a web request
journey down protocol stack complete application transport network link
putting-it-all-together synthesis goal identify review understand protocols (at all
layers) involved in seemingly simple scenario requesting www page
scenario student attaches laptop to campus network requestsreceives wwwgooglecom
Link Layer 5-89
A day in the life scenario
Comcast network
68800013
Googlersquos network
64233160019 64233169105
web server
DNS server
school network
68802024
web page
browser
router
(runs DHCP)
Link Layer 5-90
A day in the lifehellip connecting to the Internet
connecting laptop needs to get its own IP address addr of first-hop router addr of DNS server use DHCP
DHCP
UDP
IP
Eth
Phy
DHCP
DHCP
DHCP
DHCP
DHCP
DHCP
UDP
IP
Eth
Phy
DHCP
DHCP
DHCP
DHCP DHCP
DHCP request encapsulated in UDP encapsulated in IP encapsulated in 8023 Ethernet
Ethernet frame broadcast
(dest FFFFFFFFFFFF) on LAN received at router running DHCP server
Ethernet demuxed to IP demuxed UDP demuxed to DHCP
router
(runs DHCP)
Link Layer 5-91
DHCP server formulates DHCP ACK containing clientrsquos IP address IP address of first-hop router for client name amp IP address of DNS server
DHCP
UDP
IP
Eth
Phy
DHCP
DHCP
DHCP
DHCP
DHCP
UDP
IP
Eth
Phy
DHCP
DHCP
DHCP
DHCP
DHCP
encapsulation at DHCP server frame forwarded (switch learning) through LAN demultiplexing at client
Client now has IP address knows name amp addr of DNS
server IP address of its first-hop router
DHCP client receives DHCP ACK reply
A day in the lifehellip connecting to the Internet
router
(runs DHCP)
Link Layer 5-92
A day in the lifehellip ARP (before DNS before HTTP)
before sending HTTP request need IP address of wwwgooglecom DNS
DNS
UDP
IP
Eth
Phy
DNS
DNS
DNS
DNS query created encapsulated in UDP encapsulated in IP encapsulated in Eth To send frame to router need MAC address of router interface ARP
ARP query broadcast received by
router which replies with ARP reply giving MAC address of router interface
client now knows MAC address of first hop router so can now send frame containing DNS query
ARP query
Eth
Phy
ARP
ARP
ARP reply
router
(runs DHCP)
Link Layer 5-93
DNS
UDP
IP
Eth
Phy
DNS
DNS
DNS
DNS
DNS
IP datagram containing DNS query forwarded via LAN switch from client to 1st hop router
IP datagram forwarded from campus network into comcast network routed (tables created by RIP OSPF IS-IS andor BGP routing protocols) to DNS server
demuxrsquoed to DNS server
DNS server replies to client with IP address of wwwgooglecom
Comcast network
68800013
DNS server
DNS
UDP
IP
Eth
Phy
DNS
DNS
DNS
DNS
A day in the lifehellip using DNS
router
(runs DHCP)
Link Layer 5-94
A day in the lifehellipTCP connection carrying HTTP
HTTP
TCP
IP
Eth
Phy
HTTP
to send HTTP request client first opens TCP socket to web server
TCP SYN segment (step 1 in 3-way handshake) inter-domain routed to web server
TCP connection established 64233169105
web server
SYN
SYN
SYN
SYN
TCP
IP
Eth
Phy
SYN
SYN
SYN
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
web server responds with TCP SYNACK (step 2 in 3-way handshake)
router
(runs DHCP)
Link Layer 5-95
A day in the lifehellip HTTP requestreply
HTTP
TCP
IP
Eth
Phy
HTTP
HTTP request sent into TCP socket
IP datagram containing HTTP request routed to wwwgooglecom
IP datagram containing HTTP reply routed back to client
64233169105
web server
HTTP
TCP
IP
Eth
Phy web server responds with
HTTP reply (containing web page)
HTTP
HTTP
HTTP HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
web page finally () displayed
Link Layer 5-96
Chapter 5 Summary
principles behind data link layer services error detection correction
sharing a broadcast channel multiple access
link layer addressing
instantiation and implementation of various link layer technologies Ethernet
switched LANS VLANs
virtualized networks as a link layer MPLS
synthesis a day in the life of a web request
Link Layer 5-97
Chapter 5 letrsquos take a breath
journey down protocol stack complete (except PHY)
solid understanding of networking principles practice
hellip could stop here hellip but lots of interesting topics wireless
multimedia
security
network management
Link Layer 5-30
CSMA collisions
collisions can still occur propagation delay means two nodes may not hear each otherrsquos transmission
collision entire packet transmission time wasted distance amp propagation
delay play role in in determining collision probability
spatial layout of nodes
Link Layer 5-31
CSMACD (collision detection)
CSMACD carrier sensing deferral as in CSMA collisions detected within short time
colliding transmissions aborted reducing channel wastage
collision detection easy in wired LANs measure signal strengths compare
transmitted received signals
difficult in wireless LANs received signal strength overwhelmed by local transmission strength
human analogy the polite conversationalist
Link Layer 5-32
CSMACD (collision detection)
spatial layout of nodes
Link Layer 5-33
Ethernet CSMACD algorithm
1 NIC receives datagram from network layer creates frame
2 If NIC senses channel idle starts frame transmission If NIC senses channel busy waits until channel idle then transmits
3 If NIC transmits entire frame without detecting another transmission NIC is done with frame
4 If NIC detects another transmission while transmitting aborts and sends jam signal
5 After aborting NIC enters binary (exponential) backoff
after mth collision NIC chooses K at random from 012 hellip 2m-1 NIC waits K512 bit times returns to Step 2
longer backoff interval with more collisions
Link Layer 5-34
CSMACD efficiency
Tprop = max prop delay between 2 nodes in LAN
ttrans = time to transmit max-size frame
efficiency goes to 1
as tprop goes to 0
as ttrans goes to infinity
better performance than ALOHA and simple cheap decentralized
transpropttefficiency
51
1
Link Layer 5-35
ldquoTaking turnsrdquo MAC protocols
channel partitioning MAC protocols share channel efficiently and fairly at high load
inefficient at low load delay in channel access 1N bandwidth allocated even if only 1 active node
random access MAC protocols efficient at low load single node can fully utilize
channel
high load collision overhead
ldquotaking turnsrdquo protocols look for best of both worlds
Link Layer 5-36
polling master node ldquoinvitesrdquo
slave nodes to transmit in turn
typically used with ldquodumbrdquo slave devices
concerns
polling overhead
latency
single point of failure (master)
master
slaves
poll
data
data
ldquoTaking turnsrdquo MAC protocols
Link Layer 5-37
token passing control token passed
from one node to next sequentially
token message
concerns
token overhead
latency
single point of failure (token)
T
data
(nothing
to send)
T
ldquoTaking turnsrdquo MAC protocols
cable headend
CMTS
ISP
cable modem termination system
multiple 40Mbps downstream (broadcast) channels
single CMTS transmits into channels
multiple 30 Mbps upstream channels
multiple access all users contend for certain upstream channel time slots (others assigned)
Cable access network
cable modem
splitter
hellip
hellip
Internet framesTV channels control transmitted downstream at different frequencies
upstream Internet frames TV control transmitted upstream at different frequencies in time slots
Link Layer 5-39
DOCSIS data over cable service interface spec FDM over upstream downstream frequency channels
TDM upstream some slots assigned some have contention
downstream MAP frame assigns upstream slots
request for upstream slots (and data) transmitted random access (binary backoff) in selected slots
MAP frame for
Interval [t1 t2]
Residences with cable modems
Downstream channel i
Upstream channel j
t1 t2
Assigned minislots containing cable modem
upstream data frames
Minislots containing
minislots request frames
cable headend
CMTS
Cable access network
Link Layer 5-40
Summary of MAC protocols
channel partitioning by time frequency or code Time Division Frequency Division
random access (dynamic)
ALOHA S-ALOHA CSMA CSMACD
carrier sensing easy in some technologies (wire) hard in others (wireless)
CSMACD used in Ethernet
CSMACA used in 80211
taking turns
polling from central site token passing
bluetooth FDDI token ring
Link Layer 5-41
Link layer LANs outline
51 introduction services
52 error detection correction
53 multiple access protocols
54 LANs addressing ARP
Ethernet
switches
VLANS
55 link virtualization MPLS
56 data center networking
57 a day in the life of a web request
Link Layer 5-42
MAC addresses and ARP
32-bit IP address network-layer address for interface
used for layer 3 (network layer) forwarding
MAC (or LAN or physical or Ethernet) address function used lsquolocallyrdquo to get frame from one interface to
another physically-connected interface (same network in IP-addressing sense)
48 bit MAC address (for most LANs) burned in NIC ROM also sometimes software settable
eg 1A-2F-BB-76-09-AD
hexadecimal (base 16) notation
(each ldquonumberrdquo represents 4 bits)
Link Layer 5-43
LAN addresses and ARP
each adapter on LAN has unique LAN address
adapter
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN
(wired or
wireless)
Link Layer 5-44
LAN addresses (more)
MAC address allocation administered by IEEE
manufacturer buys portion of MAC address space (to assure uniqueness)
analogy MAC address like Social Security Number
IP address like postal address
MAC flat address portability can move LAN card from one LAN to another
IP hierarchical address not portable address depends on IP subnet to which node is
attached
Link Layer 5-45
ARP address resolution protocol
ARP table each IP node (host router) on LAN has table
IPMAC address mappings for some LAN nodes
lt IP address MAC address TTLgt
TTL (Time To Live) time after which address mapping will be forgotten (typically 20 min)
Question how to determine
interfacersquos MAC address
knowing its IP address
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN
137196723
137196778
137196714
137196788
Link Layer 5-46
ARP protocol same LAN
A wants to send datagram to B Brsquos MAC address not in
Arsquos ARP table
A broadcasts ARP query packet containing Bs IP address dest MAC address = FF-FF-
FF-FF-FF-FF
all nodes on LAN receive ARP query
B receives ARP packet replies to A with its (Bs) MAC address frame sent to Arsquos MAC
address (unicast)
A caches (saves) IP-to-MAC address pair in its ARP table until information becomes old (times out) soft state information that
times out (goes away) unless refreshed
ARP is ldquoplug-and-playrdquo nodes create their ARP
tables without intervention from net administrator
Link Layer 5-47
walkthrough send datagram from A to B via R
focus on addressing ndash at IP (datagram) and MAC layer (frame)
assume A knows Brsquos IP address
assume A knows IP address of first hop router R (how)
assume A knows Rrsquos MAC address (how)
Addressing routing to another LAN
R
1A-23-F9-CD-06-9B 222222222220
111111111110 E6-E9-00-17-BB-4B CC-49-DE-D0-AB-7D
111111111112
111111111111
74-29-9C-E8-FF-55
A
222222222222
49-BD-D2-C7-56-2A
222222222221 88-B2-2F-54-1A-0F
B
R
1A-23-F9-CD-06-9B 222222222220
111111111110 E6-E9-00-17-BB-4B CC-49-DE-D0-AB-7D
111111111112
111111111111
74-29-9C-E8-FF-55
A
222222222222
49-BD-D2-C7-56-2A
222222222221 88-B2-2F-54-1A-0F
B
Link Layer 5-48
Addressing routing to another LAN
IP
Eth
Phy
IP src 111111111111
IP dest 222222222222
A creates IP datagram with IP source A destination B
A creates link-layer frame with Rs MAC address as dest frame contains A-to-B IP datagram
MAC src 74-29-9C-E8-FF-55
MAC dest E6-E9-00-17-BB-4B
R
1A-23-F9-CD-06-9B 222222222220
111111111110 E6-E9-00-17-BB-4B CC-49-DE-D0-AB-7D
111111111112
111111111111
74-29-9C-E8-FF-55
A
222222222222
49-BD-D2-C7-56-2A
222222222221 88-B2-2F-54-1A-0F
B
Link Layer 5-49
Addressing routing to another LAN
IP
Eth
Phy
frame sent from A to R
IP
Eth
Phy
frame received at R datagram removed passed up to IP
MAC src 74-29-9C-E8-FF-55
MAC dest E6-E9-00-17-BB-4B
IP src 111111111111
IP dest 222222222222
IP src 111111111111
IP dest 222222222222
R
1A-23-F9-CD-06-9B 222222222220
111111111110 E6-E9-00-17-BB-4B CC-49-DE-D0-AB-7D
111111111112
111111111111
74-29-9C-E8-FF-55
A
222222222222
49-BD-D2-C7-56-2A
222222222221 88-B2-2F-54-1A-0F
B
Link Layer 5-50
Addressing routing to another LAN
IP src 111111111111
IP dest 222222222222
R forwards datagram with IP source A destination B
R creates link-layer frame with Bs MAC address as dest frame contains A-to-B IP datagram
MAC src 1A-23-F9-CD-06-9B
MAC dest 49-BD-D2-C7-56-2A
IP
Eth
Phy
IP
Eth
Phy
R
1A-23-F9-CD-06-9B 222222222220
111111111110 E6-E9-00-17-BB-4B CC-49-DE-D0-AB-7D
111111111112
111111111111
74-29-9C-E8-FF-55
A
222222222222
49-BD-D2-C7-56-2A
222222222221 88-B2-2F-54-1A-0F
B
Link Layer 5-51
Addressing routing to another LAN R forwards datagram with IP source A destination B
R creates link-layer frame with Bs MAC address as dest frame contains A-to-B IP datagram
IP src 111111111111
IP dest 222222222222
MAC src 1A-23-F9-CD-06-9B
MAC dest 49-BD-D2-C7-56-2A
IP
Eth
Phy
IP
Eth
Phy
R
1A-23-F9-CD-06-9B 222222222220
111111111110 E6-E9-00-17-BB-4B CC-49-DE-D0-AB-7D
111111111112
111111111111
74-29-9C-E8-FF-55
A
222222222222
49-BD-D2-C7-56-2A
222222222221 88-B2-2F-54-1A-0F
B
Link Layer 5-52
Addressing routing to another LAN R forwards datagram with IP source A destination B
R creates link-layer frame with Bs MAC address as dest frame contains A-to-B IP datagram
IP src 111111111111
IP dest 222222222222
MAC src 1A-23-F9-CD-06-9B
MAC dest 49-BD-D2-C7-56-2A
IP
Eth
Phy
Link Layer 5-53
Link layer LANs outline
51 introduction services
52 error detection correction
53 multiple access protocols
54 LANs addressing ARP
Ethernet
switches
VLANS
55 link virtualization MPLS
56 data center networking
57 a day in the life of a web request
Link Layer 5-54
Ethernet
ldquodominantrdquo wired LAN technology
cheap $20 for NIC
first widely used LAN technology
simpler cheaper than token LANs and ATM
kept up with speed race 10 Mbps ndash 10 Gbps
Metcalfersquos Ethernet sketch
Link Layer 5-55
Ethernet physical topology
bus popular through mid 90s all nodes in same collision domain (can collide with each
other)
star prevails today active switch in center
each ldquospokerdquo runs a (separate) Ethernet protocol (nodes
do not collide with each other)
switch
bus coaxial cable star
Link Layer 5-56
Ethernet frame structure
sending adapter encapsulates IP datagram (or other network layer protocol packet) in Ethernet frame
preamble
7 bytes with pattern 10101010 followed by one byte with pattern 10101011
used to synchronize receiver sender clock rates
dest address
source address
data (payload) CRC preamble
type
Link Layer 5-57
Ethernet frame structure (more)
addresses 6 byte source destination MAC addresses if adapter receives frame with matching destination
address or with broadcast address (eg ARP packet) it passes data in frame to network layer protocol
otherwise adapter discards frame
type indicates higher layer protocol (mostly IP but others possible eg Novell IPX AppleTalk)
CRC cyclic redundancy check at receiver error detected frame is dropped
dest address
source address
data (payload) CRC preamble
type
Link Layer 5-58
Ethernet unreliable connectionless
connectionless no handshaking between sending and receiving NICs
unreliable receiving NIC doesnt send acks or nacks to sending NIC
data in dropped frames recovered only if initial sender uses higher layer rdt (eg TCP) otherwise dropped data lost
Ethernetrsquos MAC protocol unslotted CSMACD wth binary backoff
Link Layer 5-59
8023 Ethernet standards link amp physical layers
many different Ethernet standards
common MAC protocol and frame format
different speeds 2 Mbps 10 Mbps 100 Mbps 1Gbps 10G bps
different physical layer media fiber cable
application
transport
network
link
physical
MAC protocol
and frame format
100BASE-TX
100BASE-T4
100BASE-FX 100BASE-T2
100BASE-SX 100BASE-BX
fiber physical layer copper (twister
pair) physical layer
Link Layer 5-60
Link layer LANs outline
51 introduction services
52 error detection correction
53 multiple access protocols
54 LANs addressing ARP
Ethernet
switches
VLANS
55 link virtualization MPLS
56 data center networking
57 a day in the life of a web request
Link Layer 5-61
Ethernet switch link-layer device takes an active role
store forward Ethernet frames
examine incoming framersquos MAC address selectively forward frame to one-or-more outgoing links when frame is to be forwarded on segment uses CSMACD to access segment
transparent
hosts are unaware of presence of switches
plug-and-play self-learning
switches do not need to be configured
Link Layer 5-62
Switch multiple simultaneous transmissions
hosts have dedicated direct connection to switch
switches buffer packets
Ethernet protocol used on each incoming link but no collisions full duplex
each link is its own collision domain
switching A-to-Arsquo and B-to-Brsquo can transmit simultaneously without collisions switch with six interfaces
(123456)
A
Arsquo
B
Brsquo C
Crsquo
1 2
3 4 5
6
Link Layer 5-63
Switch forwarding table
Q how does switch know Arsquo reachable via interface 4 Brsquo reachable via interface 5
switch with six interfaces
(123456)
A
Arsquo
B
Brsquo C
Crsquo
1 2
3 4 5
6 A each switch has a switch table each entry
(MAC address of host interface to
reach host time stamp)
looks like a routing table
Q how are entries created maintained in switch table
something like a routing protocol
A
Arsquo
B
Brsquo C
Crsquo
1 2
3 4 5
6
Link Layer 5-64
Switch self-learning
switch learns which hosts can be reached through which interfaces
when frame received switch ldquolearnsrdquo location of sender incoming LAN segment
records senderlocation pair in switch table
A Arsquo
Source A
Dest Arsquo
MAC addr interface TTL
Switch table
(initially empty) A 1 60
Link Layer 5-65
Switch frame filteringforwarding
when frame received at switch 1 record incoming link MAC address of sending host
2 index switch table using MAC destination address
3 if entry found for destination then
if destination on segment from which frame arrived then drop frame
else forward frame on interface indicated by entry
else flood forward on all interfaces except arriving
interface
A
Arsquo
B
Brsquo C
Crsquo
1 2
3 4 5
6
Link Layer 5-66
Self-learning forwarding example
A Arsquo
Source A
Dest Arsquo
MAC addr interface TTL
switch table
(initially empty) A 1 60
A Arsquo A Arsquo A Arsquo A Arsquo A Arsquo
frame destination Arsquo locaton unknown flood
Arsquo A
destination A location
known
Arsquo 4 60
selectively send
on just one link
Link Layer 5-67
Interconnecting switches
switches can be connected together
Q sending from A to G - how does S1 know to forward frame destined to F via S4 and S3
A self learning (works exactly the same as in single-switch case)
A
B
S1
C D
E
F
S2
S4
S3
H
I
G
Link Layer 5-68
Self-learning multi-switch example
Suppose C sends frame to I I responds to C
Q show switch tables and packet forwarding in S1 S2 S3 S4
A
B
S1
C D
E
F
S2
S4
S3
H
I
G
Link Layer 5-69
Institutional network
to external
network
router
IP subnet
mail server
web server
Link Layer 5-70
Switches vs routers
both are store-and-forward
routers network-layer devices (examine network-layer headers)
switches link-layer devices (examine link-layer headers)
both have forwarding tables
routers compute tables using routing algorithms IP addresses
switches learn forwarding table using flooding learning MAC addresses
application
transport
network
link
physical
network
link
physical
link
physical
switch
datagram
application
transport
network
link
physical
frame
frame
frame
datagram
Link Layer 5-71
VLANs motivation
consider CS user moves office to
EE but wants connect to CS switch
single broadcast domain
all layer-2 broadcast traffic (ARP DHCP unknown location of destination MAC address) must cross entire LAN
securityprivacy efficiency issues
Computer
Science Electrical
Engineering
Computer
Engineering
Link Layer 5-72
VLANs port-based VLAN switch ports
grouped (by switch management software) so that single physical switch helliphellip
switch(es) supporting
VLAN capabilities can
be configured to
define multiple virtual
LANS over single
physical LAN
infrastructure
Virtual Local
Area Network 1
8
9
16 10 2
7
hellip
Electrical Engineering
(VLAN ports 1-8)
Computer Science
(VLAN ports 9-15)
15
hellip
Electrical Engineering
(VLAN ports 1-8)
hellip
1
8 2
7 9
16 10
15
hellip
Computer Science
(VLAN ports 9-16)
hellip operates as multiple virtual switches
Link Layer 5-73
Port-based VLAN
1
8
9
16 10 2
7
hellip
Electrical Engineering
(VLAN ports 1-8)
Computer Science
(VLAN ports 9-15)
15
hellip
traffic isolation frames tofrom ports 1-8 can only reach ports 1-8 can also define VLAN based on
MAC addresses of endpoints rather than switch port
dynamic membership ports can be dynamically assigned among VLANs
router
forwarding between VLANS done via routing (just as with separate switches) in practice vendors sell combined
switches plus routers
Link Layer 5-74
VLANS spanning multiple switches
trunk port carries frames between VLANS defined over multiple physical switches frames forwarded within VLAN between switches canrsquot be vanilla
8021 frames (must carry VLAN ID info)
8021q protocol addsremoved additional header fields for frames forwarded between trunk ports
1
8
9
10 2
7
hellip
Electrical Engineering
(VLAN ports 1-8)
Computer Science
(VLAN ports 9-15)
15
hellip
2
7 3
Ports 235 belong to EE VLAN
Ports 4678 belong to CS VLAN
5
4 6 8 16
1
Link Layer 5-75
type
2-byte Tag Protocol Identifier
(value 81-00)
Tag Control Information (12 bit VLAN ID field
3 bit priority field like IP TOS)
Recomputed CRC
8021Q VLAN frame format
8021 frame
8021Q frame
dest address
source address
data (payload) CRC preamble
dest address
source address
preamble data (payload) CRC
type
Link Layer 5-76
Link layer LANs outline
51 introduction services
52 error detection correction
53 multiple access protocols
54 LANs addressing ARP
Ethernet
switches
VLANS
55 link virtualization MPLS
56 data center networking
57 a day in the life of a web request
Link Layer 5-77
Multiprotocol label switching (MPLS)
initial goal high-speed IP forwarding using fixed length label (instead of IP address) fast lookup using fixed length identifier (rather than
shortest prefix matching)
borrowing ideas from Virtual Circuit (VC) approach
but IP datagram still keeps IP address
PPP or Ethernet
header IP header remainder of link-layer frame MPLS header
label Exp S TTL
20 3 1 5
Link Layer 5-78
MPLS capable routers
aka label-switched router
forward packets to outgoing interface based only on label value (donrsquot inspect IP address) MPLS forwarding table distinct from IP forwarding tables
flexibility MPLS forwarding decisions can differ from those of IP use destination and source addresses to route flows to
same destination differently (traffic engineering)
re-route flows quickly if link fails pre-computed backup paths (useful for VoIP)
Link Layer 5-79
R2
D
R3 R5
A
R6
MPLS versus IP paths
IP router IP routing path to destination determined
by destination address alone
R4
Link Layer 5-80
R2
D
R3 R4
R5
A
R6
MPLS versus IP paths
IP-only router
IP routing path to destination determined by destination address alone
MPLS and IP router
MPLS routing path to destination can be based on source and dest address fast reroute precompute backup routes in
case of link failure
entry router (R4) can use different MPLS
routes to A based eg on source address
Link Layer 5-81
MPLS signaling
modify OSPF IS-IS link-state flooding protocols to carry info used by MPLS routing eg link bandwidth amount of ldquoreservedrdquo link bandwidth
D
R4
R5
A
R6
entry MPLS router uses RSVP-TE signaling protocol to set up MPLS forwarding at downstream routers
modified link state flooding
RSVP-TE
Link Layer 5-82
R1 R2
D
R3 R4 R5
0
1
0 0
A
R6
in out out
label label dest interface
6 - A 0
in out out
label label dest interface
10 6 A 1
12 9 D 0
in out out
label label dest interface
10 A 0
12 D 0
1
in out out
label label dest interface
8 6 A 0
0
8 A 1
MPLS forwarding tables
Link Layer 5-83
Link layer LANs outline
51 introduction services
52 error detection correction
53 multiple access protocols
54 LANs addressing ARP
Ethernet
switches
VLANS
55 link virtualization MPLS
56 data center networking
57 a day in the life of a web request
Link Layer 5-84
Data center networks
10rsquos to 100rsquos of thousands of hosts often closely coupled in close proximity e-business (eg Amazon)
content-servers (eg YouTube Akamai Apple Microsoft)
search engines data mining (eg Google)
challenges
multiple applications each serving massive numbers of clients
managingbalancing load avoiding processing networking data bottlenecks
Inside a 40-ft Microsoft container
Chicago data center
Link Layer 5-85
Server racks
TOR switches
Tier-1 switches
Tier-2 switches
Load balancer
Load balancer
B
1 2 3 4 5 6 7 8
A C
Border router
Access router
Internet
Data center networks
load balancer application-layer routing receives external client requests
directs workload within data center
returns results to external client (hiding data
center internals from client)
Server racks
TOR switches
Tier-1 switches
Tier-2 switches
1 2 3 4 5 6 7 8
Data center networks
rich interconnection among switches racks
increased throughput between racks (multiple routing
paths possible)
increased reliability via redundancy
Link Layer 5-87
Link layer LANs outline
51 introduction services
52 error detection correction
53 multiple access protocols
54 LANs addressing ARP
Ethernet
switches
VLANS
55 link virtualization MPLS
56 data center networking
57 a day in the life of a web request
Link Layer 5-88
Synthesis a day in the life of a web request
journey down protocol stack complete application transport network link
putting-it-all-together synthesis goal identify review understand protocols (at all
layers) involved in seemingly simple scenario requesting www page
scenario student attaches laptop to campus network requestsreceives wwwgooglecom
Link Layer 5-89
A day in the life scenario
Comcast network
68800013
Googlersquos network
64233160019 64233169105
web server
DNS server
school network
68802024
web page
browser
router
(runs DHCP)
Link Layer 5-90
A day in the lifehellip connecting to the Internet
connecting laptop needs to get its own IP address addr of first-hop router addr of DNS server use DHCP
DHCP
UDP
IP
Eth
Phy
DHCP
DHCP
DHCP
DHCP
DHCP
DHCP
UDP
IP
Eth
Phy
DHCP
DHCP
DHCP
DHCP DHCP
DHCP request encapsulated in UDP encapsulated in IP encapsulated in 8023 Ethernet
Ethernet frame broadcast
(dest FFFFFFFFFFFF) on LAN received at router running DHCP server
Ethernet demuxed to IP demuxed UDP demuxed to DHCP
router
(runs DHCP)
Link Layer 5-91
DHCP server formulates DHCP ACK containing clientrsquos IP address IP address of first-hop router for client name amp IP address of DNS server
DHCP
UDP
IP
Eth
Phy
DHCP
DHCP
DHCP
DHCP
DHCP
UDP
IP
Eth
Phy
DHCP
DHCP
DHCP
DHCP
DHCP
encapsulation at DHCP server frame forwarded (switch learning) through LAN demultiplexing at client
Client now has IP address knows name amp addr of DNS
server IP address of its first-hop router
DHCP client receives DHCP ACK reply
A day in the lifehellip connecting to the Internet
router
(runs DHCP)
Link Layer 5-92
A day in the lifehellip ARP (before DNS before HTTP)
before sending HTTP request need IP address of wwwgooglecom DNS
DNS
UDP
IP
Eth
Phy
DNS
DNS
DNS
DNS query created encapsulated in UDP encapsulated in IP encapsulated in Eth To send frame to router need MAC address of router interface ARP
ARP query broadcast received by
router which replies with ARP reply giving MAC address of router interface
client now knows MAC address of first hop router so can now send frame containing DNS query
ARP query
Eth
Phy
ARP
ARP
ARP reply
router
(runs DHCP)
Link Layer 5-93
DNS
UDP
IP
Eth
Phy
DNS
DNS
DNS
DNS
DNS
IP datagram containing DNS query forwarded via LAN switch from client to 1st hop router
IP datagram forwarded from campus network into comcast network routed (tables created by RIP OSPF IS-IS andor BGP routing protocols) to DNS server
demuxrsquoed to DNS server
DNS server replies to client with IP address of wwwgooglecom
Comcast network
68800013
DNS server
DNS
UDP
IP
Eth
Phy
DNS
DNS
DNS
DNS
A day in the lifehellip using DNS
router
(runs DHCP)
Link Layer 5-94
A day in the lifehellipTCP connection carrying HTTP
HTTP
TCP
IP
Eth
Phy
HTTP
to send HTTP request client first opens TCP socket to web server
TCP SYN segment (step 1 in 3-way handshake) inter-domain routed to web server
TCP connection established 64233169105
web server
SYN
SYN
SYN
SYN
TCP
IP
Eth
Phy
SYN
SYN
SYN
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
web server responds with TCP SYNACK (step 2 in 3-way handshake)
router
(runs DHCP)
Link Layer 5-95
A day in the lifehellip HTTP requestreply
HTTP
TCP
IP
Eth
Phy
HTTP
HTTP request sent into TCP socket
IP datagram containing HTTP request routed to wwwgooglecom
IP datagram containing HTTP reply routed back to client
64233169105
web server
HTTP
TCP
IP
Eth
Phy web server responds with
HTTP reply (containing web page)
HTTP
HTTP
HTTP HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
web page finally () displayed
Link Layer 5-96
Chapter 5 Summary
principles behind data link layer services error detection correction
sharing a broadcast channel multiple access
link layer addressing
instantiation and implementation of various link layer technologies Ethernet
switched LANS VLANs
virtualized networks as a link layer MPLS
synthesis a day in the life of a web request
Link Layer 5-97
Chapter 5 letrsquos take a breath
journey down protocol stack complete (except PHY)
solid understanding of networking principles practice
hellip could stop here hellip but lots of interesting topics wireless
multimedia
security
network management
Link Layer 5-31
CSMACD (collision detection)
CSMACD carrier sensing deferral as in CSMA collisions detected within short time
colliding transmissions aborted reducing channel wastage
collision detection easy in wired LANs measure signal strengths compare
transmitted received signals
difficult in wireless LANs received signal strength overwhelmed by local transmission strength
human analogy the polite conversationalist
Link Layer 5-32
CSMACD (collision detection)
spatial layout of nodes
Link Layer 5-33
Ethernet CSMACD algorithm
1 NIC receives datagram from network layer creates frame
2 If NIC senses channel idle starts frame transmission If NIC senses channel busy waits until channel idle then transmits
3 If NIC transmits entire frame without detecting another transmission NIC is done with frame
4 If NIC detects another transmission while transmitting aborts and sends jam signal
5 After aborting NIC enters binary (exponential) backoff
after mth collision NIC chooses K at random from 012 hellip 2m-1 NIC waits K512 bit times returns to Step 2
longer backoff interval with more collisions
Link Layer 5-34
CSMACD efficiency
Tprop = max prop delay between 2 nodes in LAN
ttrans = time to transmit max-size frame
efficiency goes to 1
as tprop goes to 0
as ttrans goes to infinity
better performance than ALOHA and simple cheap decentralized
transpropttefficiency
51
1
Link Layer 5-35
ldquoTaking turnsrdquo MAC protocols
channel partitioning MAC protocols share channel efficiently and fairly at high load
inefficient at low load delay in channel access 1N bandwidth allocated even if only 1 active node
random access MAC protocols efficient at low load single node can fully utilize
channel
high load collision overhead
ldquotaking turnsrdquo protocols look for best of both worlds
Link Layer 5-36
polling master node ldquoinvitesrdquo
slave nodes to transmit in turn
typically used with ldquodumbrdquo slave devices
concerns
polling overhead
latency
single point of failure (master)
master
slaves
poll
data
data
ldquoTaking turnsrdquo MAC protocols
Link Layer 5-37
token passing control token passed
from one node to next sequentially
token message
concerns
token overhead
latency
single point of failure (token)
T
data
(nothing
to send)
T
ldquoTaking turnsrdquo MAC protocols
cable headend
CMTS
ISP
cable modem termination system
multiple 40Mbps downstream (broadcast) channels
single CMTS transmits into channels
multiple 30 Mbps upstream channels
multiple access all users contend for certain upstream channel time slots (others assigned)
Cable access network
cable modem
splitter
hellip
hellip
Internet framesTV channels control transmitted downstream at different frequencies
upstream Internet frames TV control transmitted upstream at different frequencies in time slots
Link Layer 5-39
DOCSIS data over cable service interface spec FDM over upstream downstream frequency channels
TDM upstream some slots assigned some have contention
downstream MAP frame assigns upstream slots
request for upstream slots (and data) transmitted random access (binary backoff) in selected slots
MAP frame for
Interval [t1 t2]
Residences with cable modems
Downstream channel i
Upstream channel j
t1 t2
Assigned minislots containing cable modem
upstream data frames
Minislots containing
minislots request frames
cable headend
CMTS
Cable access network
Link Layer 5-40
Summary of MAC protocols
channel partitioning by time frequency or code Time Division Frequency Division
random access (dynamic)
ALOHA S-ALOHA CSMA CSMACD
carrier sensing easy in some technologies (wire) hard in others (wireless)
CSMACD used in Ethernet
CSMACA used in 80211
taking turns
polling from central site token passing
bluetooth FDDI token ring
Link Layer 5-41
Link layer LANs outline
51 introduction services
52 error detection correction
53 multiple access protocols
54 LANs addressing ARP
Ethernet
switches
VLANS
55 link virtualization MPLS
56 data center networking
57 a day in the life of a web request
Link Layer 5-42
MAC addresses and ARP
32-bit IP address network-layer address for interface
used for layer 3 (network layer) forwarding
MAC (or LAN or physical or Ethernet) address function used lsquolocallyrdquo to get frame from one interface to
another physically-connected interface (same network in IP-addressing sense)
48 bit MAC address (for most LANs) burned in NIC ROM also sometimes software settable
eg 1A-2F-BB-76-09-AD
hexadecimal (base 16) notation
(each ldquonumberrdquo represents 4 bits)
Link Layer 5-43
LAN addresses and ARP
each adapter on LAN has unique LAN address
adapter
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN
(wired or
wireless)
Link Layer 5-44
LAN addresses (more)
MAC address allocation administered by IEEE
manufacturer buys portion of MAC address space (to assure uniqueness)
analogy MAC address like Social Security Number
IP address like postal address
MAC flat address portability can move LAN card from one LAN to another
IP hierarchical address not portable address depends on IP subnet to which node is
attached
Link Layer 5-45
ARP address resolution protocol
ARP table each IP node (host router) on LAN has table
IPMAC address mappings for some LAN nodes
lt IP address MAC address TTLgt
TTL (Time To Live) time after which address mapping will be forgotten (typically 20 min)
Question how to determine
interfacersquos MAC address
knowing its IP address
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN
137196723
137196778
137196714
137196788
Link Layer 5-46
ARP protocol same LAN
A wants to send datagram to B Brsquos MAC address not in
Arsquos ARP table
A broadcasts ARP query packet containing Bs IP address dest MAC address = FF-FF-
FF-FF-FF-FF
all nodes on LAN receive ARP query
B receives ARP packet replies to A with its (Bs) MAC address frame sent to Arsquos MAC
address (unicast)
A caches (saves) IP-to-MAC address pair in its ARP table until information becomes old (times out) soft state information that
times out (goes away) unless refreshed
ARP is ldquoplug-and-playrdquo nodes create their ARP
tables without intervention from net administrator
Link Layer 5-47
walkthrough send datagram from A to B via R
focus on addressing ndash at IP (datagram) and MAC layer (frame)
assume A knows Brsquos IP address
assume A knows IP address of first hop router R (how)
assume A knows Rrsquos MAC address (how)
Addressing routing to another LAN
R
1A-23-F9-CD-06-9B 222222222220
111111111110 E6-E9-00-17-BB-4B CC-49-DE-D0-AB-7D
111111111112
111111111111
74-29-9C-E8-FF-55
A
222222222222
49-BD-D2-C7-56-2A
222222222221 88-B2-2F-54-1A-0F
B
R
1A-23-F9-CD-06-9B 222222222220
111111111110 E6-E9-00-17-BB-4B CC-49-DE-D0-AB-7D
111111111112
111111111111
74-29-9C-E8-FF-55
A
222222222222
49-BD-D2-C7-56-2A
222222222221 88-B2-2F-54-1A-0F
B
Link Layer 5-48
Addressing routing to another LAN
IP
Eth
Phy
IP src 111111111111
IP dest 222222222222
A creates IP datagram with IP source A destination B
A creates link-layer frame with Rs MAC address as dest frame contains A-to-B IP datagram
MAC src 74-29-9C-E8-FF-55
MAC dest E6-E9-00-17-BB-4B
R
1A-23-F9-CD-06-9B 222222222220
111111111110 E6-E9-00-17-BB-4B CC-49-DE-D0-AB-7D
111111111112
111111111111
74-29-9C-E8-FF-55
A
222222222222
49-BD-D2-C7-56-2A
222222222221 88-B2-2F-54-1A-0F
B
Link Layer 5-49
Addressing routing to another LAN
IP
Eth
Phy
frame sent from A to R
IP
Eth
Phy
frame received at R datagram removed passed up to IP
MAC src 74-29-9C-E8-FF-55
MAC dest E6-E9-00-17-BB-4B
IP src 111111111111
IP dest 222222222222
IP src 111111111111
IP dest 222222222222
R
1A-23-F9-CD-06-9B 222222222220
111111111110 E6-E9-00-17-BB-4B CC-49-DE-D0-AB-7D
111111111112
111111111111
74-29-9C-E8-FF-55
A
222222222222
49-BD-D2-C7-56-2A
222222222221 88-B2-2F-54-1A-0F
B
Link Layer 5-50
Addressing routing to another LAN
IP src 111111111111
IP dest 222222222222
R forwards datagram with IP source A destination B
R creates link-layer frame with Bs MAC address as dest frame contains A-to-B IP datagram
MAC src 1A-23-F9-CD-06-9B
MAC dest 49-BD-D2-C7-56-2A
IP
Eth
Phy
IP
Eth
Phy
R
1A-23-F9-CD-06-9B 222222222220
111111111110 E6-E9-00-17-BB-4B CC-49-DE-D0-AB-7D
111111111112
111111111111
74-29-9C-E8-FF-55
A
222222222222
49-BD-D2-C7-56-2A
222222222221 88-B2-2F-54-1A-0F
B
Link Layer 5-51
Addressing routing to another LAN R forwards datagram with IP source A destination B
R creates link-layer frame with Bs MAC address as dest frame contains A-to-B IP datagram
IP src 111111111111
IP dest 222222222222
MAC src 1A-23-F9-CD-06-9B
MAC dest 49-BD-D2-C7-56-2A
IP
Eth
Phy
IP
Eth
Phy
R
1A-23-F9-CD-06-9B 222222222220
111111111110 E6-E9-00-17-BB-4B CC-49-DE-D0-AB-7D
111111111112
111111111111
74-29-9C-E8-FF-55
A
222222222222
49-BD-D2-C7-56-2A
222222222221 88-B2-2F-54-1A-0F
B
Link Layer 5-52
Addressing routing to another LAN R forwards datagram with IP source A destination B
R creates link-layer frame with Bs MAC address as dest frame contains A-to-B IP datagram
IP src 111111111111
IP dest 222222222222
MAC src 1A-23-F9-CD-06-9B
MAC dest 49-BD-D2-C7-56-2A
IP
Eth
Phy
Link Layer 5-53
Link layer LANs outline
51 introduction services
52 error detection correction
53 multiple access protocols
54 LANs addressing ARP
Ethernet
switches
VLANS
55 link virtualization MPLS
56 data center networking
57 a day in the life of a web request
Link Layer 5-54
Ethernet
ldquodominantrdquo wired LAN technology
cheap $20 for NIC
first widely used LAN technology
simpler cheaper than token LANs and ATM
kept up with speed race 10 Mbps ndash 10 Gbps
Metcalfersquos Ethernet sketch
Link Layer 5-55
Ethernet physical topology
bus popular through mid 90s all nodes in same collision domain (can collide with each
other)
star prevails today active switch in center
each ldquospokerdquo runs a (separate) Ethernet protocol (nodes
do not collide with each other)
switch
bus coaxial cable star
Link Layer 5-56
Ethernet frame structure
sending adapter encapsulates IP datagram (or other network layer protocol packet) in Ethernet frame
preamble
7 bytes with pattern 10101010 followed by one byte with pattern 10101011
used to synchronize receiver sender clock rates
dest address
source address
data (payload) CRC preamble
type
Link Layer 5-57
Ethernet frame structure (more)
addresses 6 byte source destination MAC addresses if adapter receives frame with matching destination
address or with broadcast address (eg ARP packet) it passes data in frame to network layer protocol
otherwise adapter discards frame
type indicates higher layer protocol (mostly IP but others possible eg Novell IPX AppleTalk)
CRC cyclic redundancy check at receiver error detected frame is dropped
dest address
source address
data (payload) CRC preamble
type
Link Layer 5-58
Ethernet unreliable connectionless
connectionless no handshaking between sending and receiving NICs
unreliable receiving NIC doesnt send acks or nacks to sending NIC
data in dropped frames recovered only if initial sender uses higher layer rdt (eg TCP) otherwise dropped data lost
Ethernetrsquos MAC protocol unslotted CSMACD wth binary backoff
Link Layer 5-59
8023 Ethernet standards link amp physical layers
many different Ethernet standards
common MAC protocol and frame format
different speeds 2 Mbps 10 Mbps 100 Mbps 1Gbps 10G bps
different physical layer media fiber cable
application
transport
network
link
physical
MAC protocol
and frame format
100BASE-TX
100BASE-T4
100BASE-FX 100BASE-T2
100BASE-SX 100BASE-BX
fiber physical layer copper (twister
pair) physical layer
Link Layer 5-60
Link layer LANs outline
51 introduction services
52 error detection correction
53 multiple access protocols
54 LANs addressing ARP
Ethernet
switches
VLANS
55 link virtualization MPLS
56 data center networking
57 a day in the life of a web request
Link Layer 5-61
Ethernet switch link-layer device takes an active role
store forward Ethernet frames
examine incoming framersquos MAC address selectively forward frame to one-or-more outgoing links when frame is to be forwarded on segment uses CSMACD to access segment
transparent
hosts are unaware of presence of switches
plug-and-play self-learning
switches do not need to be configured
Link Layer 5-62
Switch multiple simultaneous transmissions
hosts have dedicated direct connection to switch
switches buffer packets
Ethernet protocol used on each incoming link but no collisions full duplex
each link is its own collision domain
switching A-to-Arsquo and B-to-Brsquo can transmit simultaneously without collisions switch with six interfaces
(123456)
A
Arsquo
B
Brsquo C
Crsquo
1 2
3 4 5
6
Link Layer 5-63
Switch forwarding table
Q how does switch know Arsquo reachable via interface 4 Brsquo reachable via interface 5
switch with six interfaces
(123456)
A
Arsquo
B
Brsquo C
Crsquo
1 2
3 4 5
6 A each switch has a switch table each entry
(MAC address of host interface to
reach host time stamp)
looks like a routing table
Q how are entries created maintained in switch table
something like a routing protocol
A
Arsquo
B
Brsquo C
Crsquo
1 2
3 4 5
6
Link Layer 5-64
Switch self-learning
switch learns which hosts can be reached through which interfaces
when frame received switch ldquolearnsrdquo location of sender incoming LAN segment
records senderlocation pair in switch table
A Arsquo
Source A
Dest Arsquo
MAC addr interface TTL
Switch table
(initially empty) A 1 60
Link Layer 5-65
Switch frame filteringforwarding
when frame received at switch 1 record incoming link MAC address of sending host
2 index switch table using MAC destination address
3 if entry found for destination then
if destination on segment from which frame arrived then drop frame
else forward frame on interface indicated by entry
else flood forward on all interfaces except arriving
interface
A
Arsquo
B
Brsquo C
Crsquo
1 2
3 4 5
6
Link Layer 5-66
Self-learning forwarding example
A Arsquo
Source A
Dest Arsquo
MAC addr interface TTL
switch table
(initially empty) A 1 60
A Arsquo A Arsquo A Arsquo A Arsquo A Arsquo
frame destination Arsquo locaton unknown flood
Arsquo A
destination A location
known
Arsquo 4 60
selectively send
on just one link
Link Layer 5-67
Interconnecting switches
switches can be connected together
Q sending from A to G - how does S1 know to forward frame destined to F via S4 and S3
A self learning (works exactly the same as in single-switch case)
A
B
S1
C D
E
F
S2
S4
S3
H
I
G
Link Layer 5-68
Self-learning multi-switch example
Suppose C sends frame to I I responds to C
Q show switch tables and packet forwarding in S1 S2 S3 S4
A
B
S1
C D
E
F
S2
S4
S3
H
I
G
Link Layer 5-69
Institutional network
to external
network
router
IP subnet
mail server
web server
Link Layer 5-70
Switches vs routers
both are store-and-forward
routers network-layer devices (examine network-layer headers)
switches link-layer devices (examine link-layer headers)
both have forwarding tables
routers compute tables using routing algorithms IP addresses
switches learn forwarding table using flooding learning MAC addresses
application
transport
network
link
physical
network
link
physical
link
physical
switch
datagram
application
transport
network
link
physical
frame
frame
frame
datagram
Link Layer 5-71
VLANs motivation
consider CS user moves office to
EE but wants connect to CS switch
single broadcast domain
all layer-2 broadcast traffic (ARP DHCP unknown location of destination MAC address) must cross entire LAN
securityprivacy efficiency issues
Computer
Science Electrical
Engineering
Computer
Engineering
Link Layer 5-72
VLANs port-based VLAN switch ports
grouped (by switch management software) so that single physical switch helliphellip
switch(es) supporting
VLAN capabilities can
be configured to
define multiple virtual
LANS over single
physical LAN
infrastructure
Virtual Local
Area Network 1
8
9
16 10 2
7
hellip
Electrical Engineering
(VLAN ports 1-8)
Computer Science
(VLAN ports 9-15)
15
hellip
Electrical Engineering
(VLAN ports 1-8)
hellip
1
8 2
7 9
16 10
15
hellip
Computer Science
(VLAN ports 9-16)
hellip operates as multiple virtual switches
Link Layer 5-73
Port-based VLAN
1
8
9
16 10 2
7
hellip
Electrical Engineering
(VLAN ports 1-8)
Computer Science
(VLAN ports 9-15)
15
hellip
traffic isolation frames tofrom ports 1-8 can only reach ports 1-8 can also define VLAN based on
MAC addresses of endpoints rather than switch port
dynamic membership ports can be dynamically assigned among VLANs
router
forwarding between VLANS done via routing (just as with separate switches) in practice vendors sell combined
switches plus routers
Link Layer 5-74
VLANS spanning multiple switches
trunk port carries frames between VLANS defined over multiple physical switches frames forwarded within VLAN between switches canrsquot be vanilla
8021 frames (must carry VLAN ID info)
8021q protocol addsremoved additional header fields for frames forwarded between trunk ports
1
8
9
10 2
7
hellip
Electrical Engineering
(VLAN ports 1-8)
Computer Science
(VLAN ports 9-15)
15
hellip
2
7 3
Ports 235 belong to EE VLAN
Ports 4678 belong to CS VLAN
5
4 6 8 16
1
Link Layer 5-75
type
2-byte Tag Protocol Identifier
(value 81-00)
Tag Control Information (12 bit VLAN ID field
3 bit priority field like IP TOS)
Recomputed CRC
8021Q VLAN frame format
8021 frame
8021Q frame
dest address
source address
data (payload) CRC preamble
dest address
source address
preamble data (payload) CRC
type
Link Layer 5-76
Link layer LANs outline
51 introduction services
52 error detection correction
53 multiple access protocols
54 LANs addressing ARP
Ethernet
switches
VLANS
55 link virtualization MPLS
56 data center networking
57 a day in the life of a web request
Link Layer 5-77
Multiprotocol label switching (MPLS)
initial goal high-speed IP forwarding using fixed length label (instead of IP address) fast lookup using fixed length identifier (rather than
shortest prefix matching)
borrowing ideas from Virtual Circuit (VC) approach
but IP datagram still keeps IP address
PPP or Ethernet
header IP header remainder of link-layer frame MPLS header
label Exp S TTL
20 3 1 5
Link Layer 5-78
MPLS capable routers
aka label-switched router
forward packets to outgoing interface based only on label value (donrsquot inspect IP address) MPLS forwarding table distinct from IP forwarding tables
flexibility MPLS forwarding decisions can differ from those of IP use destination and source addresses to route flows to
same destination differently (traffic engineering)
re-route flows quickly if link fails pre-computed backup paths (useful for VoIP)
Link Layer 5-79
R2
D
R3 R5
A
R6
MPLS versus IP paths
IP router IP routing path to destination determined
by destination address alone
R4
Link Layer 5-80
R2
D
R3 R4
R5
A
R6
MPLS versus IP paths
IP-only router
IP routing path to destination determined by destination address alone
MPLS and IP router
MPLS routing path to destination can be based on source and dest address fast reroute precompute backup routes in
case of link failure
entry router (R4) can use different MPLS
routes to A based eg on source address
Link Layer 5-81
MPLS signaling
modify OSPF IS-IS link-state flooding protocols to carry info used by MPLS routing eg link bandwidth amount of ldquoreservedrdquo link bandwidth
D
R4
R5
A
R6
entry MPLS router uses RSVP-TE signaling protocol to set up MPLS forwarding at downstream routers
modified link state flooding
RSVP-TE
Link Layer 5-82
R1 R2
D
R3 R4 R5
0
1
0 0
A
R6
in out out
label label dest interface
6 - A 0
in out out
label label dest interface
10 6 A 1
12 9 D 0
in out out
label label dest interface
10 A 0
12 D 0
1
in out out
label label dest interface
8 6 A 0
0
8 A 1
MPLS forwarding tables
Link Layer 5-83
Link layer LANs outline
51 introduction services
52 error detection correction
53 multiple access protocols
54 LANs addressing ARP
Ethernet
switches
VLANS
55 link virtualization MPLS
56 data center networking
57 a day in the life of a web request
Link Layer 5-84
Data center networks
10rsquos to 100rsquos of thousands of hosts often closely coupled in close proximity e-business (eg Amazon)
content-servers (eg YouTube Akamai Apple Microsoft)
search engines data mining (eg Google)
challenges
multiple applications each serving massive numbers of clients
managingbalancing load avoiding processing networking data bottlenecks
Inside a 40-ft Microsoft container
Chicago data center
Link Layer 5-85
Server racks
TOR switches
Tier-1 switches
Tier-2 switches
Load balancer
Load balancer
B
1 2 3 4 5 6 7 8
A C
Border router
Access router
Internet
Data center networks
load balancer application-layer routing receives external client requests
directs workload within data center
returns results to external client (hiding data
center internals from client)
Server racks
TOR switches
Tier-1 switches
Tier-2 switches
1 2 3 4 5 6 7 8
Data center networks
rich interconnection among switches racks
increased throughput between racks (multiple routing
paths possible)
increased reliability via redundancy
Link Layer 5-87
Link layer LANs outline
51 introduction services
52 error detection correction
53 multiple access protocols
54 LANs addressing ARP
Ethernet
switches
VLANS
55 link virtualization MPLS
56 data center networking
57 a day in the life of a web request
Link Layer 5-88
Synthesis a day in the life of a web request
journey down protocol stack complete application transport network link
putting-it-all-together synthesis goal identify review understand protocols (at all
layers) involved in seemingly simple scenario requesting www page
scenario student attaches laptop to campus network requestsreceives wwwgooglecom
Link Layer 5-89
A day in the life scenario
Comcast network
68800013
Googlersquos network
64233160019 64233169105
web server
DNS server
school network
68802024
web page
browser
router
(runs DHCP)
Link Layer 5-90
A day in the lifehellip connecting to the Internet
connecting laptop needs to get its own IP address addr of first-hop router addr of DNS server use DHCP
DHCP
UDP
IP
Eth
Phy
DHCP
DHCP
DHCP
DHCP
DHCP
DHCP
UDP
IP
Eth
Phy
DHCP
DHCP
DHCP
DHCP DHCP
DHCP request encapsulated in UDP encapsulated in IP encapsulated in 8023 Ethernet
Ethernet frame broadcast
(dest FFFFFFFFFFFF) on LAN received at router running DHCP server
Ethernet demuxed to IP demuxed UDP demuxed to DHCP
router
(runs DHCP)
Link Layer 5-91
DHCP server formulates DHCP ACK containing clientrsquos IP address IP address of first-hop router for client name amp IP address of DNS server
DHCP
UDP
IP
Eth
Phy
DHCP
DHCP
DHCP
DHCP
DHCP
UDP
IP
Eth
Phy
DHCP
DHCP
DHCP
DHCP
DHCP
encapsulation at DHCP server frame forwarded (switch learning) through LAN demultiplexing at client
Client now has IP address knows name amp addr of DNS
server IP address of its first-hop router
DHCP client receives DHCP ACK reply
A day in the lifehellip connecting to the Internet
router
(runs DHCP)
Link Layer 5-92
A day in the lifehellip ARP (before DNS before HTTP)
before sending HTTP request need IP address of wwwgooglecom DNS
DNS
UDP
IP
Eth
Phy
DNS
DNS
DNS
DNS query created encapsulated in UDP encapsulated in IP encapsulated in Eth To send frame to router need MAC address of router interface ARP
ARP query broadcast received by
router which replies with ARP reply giving MAC address of router interface
client now knows MAC address of first hop router so can now send frame containing DNS query
ARP query
Eth
Phy
ARP
ARP
ARP reply
router
(runs DHCP)
Link Layer 5-93
DNS
UDP
IP
Eth
Phy
DNS
DNS
DNS
DNS
DNS
IP datagram containing DNS query forwarded via LAN switch from client to 1st hop router
IP datagram forwarded from campus network into comcast network routed (tables created by RIP OSPF IS-IS andor BGP routing protocols) to DNS server
demuxrsquoed to DNS server
DNS server replies to client with IP address of wwwgooglecom
Comcast network
68800013
DNS server
DNS
UDP
IP
Eth
Phy
DNS
DNS
DNS
DNS
A day in the lifehellip using DNS
router
(runs DHCP)
Link Layer 5-94
A day in the lifehellipTCP connection carrying HTTP
HTTP
TCP
IP
Eth
Phy
HTTP
to send HTTP request client first opens TCP socket to web server
TCP SYN segment (step 1 in 3-way handshake) inter-domain routed to web server
TCP connection established 64233169105
web server
SYN
SYN
SYN
SYN
TCP
IP
Eth
Phy
SYN
SYN
SYN
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
web server responds with TCP SYNACK (step 2 in 3-way handshake)
router
(runs DHCP)
Link Layer 5-95
A day in the lifehellip HTTP requestreply
HTTP
TCP
IP
Eth
Phy
HTTP
HTTP request sent into TCP socket
IP datagram containing HTTP request routed to wwwgooglecom
IP datagram containing HTTP reply routed back to client
64233169105
web server
HTTP
TCP
IP
Eth
Phy web server responds with
HTTP reply (containing web page)
HTTP
HTTP
HTTP HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
web page finally () displayed
Link Layer 5-96
Chapter 5 Summary
principles behind data link layer services error detection correction
sharing a broadcast channel multiple access
link layer addressing
instantiation and implementation of various link layer technologies Ethernet
switched LANS VLANs
virtualized networks as a link layer MPLS
synthesis a day in the life of a web request
Link Layer 5-97
Chapter 5 letrsquos take a breath
journey down protocol stack complete (except PHY)
solid understanding of networking principles practice
hellip could stop here hellip but lots of interesting topics wireless
multimedia
security
network management
Link Layer 5-32
CSMACD (collision detection)
spatial layout of nodes
Link Layer 5-33
Ethernet CSMACD algorithm
1 NIC receives datagram from network layer creates frame
2 If NIC senses channel idle starts frame transmission If NIC senses channel busy waits until channel idle then transmits
3 If NIC transmits entire frame without detecting another transmission NIC is done with frame
4 If NIC detects another transmission while transmitting aborts and sends jam signal
5 After aborting NIC enters binary (exponential) backoff
after mth collision NIC chooses K at random from 012 hellip 2m-1 NIC waits K512 bit times returns to Step 2
longer backoff interval with more collisions
Link Layer 5-34
CSMACD efficiency
Tprop = max prop delay between 2 nodes in LAN
ttrans = time to transmit max-size frame
efficiency goes to 1
as tprop goes to 0
as ttrans goes to infinity
better performance than ALOHA and simple cheap decentralized
transpropttefficiency
51
1
Link Layer 5-35
ldquoTaking turnsrdquo MAC protocols
channel partitioning MAC protocols share channel efficiently and fairly at high load
inefficient at low load delay in channel access 1N bandwidth allocated even if only 1 active node
random access MAC protocols efficient at low load single node can fully utilize
channel
high load collision overhead
ldquotaking turnsrdquo protocols look for best of both worlds
Link Layer 5-36
polling master node ldquoinvitesrdquo
slave nodes to transmit in turn
typically used with ldquodumbrdquo slave devices
concerns
polling overhead
latency
single point of failure (master)
master
slaves
poll
data
data
ldquoTaking turnsrdquo MAC protocols
Link Layer 5-37
token passing control token passed
from one node to next sequentially
token message
concerns
token overhead
latency
single point of failure (token)
T
data
(nothing
to send)
T
ldquoTaking turnsrdquo MAC protocols
cable headend
CMTS
ISP
cable modem termination system
multiple 40Mbps downstream (broadcast) channels
single CMTS transmits into channels
multiple 30 Mbps upstream channels
multiple access all users contend for certain upstream channel time slots (others assigned)
Cable access network
cable modem
splitter
hellip
hellip
Internet framesTV channels control transmitted downstream at different frequencies
upstream Internet frames TV control transmitted upstream at different frequencies in time slots
Link Layer 5-39
DOCSIS data over cable service interface spec FDM over upstream downstream frequency channels
TDM upstream some slots assigned some have contention
downstream MAP frame assigns upstream slots
request for upstream slots (and data) transmitted random access (binary backoff) in selected slots
MAP frame for
Interval [t1 t2]
Residences with cable modems
Downstream channel i
Upstream channel j
t1 t2
Assigned minislots containing cable modem
upstream data frames
Minislots containing
minislots request frames
cable headend
CMTS
Cable access network
Link Layer 5-40
Summary of MAC protocols
channel partitioning by time frequency or code Time Division Frequency Division
random access (dynamic)
ALOHA S-ALOHA CSMA CSMACD
carrier sensing easy in some technologies (wire) hard in others (wireless)
CSMACD used in Ethernet
CSMACA used in 80211
taking turns
polling from central site token passing
bluetooth FDDI token ring
Link Layer 5-41
Link layer LANs outline
51 introduction services
52 error detection correction
53 multiple access protocols
54 LANs addressing ARP
Ethernet
switches
VLANS
55 link virtualization MPLS
56 data center networking
57 a day in the life of a web request
Link Layer 5-42
MAC addresses and ARP
32-bit IP address network-layer address for interface
used for layer 3 (network layer) forwarding
MAC (or LAN or physical or Ethernet) address function used lsquolocallyrdquo to get frame from one interface to
another physically-connected interface (same network in IP-addressing sense)
48 bit MAC address (for most LANs) burned in NIC ROM also sometimes software settable
eg 1A-2F-BB-76-09-AD
hexadecimal (base 16) notation
(each ldquonumberrdquo represents 4 bits)
Link Layer 5-43
LAN addresses and ARP
each adapter on LAN has unique LAN address
adapter
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN
(wired or
wireless)
Link Layer 5-44
LAN addresses (more)
MAC address allocation administered by IEEE
manufacturer buys portion of MAC address space (to assure uniqueness)
analogy MAC address like Social Security Number
IP address like postal address
MAC flat address portability can move LAN card from one LAN to another
IP hierarchical address not portable address depends on IP subnet to which node is
attached
Link Layer 5-45
ARP address resolution protocol
ARP table each IP node (host router) on LAN has table
IPMAC address mappings for some LAN nodes
lt IP address MAC address TTLgt
TTL (Time To Live) time after which address mapping will be forgotten (typically 20 min)
Question how to determine
interfacersquos MAC address
knowing its IP address
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN
137196723
137196778
137196714
137196788
Link Layer 5-46
ARP protocol same LAN
A wants to send datagram to B Brsquos MAC address not in
Arsquos ARP table
A broadcasts ARP query packet containing Bs IP address dest MAC address = FF-FF-
FF-FF-FF-FF
all nodes on LAN receive ARP query
B receives ARP packet replies to A with its (Bs) MAC address frame sent to Arsquos MAC
address (unicast)
A caches (saves) IP-to-MAC address pair in its ARP table until information becomes old (times out) soft state information that
times out (goes away) unless refreshed
ARP is ldquoplug-and-playrdquo nodes create their ARP
tables without intervention from net administrator
Link Layer 5-47
walkthrough send datagram from A to B via R
focus on addressing ndash at IP (datagram) and MAC layer (frame)
assume A knows Brsquos IP address
assume A knows IP address of first hop router R (how)
assume A knows Rrsquos MAC address (how)
Addressing routing to another LAN
R
1A-23-F9-CD-06-9B 222222222220
111111111110 E6-E9-00-17-BB-4B CC-49-DE-D0-AB-7D
111111111112
111111111111
74-29-9C-E8-FF-55
A
222222222222
49-BD-D2-C7-56-2A
222222222221 88-B2-2F-54-1A-0F
B
R
1A-23-F9-CD-06-9B 222222222220
111111111110 E6-E9-00-17-BB-4B CC-49-DE-D0-AB-7D
111111111112
111111111111
74-29-9C-E8-FF-55
A
222222222222
49-BD-D2-C7-56-2A
222222222221 88-B2-2F-54-1A-0F
B
Link Layer 5-48
Addressing routing to another LAN
IP
Eth
Phy
IP src 111111111111
IP dest 222222222222
A creates IP datagram with IP source A destination B
A creates link-layer frame with Rs MAC address as dest frame contains A-to-B IP datagram
MAC src 74-29-9C-E8-FF-55
MAC dest E6-E9-00-17-BB-4B
R
1A-23-F9-CD-06-9B 222222222220
111111111110 E6-E9-00-17-BB-4B CC-49-DE-D0-AB-7D
111111111112
111111111111
74-29-9C-E8-FF-55
A
222222222222
49-BD-D2-C7-56-2A
222222222221 88-B2-2F-54-1A-0F
B
Link Layer 5-49
Addressing routing to another LAN
IP
Eth
Phy
frame sent from A to R
IP
Eth
Phy
frame received at R datagram removed passed up to IP
MAC src 74-29-9C-E8-FF-55
MAC dest E6-E9-00-17-BB-4B
IP src 111111111111
IP dest 222222222222
IP src 111111111111
IP dest 222222222222
R
1A-23-F9-CD-06-9B 222222222220
111111111110 E6-E9-00-17-BB-4B CC-49-DE-D0-AB-7D
111111111112
111111111111
74-29-9C-E8-FF-55
A
222222222222
49-BD-D2-C7-56-2A
222222222221 88-B2-2F-54-1A-0F
B
Link Layer 5-50
Addressing routing to another LAN
IP src 111111111111
IP dest 222222222222
R forwards datagram with IP source A destination B
R creates link-layer frame with Bs MAC address as dest frame contains A-to-B IP datagram
MAC src 1A-23-F9-CD-06-9B
MAC dest 49-BD-D2-C7-56-2A
IP
Eth
Phy
IP
Eth
Phy
R
1A-23-F9-CD-06-9B 222222222220
111111111110 E6-E9-00-17-BB-4B CC-49-DE-D0-AB-7D
111111111112
111111111111
74-29-9C-E8-FF-55
A
222222222222
49-BD-D2-C7-56-2A
222222222221 88-B2-2F-54-1A-0F
B
Link Layer 5-51
Addressing routing to another LAN R forwards datagram with IP source A destination B
R creates link-layer frame with Bs MAC address as dest frame contains A-to-B IP datagram
IP src 111111111111
IP dest 222222222222
MAC src 1A-23-F9-CD-06-9B
MAC dest 49-BD-D2-C7-56-2A
IP
Eth
Phy
IP
Eth
Phy
R
1A-23-F9-CD-06-9B 222222222220
111111111110 E6-E9-00-17-BB-4B CC-49-DE-D0-AB-7D
111111111112
111111111111
74-29-9C-E8-FF-55
A
222222222222
49-BD-D2-C7-56-2A
222222222221 88-B2-2F-54-1A-0F
B
Link Layer 5-52
Addressing routing to another LAN R forwards datagram with IP source A destination B
R creates link-layer frame with Bs MAC address as dest frame contains A-to-B IP datagram
IP src 111111111111
IP dest 222222222222
MAC src 1A-23-F9-CD-06-9B
MAC dest 49-BD-D2-C7-56-2A
IP
Eth
Phy
Link Layer 5-53
Link layer LANs outline
51 introduction services
52 error detection correction
53 multiple access protocols
54 LANs addressing ARP
Ethernet
switches
VLANS
55 link virtualization MPLS
56 data center networking
57 a day in the life of a web request
Link Layer 5-54
Ethernet
ldquodominantrdquo wired LAN technology
cheap $20 for NIC
first widely used LAN technology
simpler cheaper than token LANs and ATM
kept up with speed race 10 Mbps ndash 10 Gbps
Metcalfersquos Ethernet sketch
Link Layer 5-55
Ethernet physical topology
bus popular through mid 90s all nodes in same collision domain (can collide with each
other)
star prevails today active switch in center
each ldquospokerdquo runs a (separate) Ethernet protocol (nodes
do not collide with each other)
switch
bus coaxial cable star
Link Layer 5-56
Ethernet frame structure
sending adapter encapsulates IP datagram (or other network layer protocol packet) in Ethernet frame
preamble
7 bytes with pattern 10101010 followed by one byte with pattern 10101011
used to synchronize receiver sender clock rates
dest address
source address
data (payload) CRC preamble
type
Link Layer 5-57
Ethernet frame structure (more)
addresses 6 byte source destination MAC addresses if adapter receives frame with matching destination
address or with broadcast address (eg ARP packet) it passes data in frame to network layer protocol
otherwise adapter discards frame
type indicates higher layer protocol (mostly IP but others possible eg Novell IPX AppleTalk)
CRC cyclic redundancy check at receiver error detected frame is dropped
dest address
source address
data (payload) CRC preamble
type
Link Layer 5-58
Ethernet unreliable connectionless
connectionless no handshaking between sending and receiving NICs
unreliable receiving NIC doesnt send acks or nacks to sending NIC
data in dropped frames recovered only if initial sender uses higher layer rdt (eg TCP) otherwise dropped data lost
Ethernetrsquos MAC protocol unslotted CSMACD wth binary backoff
Link Layer 5-59
8023 Ethernet standards link amp physical layers
many different Ethernet standards
common MAC protocol and frame format
different speeds 2 Mbps 10 Mbps 100 Mbps 1Gbps 10G bps
different physical layer media fiber cable
application
transport
network
link
physical
MAC protocol
and frame format
100BASE-TX
100BASE-T4
100BASE-FX 100BASE-T2
100BASE-SX 100BASE-BX
fiber physical layer copper (twister
pair) physical layer
Link Layer 5-60
Link layer LANs outline
51 introduction services
52 error detection correction
53 multiple access protocols
54 LANs addressing ARP
Ethernet
switches
VLANS
55 link virtualization MPLS
56 data center networking
57 a day in the life of a web request
Link Layer 5-61
Ethernet switch link-layer device takes an active role
store forward Ethernet frames
examine incoming framersquos MAC address selectively forward frame to one-or-more outgoing links when frame is to be forwarded on segment uses CSMACD to access segment
transparent
hosts are unaware of presence of switches
plug-and-play self-learning
switches do not need to be configured
Link Layer 5-62
Switch multiple simultaneous transmissions
hosts have dedicated direct connection to switch
switches buffer packets
Ethernet protocol used on each incoming link but no collisions full duplex
each link is its own collision domain
switching A-to-Arsquo and B-to-Brsquo can transmit simultaneously without collisions switch with six interfaces
(123456)
A
Arsquo
B
Brsquo C
Crsquo
1 2
3 4 5
6
Link Layer 5-63
Switch forwarding table
Q how does switch know Arsquo reachable via interface 4 Brsquo reachable via interface 5
switch with six interfaces
(123456)
A
Arsquo
B
Brsquo C
Crsquo
1 2
3 4 5
6 A each switch has a switch table each entry
(MAC address of host interface to
reach host time stamp)
looks like a routing table
Q how are entries created maintained in switch table
something like a routing protocol
A
Arsquo
B
Brsquo C
Crsquo
1 2
3 4 5
6
Link Layer 5-64
Switch self-learning
switch learns which hosts can be reached through which interfaces
when frame received switch ldquolearnsrdquo location of sender incoming LAN segment
records senderlocation pair in switch table
A Arsquo
Source A
Dest Arsquo
MAC addr interface TTL
Switch table
(initially empty) A 1 60
Link Layer 5-65
Switch frame filteringforwarding
when frame received at switch 1 record incoming link MAC address of sending host
2 index switch table using MAC destination address
3 if entry found for destination then
if destination on segment from which frame arrived then drop frame
else forward frame on interface indicated by entry
else flood forward on all interfaces except arriving
interface
A
Arsquo
B
Brsquo C
Crsquo
1 2
3 4 5
6
Link Layer 5-66
Self-learning forwarding example
A Arsquo
Source A
Dest Arsquo
MAC addr interface TTL
switch table
(initially empty) A 1 60
A Arsquo A Arsquo A Arsquo A Arsquo A Arsquo
frame destination Arsquo locaton unknown flood
Arsquo A
destination A location
known
Arsquo 4 60
selectively send
on just one link
Link Layer 5-67
Interconnecting switches
switches can be connected together
Q sending from A to G - how does S1 know to forward frame destined to F via S4 and S3
A self learning (works exactly the same as in single-switch case)
A
B
S1
C D
E
F
S2
S4
S3
H
I
G
Link Layer 5-68
Self-learning multi-switch example
Suppose C sends frame to I I responds to C
Q show switch tables and packet forwarding in S1 S2 S3 S4
A
B
S1
C D
E
F
S2
S4
S3
H
I
G
Link Layer 5-69
Institutional network
to external
network
router
IP subnet
mail server
web server
Link Layer 5-70
Switches vs routers
both are store-and-forward
routers network-layer devices (examine network-layer headers)
switches link-layer devices (examine link-layer headers)
both have forwarding tables
routers compute tables using routing algorithms IP addresses
switches learn forwarding table using flooding learning MAC addresses
application
transport
network
link
physical
network
link
physical
link
physical
switch
datagram
application
transport
network
link
physical
frame
frame
frame
datagram
Link Layer 5-71
VLANs motivation
consider CS user moves office to
EE but wants connect to CS switch
single broadcast domain
all layer-2 broadcast traffic (ARP DHCP unknown location of destination MAC address) must cross entire LAN
securityprivacy efficiency issues
Computer
Science Electrical
Engineering
Computer
Engineering
Link Layer 5-72
VLANs port-based VLAN switch ports
grouped (by switch management software) so that single physical switch helliphellip
switch(es) supporting
VLAN capabilities can
be configured to
define multiple virtual
LANS over single
physical LAN
infrastructure
Virtual Local
Area Network 1
8
9
16 10 2
7
hellip
Electrical Engineering
(VLAN ports 1-8)
Computer Science
(VLAN ports 9-15)
15
hellip
Electrical Engineering
(VLAN ports 1-8)
hellip
1
8 2
7 9
16 10
15
hellip
Computer Science
(VLAN ports 9-16)
hellip operates as multiple virtual switches
Link Layer 5-73
Port-based VLAN
1
8
9
16 10 2
7
hellip
Electrical Engineering
(VLAN ports 1-8)
Computer Science
(VLAN ports 9-15)
15
hellip
traffic isolation frames tofrom ports 1-8 can only reach ports 1-8 can also define VLAN based on
MAC addresses of endpoints rather than switch port
dynamic membership ports can be dynamically assigned among VLANs
router
forwarding between VLANS done via routing (just as with separate switches) in practice vendors sell combined
switches plus routers
Link Layer 5-74
VLANS spanning multiple switches
trunk port carries frames between VLANS defined over multiple physical switches frames forwarded within VLAN between switches canrsquot be vanilla
8021 frames (must carry VLAN ID info)
8021q protocol addsremoved additional header fields for frames forwarded between trunk ports
1
8
9
10 2
7
hellip
Electrical Engineering
(VLAN ports 1-8)
Computer Science
(VLAN ports 9-15)
15
hellip
2
7 3
Ports 235 belong to EE VLAN
Ports 4678 belong to CS VLAN
5
4 6 8 16
1
Link Layer 5-75
type
2-byte Tag Protocol Identifier
(value 81-00)
Tag Control Information (12 bit VLAN ID field
3 bit priority field like IP TOS)
Recomputed CRC
8021Q VLAN frame format
8021 frame
8021Q frame
dest address
source address
data (payload) CRC preamble
dest address
source address
preamble data (payload) CRC
type
Link Layer 5-76
Link layer LANs outline
51 introduction services
52 error detection correction
53 multiple access protocols
54 LANs addressing ARP
Ethernet
switches
VLANS
55 link virtualization MPLS
56 data center networking
57 a day in the life of a web request
Link Layer 5-77
Multiprotocol label switching (MPLS)
initial goal high-speed IP forwarding using fixed length label (instead of IP address) fast lookup using fixed length identifier (rather than
shortest prefix matching)
borrowing ideas from Virtual Circuit (VC) approach
but IP datagram still keeps IP address
PPP or Ethernet
header IP header remainder of link-layer frame MPLS header
label Exp S TTL
20 3 1 5
Link Layer 5-78
MPLS capable routers
aka label-switched router
forward packets to outgoing interface based only on label value (donrsquot inspect IP address) MPLS forwarding table distinct from IP forwarding tables
flexibility MPLS forwarding decisions can differ from those of IP use destination and source addresses to route flows to
same destination differently (traffic engineering)
re-route flows quickly if link fails pre-computed backup paths (useful for VoIP)
Link Layer 5-79
R2
D
R3 R5
A
R6
MPLS versus IP paths
IP router IP routing path to destination determined
by destination address alone
R4
Link Layer 5-80
R2
D
R3 R4
R5
A
R6
MPLS versus IP paths
IP-only router
IP routing path to destination determined by destination address alone
MPLS and IP router
MPLS routing path to destination can be based on source and dest address fast reroute precompute backup routes in
case of link failure
entry router (R4) can use different MPLS
routes to A based eg on source address
Link Layer 5-81
MPLS signaling
modify OSPF IS-IS link-state flooding protocols to carry info used by MPLS routing eg link bandwidth amount of ldquoreservedrdquo link bandwidth
D
R4
R5
A
R6
entry MPLS router uses RSVP-TE signaling protocol to set up MPLS forwarding at downstream routers
modified link state flooding
RSVP-TE
Link Layer 5-82
R1 R2
D
R3 R4 R5
0
1
0 0
A
R6
in out out
label label dest interface
6 - A 0
in out out
label label dest interface
10 6 A 1
12 9 D 0
in out out
label label dest interface
10 A 0
12 D 0
1
in out out
label label dest interface
8 6 A 0
0
8 A 1
MPLS forwarding tables
Link Layer 5-83
Link layer LANs outline
51 introduction services
52 error detection correction
53 multiple access protocols
54 LANs addressing ARP
Ethernet
switches
VLANS
55 link virtualization MPLS
56 data center networking
57 a day in the life of a web request
Link Layer 5-84
Data center networks
10rsquos to 100rsquos of thousands of hosts often closely coupled in close proximity e-business (eg Amazon)
content-servers (eg YouTube Akamai Apple Microsoft)
search engines data mining (eg Google)
challenges
multiple applications each serving massive numbers of clients
managingbalancing load avoiding processing networking data bottlenecks
Inside a 40-ft Microsoft container
Chicago data center
Link Layer 5-85
Server racks
TOR switches
Tier-1 switches
Tier-2 switches
Load balancer
Load balancer
B
1 2 3 4 5 6 7 8
A C
Border router
Access router
Internet
Data center networks
load balancer application-layer routing receives external client requests
directs workload within data center
returns results to external client (hiding data
center internals from client)
Server racks
TOR switches
Tier-1 switches
Tier-2 switches
1 2 3 4 5 6 7 8
Data center networks
rich interconnection among switches racks
increased throughput between racks (multiple routing
paths possible)
increased reliability via redundancy
Link Layer 5-87
Link layer LANs outline
51 introduction services
52 error detection correction
53 multiple access protocols
54 LANs addressing ARP
Ethernet
switches
VLANS
55 link virtualization MPLS
56 data center networking
57 a day in the life of a web request
Link Layer 5-88
Synthesis a day in the life of a web request
journey down protocol stack complete application transport network link
putting-it-all-together synthesis goal identify review understand protocols (at all
layers) involved in seemingly simple scenario requesting www page
scenario student attaches laptop to campus network requestsreceives wwwgooglecom
Link Layer 5-89
A day in the life scenario
Comcast network
68800013
Googlersquos network
64233160019 64233169105
web server
DNS server
school network
68802024
web page
browser
router
(runs DHCP)
Link Layer 5-90
A day in the lifehellip connecting to the Internet
connecting laptop needs to get its own IP address addr of first-hop router addr of DNS server use DHCP
DHCP
UDP
IP
Eth
Phy
DHCP
DHCP
DHCP
DHCP
DHCP
DHCP
UDP
IP
Eth
Phy
DHCP
DHCP
DHCP
DHCP DHCP
DHCP request encapsulated in UDP encapsulated in IP encapsulated in 8023 Ethernet
Ethernet frame broadcast
(dest FFFFFFFFFFFF) on LAN received at router running DHCP server
Ethernet demuxed to IP demuxed UDP demuxed to DHCP
router
(runs DHCP)
Link Layer 5-91
DHCP server formulates DHCP ACK containing clientrsquos IP address IP address of first-hop router for client name amp IP address of DNS server
DHCP
UDP
IP
Eth
Phy
DHCP
DHCP
DHCP
DHCP
DHCP
UDP
IP
Eth
Phy
DHCP
DHCP
DHCP
DHCP
DHCP
encapsulation at DHCP server frame forwarded (switch learning) through LAN demultiplexing at client
Client now has IP address knows name amp addr of DNS
server IP address of its first-hop router
DHCP client receives DHCP ACK reply
A day in the lifehellip connecting to the Internet
router
(runs DHCP)
Link Layer 5-92
A day in the lifehellip ARP (before DNS before HTTP)
before sending HTTP request need IP address of wwwgooglecom DNS
DNS
UDP
IP
Eth
Phy
DNS
DNS
DNS
DNS query created encapsulated in UDP encapsulated in IP encapsulated in Eth To send frame to router need MAC address of router interface ARP
ARP query broadcast received by
router which replies with ARP reply giving MAC address of router interface
client now knows MAC address of first hop router so can now send frame containing DNS query
ARP query
Eth
Phy
ARP
ARP
ARP reply
router
(runs DHCP)
Link Layer 5-93
DNS
UDP
IP
Eth
Phy
DNS
DNS
DNS
DNS
DNS
IP datagram containing DNS query forwarded via LAN switch from client to 1st hop router
IP datagram forwarded from campus network into comcast network routed (tables created by RIP OSPF IS-IS andor BGP routing protocols) to DNS server
demuxrsquoed to DNS server
DNS server replies to client with IP address of wwwgooglecom
Comcast network
68800013
DNS server
DNS
UDP
IP
Eth
Phy
DNS
DNS
DNS
DNS
A day in the lifehellip using DNS
router
(runs DHCP)
Link Layer 5-94
A day in the lifehellipTCP connection carrying HTTP
HTTP
TCP
IP
Eth
Phy
HTTP
to send HTTP request client first opens TCP socket to web server
TCP SYN segment (step 1 in 3-way handshake) inter-domain routed to web server
TCP connection established 64233169105
web server
SYN
SYN
SYN
SYN
TCP
IP
Eth
Phy
SYN
SYN
SYN
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
web server responds with TCP SYNACK (step 2 in 3-way handshake)
router
(runs DHCP)
Link Layer 5-95
A day in the lifehellip HTTP requestreply
HTTP
TCP
IP
Eth
Phy
HTTP
HTTP request sent into TCP socket
IP datagram containing HTTP request routed to wwwgooglecom
IP datagram containing HTTP reply routed back to client
64233169105
web server
HTTP
TCP
IP
Eth
Phy web server responds with
HTTP reply (containing web page)
HTTP
HTTP
HTTP HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
web page finally () displayed
Link Layer 5-96
Chapter 5 Summary
principles behind data link layer services error detection correction
sharing a broadcast channel multiple access
link layer addressing
instantiation and implementation of various link layer technologies Ethernet
switched LANS VLANs
virtualized networks as a link layer MPLS
synthesis a day in the life of a web request
Link Layer 5-97
Chapter 5 letrsquos take a breath
journey down protocol stack complete (except PHY)
solid understanding of networking principles practice
hellip could stop here hellip but lots of interesting topics wireless
multimedia
security
network management
Link Layer 5-33
Ethernet CSMACD algorithm
1 NIC receives datagram from network layer creates frame
2 If NIC senses channel idle starts frame transmission If NIC senses channel busy waits until channel idle then transmits
3 If NIC transmits entire frame without detecting another transmission NIC is done with frame
4 If NIC detects another transmission while transmitting aborts and sends jam signal
5 After aborting NIC enters binary (exponential) backoff
after mth collision NIC chooses K at random from 012 hellip 2m-1 NIC waits K512 bit times returns to Step 2
longer backoff interval with more collisions
Link Layer 5-34
CSMACD efficiency
Tprop = max prop delay between 2 nodes in LAN
ttrans = time to transmit max-size frame
efficiency goes to 1
as tprop goes to 0
as ttrans goes to infinity
better performance than ALOHA and simple cheap decentralized
transpropttefficiency
51
1
Link Layer 5-35
ldquoTaking turnsrdquo MAC protocols
channel partitioning MAC protocols share channel efficiently and fairly at high load
inefficient at low load delay in channel access 1N bandwidth allocated even if only 1 active node
random access MAC protocols efficient at low load single node can fully utilize
channel
high load collision overhead
ldquotaking turnsrdquo protocols look for best of both worlds
Link Layer 5-36
polling master node ldquoinvitesrdquo
slave nodes to transmit in turn
typically used with ldquodumbrdquo slave devices
concerns
polling overhead
latency
single point of failure (master)
master
slaves
poll
data
data
ldquoTaking turnsrdquo MAC protocols
Link Layer 5-37
token passing control token passed
from one node to next sequentially
token message
concerns
token overhead
latency
single point of failure (token)
T
data
(nothing
to send)
T
ldquoTaking turnsrdquo MAC protocols
cable headend
CMTS
ISP
cable modem termination system
multiple 40Mbps downstream (broadcast) channels
single CMTS transmits into channels
multiple 30 Mbps upstream channels
multiple access all users contend for certain upstream channel time slots (others assigned)
Cable access network
cable modem
splitter
hellip
hellip
Internet framesTV channels control transmitted downstream at different frequencies
upstream Internet frames TV control transmitted upstream at different frequencies in time slots
Link Layer 5-39
DOCSIS data over cable service interface spec FDM over upstream downstream frequency channels
TDM upstream some slots assigned some have contention
downstream MAP frame assigns upstream slots
request for upstream slots (and data) transmitted random access (binary backoff) in selected slots
MAP frame for
Interval [t1 t2]
Residences with cable modems
Downstream channel i
Upstream channel j
t1 t2
Assigned minislots containing cable modem
upstream data frames
Minislots containing
minislots request frames
cable headend
CMTS
Cable access network
Link Layer 5-40
Summary of MAC protocols
channel partitioning by time frequency or code Time Division Frequency Division
random access (dynamic)
ALOHA S-ALOHA CSMA CSMACD
carrier sensing easy in some technologies (wire) hard in others (wireless)
CSMACD used in Ethernet
CSMACA used in 80211
taking turns
polling from central site token passing
bluetooth FDDI token ring
Link Layer 5-41
Link layer LANs outline
51 introduction services
52 error detection correction
53 multiple access protocols
54 LANs addressing ARP
Ethernet
switches
VLANS
55 link virtualization MPLS
56 data center networking
57 a day in the life of a web request
Link Layer 5-42
MAC addresses and ARP
32-bit IP address network-layer address for interface
used for layer 3 (network layer) forwarding
MAC (or LAN or physical or Ethernet) address function used lsquolocallyrdquo to get frame from one interface to
another physically-connected interface (same network in IP-addressing sense)
48 bit MAC address (for most LANs) burned in NIC ROM also sometimes software settable
eg 1A-2F-BB-76-09-AD
hexadecimal (base 16) notation
(each ldquonumberrdquo represents 4 bits)
Link Layer 5-43
LAN addresses and ARP
each adapter on LAN has unique LAN address
adapter
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN
(wired or
wireless)
Link Layer 5-44
LAN addresses (more)
MAC address allocation administered by IEEE
manufacturer buys portion of MAC address space (to assure uniqueness)
analogy MAC address like Social Security Number
IP address like postal address
MAC flat address portability can move LAN card from one LAN to another
IP hierarchical address not portable address depends on IP subnet to which node is
attached
Link Layer 5-45
ARP address resolution protocol
ARP table each IP node (host router) on LAN has table
IPMAC address mappings for some LAN nodes
lt IP address MAC address TTLgt
TTL (Time To Live) time after which address mapping will be forgotten (typically 20 min)
Question how to determine
interfacersquos MAC address
knowing its IP address
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN
137196723
137196778
137196714
137196788
Link Layer 5-46
ARP protocol same LAN
A wants to send datagram to B Brsquos MAC address not in
Arsquos ARP table
A broadcasts ARP query packet containing Bs IP address dest MAC address = FF-FF-
FF-FF-FF-FF
all nodes on LAN receive ARP query
B receives ARP packet replies to A with its (Bs) MAC address frame sent to Arsquos MAC
address (unicast)
A caches (saves) IP-to-MAC address pair in its ARP table until information becomes old (times out) soft state information that
times out (goes away) unless refreshed
ARP is ldquoplug-and-playrdquo nodes create their ARP
tables without intervention from net administrator
Link Layer 5-47
walkthrough send datagram from A to B via R
focus on addressing ndash at IP (datagram) and MAC layer (frame)
assume A knows Brsquos IP address
assume A knows IP address of first hop router R (how)
assume A knows Rrsquos MAC address (how)
Addressing routing to another LAN
R
1A-23-F9-CD-06-9B 222222222220
111111111110 E6-E9-00-17-BB-4B CC-49-DE-D0-AB-7D
111111111112
111111111111
74-29-9C-E8-FF-55
A
222222222222
49-BD-D2-C7-56-2A
222222222221 88-B2-2F-54-1A-0F
B
R
1A-23-F9-CD-06-9B 222222222220
111111111110 E6-E9-00-17-BB-4B CC-49-DE-D0-AB-7D
111111111112
111111111111
74-29-9C-E8-FF-55
A
222222222222
49-BD-D2-C7-56-2A
222222222221 88-B2-2F-54-1A-0F
B
Link Layer 5-48
Addressing routing to another LAN
IP
Eth
Phy
IP src 111111111111
IP dest 222222222222
A creates IP datagram with IP source A destination B
A creates link-layer frame with Rs MAC address as dest frame contains A-to-B IP datagram
MAC src 74-29-9C-E8-FF-55
MAC dest E6-E9-00-17-BB-4B
R
1A-23-F9-CD-06-9B 222222222220
111111111110 E6-E9-00-17-BB-4B CC-49-DE-D0-AB-7D
111111111112
111111111111
74-29-9C-E8-FF-55
A
222222222222
49-BD-D2-C7-56-2A
222222222221 88-B2-2F-54-1A-0F
B
Link Layer 5-49
Addressing routing to another LAN
IP
Eth
Phy
frame sent from A to R
IP
Eth
Phy
frame received at R datagram removed passed up to IP
MAC src 74-29-9C-E8-FF-55
MAC dest E6-E9-00-17-BB-4B
IP src 111111111111
IP dest 222222222222
IP src 111111111111
IP dest 222222222222
R
1A-23-F9-CD-06-9B 222222222220
111111111110 E6-E9-00-17-BB-4B CC-49-DE-D0-AB-7D
111111111112
111111111111
74-29-9C-E8-FF-55
A
222222222222
49-BD-D2-C7-56-2A
222222222221 88-B2-2F-54-1A-0F
B
Link Layer 5-50
Addressing routing to another LAN
IP src 111111111111
IP dest 222222222222
R forwards datagram with IP source A destination B
R creates link-layer frame with Bs MAC address as dest frame contains A-to-B IP datagram
MAC src 1A-23-F9-CD-06-9B
MAC dest 49-BD-D2-C7-56-2A
IP
Eth
Phy
IP
Eth
Phy
R
1A-23-F9-CD-06-9B 222222222220
111111111110 E6-E9-00-17-BB-4B CC-49-DE-D0-AB-7D
111111111112
111111111111
74-29-9C-E8-FF-55
A
222222222222
49-BD-D2-C7-56-2A
222222222221 88-B2-2F-54-1A-0F
B
Link Layer 5-51
Addressing routing to another LAN R forwards datagram with IP source A destination B
R creates link-layer frame with Bs MAC address as dest frame contains A-to-B IP datagram
IP src 111111111111
IP dest 222222222222
MAC src 1A-23-F9-CD-06-9B
MAC dest 49-BD-D2-C7-56-2A
IP
Eth
Phy
IP
Eth
Phy
R
1A-23-F9-CD-06-9B 222222222220
111111111110 E6-E9-00-17-BB-4B CC-49-DE-D0-AB-7D
111111111112
111111111111
74-29-9C-E8-FF-55
A
222222222222
49-BD-D2-C7-56-2A
222222222221 88-B2-2F-54-1A-0F
B
Link Layer 5-52
Addressing routing to another LAN R forwards datagram with IP source A destination B
R creates link-layer frame with Bs MAC address as dest frame contains A-to-B IP datagram
IP src 111111111111
IP dest 222222222222
MAC src 1A-23-F9-CD-06-9B
MAC dest 49-BD-D2-C7-56-2A
IP
Eth
Phy
Link Layer 5-53
Link layer LANs outline
51 introduction services
52 error detection correction
53 multiple access protocols
54 LANs addressing ARP
Ethernet
switches
VLANS
55 link virtualization MPLS
56 data center networking
57 a day in the life of a web request
Link Layer 5-54
Ethernet
ldquodominantrdquo wired LAN technology
cheap $20 for NIC
first widely used LAN technology
simpler cheaper than token LANs and ATM
kept up with speed race 10 Mbps ndash 10 Gbps
Metcalfersquos Ethernet sketch
Link Layer 5-55
Ethernet physical topology
bus popular through mid 90s all nodes in same collision domain (can collide with each
other)
star prevails today active switch in center
each ldquospokerdquo runs a (separate) Ethernet protocol (nodes
do not collide with each other)
switch
bus coaxial cable star
Link Layer 5-56
Ethernet frame structure
sending adapter encapsulates IP datagram (or other network layer protocol packet) in Ethernet frame
preamble
7 bytes with pattern 10101010 followed by one byte with pattern 10101011
used to synchronize receiver sender clock rates
dest address
source address
data (payload) CRC preamble
type
Link Layer 5-57
Ethernet frame structure (more)
addresses 6 byte source destination MAC addresses if adapter receives frame with matching destination
address or with broadcast address (eg ARP packet) it passes data in frame to network layer protocol
otherwise adapter discards frame
type indicates higher layer protocol (mostly IP but others possible eg Novell IPX AppleTalk)
CRC cyclic redundancy check at receiver error detected frame is dropped
dest address
source address
data (payload) CRC preamble
type
Link Layer 5-58
Ethernet unreliable connectionless
connectionless no handshaking between sending and receiving NICs
unreliable receiving NIC doesnt send acks or nacks to sending NIC
data in dropped frames recovered only if initial sender uses higher layer rdt (eg TCP) otherwise dropped data lost
Ethernetrsquos MAC protocol unslotted CSMACD wth binary backoff
Link Layer 5-59
8023 Ethernet standards link amp physical layers
many different Ethernet standards
common MAC protocol and frame format
different speeds 2 Mbps 10 Mbps 100 Mbps 1Gbps 10G bps
different physical layer media fiber cable
application
transport
network
link
physical
MAC protocol
and frame format
100BASE-TX
100BASE-T4
100BASE-FX 100BASE-T2
100BASE-SX 100BASE-BX
fiber physical layer copper (twister
pair) physical layer
Link Layer 5-60
Link layer LANs outline
51 introduction services
52 error detection correction
53 multiple access protocols
54 LANs addressing ARP
Ethernet
switches
VLANS
55 link virtualization MPLS
56 data center networking
57 a day in the life of a web request
Link Layer 5-61
Ethernet switch link-layer device takes an active role
store forward Ethernet frames
examine incoming framersquos MAC address selectively forward frame to one-or-more outgoing links when frame is to be forwarded on segment uses CSMACD to access segment
transparent
hosts are unaware of presence of switches
plug-and-play self-learning
switches do not need to be configured
Link Layer 5-62
Switch multiple simultaneous transmissions
hosts have dedicated direct connection to switch
switches buffer packets
Ethernet protocol used on each incoming link but no collisions full duplex
each link is its own collision domain
switching A-to-Arsquo and B-to-Brsquo can transmit simultaneously without collisions switch with six interfaces
(123456)
A
Arsquo
B
Brsquo C
Crsquo
1 2
3 4 5
6
Link Layer 5-63
Switch forwarding table
Q how does switch know Arsquo reachable via interface 4 Brsquo reachable via interface 5
switch with six interfaces
(123456)
A
Arsquo
B
Brsquo C
Crsquo
1 2
3 4 5
6 A each switch has a switch table each entry
(MAC address of host interface to
reach host time stamp)
looks like a routing table
Q how are entries created maintained in switch table
something like a routing protocol
A
Arsquo
B
Brsquo C
Crsquo
1 2
3 4 5
6
Link Layer 5-64
Switch self-learning
switch learns which hosts can be reached through which interfaces
when frame received switch ldquolearnsrdquo location of sender incoming LAN segment
records senderlocation pair in switch table
A Arsquo
Source A
Dest Arsquo
MAC addr interface TTL
Switch table
(initially empty) A 1 60
Link Layer 5-65
Switch frame filteringforwarding
when frame received at switch 1 record incoming link MAC address of sending host
2 index switch table using MAC destination address
3 if entry found for destination then
if destination on segment from which frame arrived then drop frame
else forward frame on interface indicated by entry
else flood forward on all interfaces except arriving
interface
A
Arsquo
B
Brsquo C
Crsquo
1 2
3 4 5
6
Link Layer 5-66
Self-learning forwarding example
A Arsquo
Source A
Dest Arsquo
MAC addr interface TTL
switch table
(initially empty) A 1 60
A Arsquo A Arsquo A Arsquo A Arsquo A Arsquo
frame destination Arsquo locaton unknown flood
Arsquo A
destination A location
known
Arsquo 4 60
selectively send
on just one link
Link Layer 5-67
Interconnecting switches
switches can be connected together
Q sending from A to G - how does S1 know to forward frame destined to F via S4 and S3
A self learning (works exactly the same as in single-switch case)
A
B
S1
C D
E
F
S2
S4
S3
H
I
G
Link Layer 5-68
Self-learning multi-switch example
Suppose C sends frame to I I responds to C
Q show switch tables and packet forwarding in S1 S2 S3 S4
A
B
S1
C D
E
F
S2
S4
S3
H
I
G
Link Layer 5-69
Institutional network
to external
network
router
IP subnet
mail server
web server
Link Layer 5-70
Switches vs routers
both are store-and-forward
routers network-layer devices (examine network-layer headers)
switches link-layer devices (examine link-layer headers)
both have forwarding tables
routers compute tables using routing algorithms IP addresses
switches learn forwarding table using flooding learning MAC addresses
application
transport
network
link
physical
network
link
physical
link
physical
switch
datagram
application
transport
network
link
physical
frame
frame
frame
datagram
Link Layer 5-71
VLANs motivation
consider CS user moves office to
EE but wants connect to CS switch
single broadcast domain
all layer-2 broadcast traffic (ARP DHCP unknown location of destination MAC address) must cross entire LAN
securityprivacy efficiency issues
Computer
Science Electrical
Engineering
Computer
Engineering
Link Layer 5-72
VLANs port-based VLAN switch ports
grouped (by switch management software) so that single physical switch helliphellip
switch(es) supporting
VLAN capabilities can
be configured to
define multiple virtual
LANS over single
physical LAN
infrastructure
Virtual Local
Area Network 1
8
9
16 10 2
7
hellip
Electrical Engineering
(VLAN ports 1-8)
Computer Science
(VLAN ports 9-15)
15
hellip
Electrical Engineering
(VLAN ports 1-8)
hellip
1
8 2
7 9
16 10
15
hellip
Computer Science
(VLAN ports 9-16)
hellip operates as multiple virtual switches
Link Layer 5-73
Port-based VLAN
1
8
9
16 10 2
7
hellip
Electrical Engineering
(VLAN ports 1-8)
Computer Science
(VLAN ports 9-15)
15
hellip
traffic isolation frames tofrom ports 1-8 can only reach ports 1-8 can also define VLAN based on
MAC addresses of endpoints rather than switch port
dynamic membership ports can be dynamically assigned among VLANs
router
forwarding between VLANS done via routing (just as with separate switches) in practice vendors sell combined
switches plus routers
Link Layer 5-74
VLANS spanning multiple switches
trunk port carries frames between VLANS defined over multiple physical switches frames forwarded within VLAN between switches canrsquot be vanilla
8021 frames (must carry VLAN ID info)
8021q protocol addsremoved additional header fields for frames forwarded between trunk ports
1
8
9
10 2
7
hellip
Electrical Engineering
(VLAN ports 1-8)
Computer Science
(VLAN ports 9-15)
15
hellip
2
7 3
Ports 235 belong to EE VLAN
Ports 4678 belong to CS VLAN
5
4 6 8 16
1
Link Layer 5-75
type
2-byte Tag Protocol Identifier
(value 81-00)
Tag Control Information (12 bit VLAN ID field
3 bit priority field like IP TOS)
Recomputed CRC
8021Q VLAN frame format
8021 frame
8021Q frame
dest address
source address
data (payload) CRC preamble
dest address
source address
preamble data (payload) CRC
type
Link Layer 5-76
Link layer LANs outline
51 introduction services
52 error detection correction
53 multiple access protocols
54 LANs addressing ARP
Ethernet
switches
VLANS
55 link virtualization MPLS
56 data center networking
57 a day in the life of a web request
Link Layer 5-77
Multiprotocol label switching (MPLS)
initial goal high-speed IP forwarding using fixed length label (instead of IP address) fast lookup using fixed length identifier (rather than
shortest prefix matching)
borrowing ideas from Virtual Circuit (VC) approach
but IP datagram still keeps IP address
PPP or Ethernet
header IP header remainder of link-layer frame MPLS header
label Exp S TTL
20 3 1 5
Link Layer 5-78
MPLS capable routers
aka label-switched router
forward packets to outgoing interface based only on label value (donrsquot inspect IP address) MPLS forwarding table distinct from IP forwarding tables
flexibility MPLS forwarding decisions can differ from those of IP use destination and source addresses to route flows to
same destination differently (traffic engineering)
re-route flows quickly if link fails pre-computed backup paths (useful for VoIP)
Link Layer 5-79
R2
D
R3 R5
A
R6
MPLS versus IP paths
IP router IP routing path to destination determined
by destination address alone
R4
Link Layer 5-80
R2
D
R3 R4
R5
A
R6
MPLS versus IP paths
IP-only router
IP routing path to destination determined by destination address alone
MPLS and IP router
MPLS routing path to destination can be based on source and dest address fast reroute precompute backup routes in
case of link failure
entry router (R4) can use different MPLS
routes to A based eg on source address
Link Layer 5-81
MPLS signaling
modify OSPF IS-IS link-state flooding protocols to carry info used by MPLS routing eg link bandwidth amount of ldquoreservedrdquo link bandwidth
D
R4
R5
A
R6
entry MPLS router uses RSVP-TE signaling protocol to set up MPLS forwarding at downstream routers
modified link state flooding
RSVP-TE
Link Layer 5-82
R1 R2
D
R3 R4 R5
0
1
0 0
A
R6
in out out
label label dest interface
6 - A 0
in out out
label label dest interface
10 6 A 1
12 9 D 0
in out out
label label dest interface
10 A 0
12 D 0
1
in out out
label label dest interface
8 6 A 0
0
8 A 1
MPLS forwarding tables
Link Layer 5-83
Link layer LANs outline
51 introduction services
52 error detection correction
53 multiple access protocols
54 LANs addressing ARP
Ethernet
switches
VLANS
55 link virtualization MPLS
56 data center networking
57 a day in the life of a web request
Link Layer 5-84
Data center networks
10rsquos to 100rsquos of thousands of hosts often closely coupled in close proximity e-business (eg Amazon)
content-servers (eg YouTube Akamai Apple Microsoft)
search engines data mining (eg Google)
challenges
multiple applications each serving massive numbers of clients
managingbalancing load avoiding processing networking data bottlenecks
Inside a 40-ft Microsoft container
Chicago data center
Link Layer 5-85
Server racks
TOR switches
Tier-1 switches
Tier-2 switches
Load balancer
Load balancer
B
1 2 3 4 5 6 7 8
A C
Border router
Access router
Internet
Data center networks
load balancer application-layer routing receives external client requests
directs workload within data center
returns results to external client (hiding data
center internals from client)
Server racks
TOR switches
Tier-1 switches
Tier-2 switches
1 2 3 4 5 6 7 8
Data center networks
rich interconnection among switches racks
increased throughput between racks (multiple routing
paths possible)
increased reliability via redundancy
Link Layer 5-87
Link layer LANs outline
51 introduction services
52 error detection correction
53 multiple access protocols
54 LANs addressing ARP
Ethernet
switches
VLANS
55 link virtualization MPLS
56 data center networking
57 a day in the life of a web request
Link Layer 5-88
Synthesis a day in the life of a web request
journey down protocol stack complete application transport network link
putting-it-all-together synthesis goal identify review understand protocols (at all
layers) involved in seemingly simple scenario requesting www page
scenario student attaches laptop to campus network requestsreceives wwwgooglecom
Link Layer 5-89
A day in the life scenario
Comcast network
68800013
Googlersquos network
64233160019 64233169105
web server
DNS server
school network
68802024
web page
browser
router
(runs DHCP)
Link Layer 5-90
A day in the lifehellip connecting to the Internet
connecting laptop needs to get its own IP address addr of first-hop router addr of DNS server use DHCP
DHCP
UDP
IP
Eth
Phy
DHCP
DHCP
DHCP
DHCP
DHCP
DHCP
UDP
IP
Eth
Phy
DHCP
DHCP
DHCP
DHCP DHCP
DHCP request encapsulated in UDP encapsulated in IP encapsulated in 8023 Ethernet
Ethernet frame broadcast
(dest FFFFFFFFFFFF) on LAN received at router running DHCP server
Ethernet demuxed to IP demuxed UDP demuxed to DHCP
router
(runs DHCP)
Link Layer 5-91
DHCP server formulates DHCP ACK containing clientrsquos IP address IP address of first-hop router for client name amp IP address of DNS server
DHCP
UDP
IP
Eth
Phy
DHCP
DHCP
DHCP
DHCP
DHCP
UDP
IP
Eth
Phy
DHCP
DHCP
DHCP
DHCP
DHCP
encapsulation at DHCP server frame forwarded (switch learning) through LAN demultiplexing at client
Client now has IP address knows name amp addr of DNS
server IP address of its first-hop router
DHCP client receives DHCP ACK reply
A day in the lifehellip connecting to the Internet
router
(runs DHCP)
Link Layer 5-92
A day in the lifehellip ARP (before DNS before HTTP)
before sending HTTP request need IP address of wwwgooglecom DNS
DNS
UDP
IP
Eth
Phy
DNS
DNS
DNS
DNS query created encapsulated in UDP encapsulated in IP encapsulated in Eth To send frame to router need MAC address of router interface ARP
ARP query broadcast received by
router which replies with ARP reply giving MAC address of router interface
client now knows MAC address of first hop router so can now send frame containing DNS query
ARP query
Eth
Phy
ARP
ARP
ARP reply
router
(runs DHCP)
Link Layer 5-93
DNS
UDP
IP
Eth
Phy
DNS
DNS
DNS
DNS
DNS
IP datagram containing DNS query forwarded via LAN switch from client to 1st hop router
IP datagram forwarded from campus network into comcast network routed (tables created by RIP OSPF IS-IS andor BGP routing protocols) to DNS server
demuxrsquoed to DNS server
DNS server replies to client with IP address of wwwgooglecom
Comcast network
68800013
DNS server
DNS
UDP
IP
Eth
Phy
DNS
DNS
DNS
DNS
A day in the lifehellip using DNS
router
(runs DHCP)
Link Layer 5-94
A day in the lifehellipTCP connection carrying HTTP
HTTP
TCP
IP
Eth
Phy
HTTP
to send HTTP request client first opens TCP socket to web server
TCP SYN segment (step 1 in 3-way handshake) inter-domain routed to web server
TCP connection established 64233169105
web server
SYN
SYN
SYN
SYN
TCP
IP
Eth
Phy
SYN
SYN
SYN
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
web server responds with TCP SYNACK (step 2 in 3-way handshake)
router
(runs DHCP)
Link Layer 5-95
A day in the lifehellip HTTP requestreply
HTTP
TCP
IP
Eth
Phy
HTTP
HTTP request sent into TCP socket
IP datagram containing HTTP request routed to wwwgooglecom
IP datagram containing HTTP reply routed back to client
64233169105
web server
HTTP
TCP
IP
Eth
Phy web server responds with
HTTP reply (containing web page)
HTTP
HTTP
HTTP HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
web page finally () displayed
Link Layer 5-96
Chapter 5 Summary
principles behind data link layer services error detection correction
sharing a broadcast channel multiple access
link layer addressing
instantiation and implementation of various link layer technologies Ethernet
switched LANS VLANs
virtualized networks as a link layer MPLS
synthesis a day in the life of a web request
Link Layer 5-97
Chapter 5 letrsquos take a breath
journey down protocol stack complete (except PHY)
solid understanding of networking principles practice
hellip could stop here hellip but lots of interesting topics wireless
multimedia
security
network management
Link Layer 5-34
CSMACD efficiency
Tprop = max prop delay between 2 nodes in LAN
ttrans = time to transmit max-size frame
efficiency goes to 1
as tprop goes to 0
as ttrans goes to infinity
better performance than ALOHA and simple cheap decentralized
transpropttefficiency
51
1
Link Layer 5-35
ldquoTaking turnsrdquo MAC protocols
channel partitioning MAC protocols share channel efficiently and fairly at high load
inefficient at low load delay in channel access 1N bandwidth allocated even if only 1 active node
random access MAC protocols efficient at low load single node can fully utilize
channel
high load collision overhead
ldquotaking turnsrdquo protocols look for best of both worlds
Link Layer 5-36
polling master node ldquoinvitesrdquo
slave nodes to transmit in turn
typically used with ldquodumbrdquo slave devices
concerns
polling overhead
latency
single point of failure (master)
master
slaves
poll
data
data
ldquoTaking turnsrdquo MAC protocols
Link Layer 5-37
token passing control token passed
from one node to next sequentially
token message
concerns
token overhead
latency
single point of failure (token)
T
data
(nothing
to send)
T
ldquoTaking turnsrdquo MAC protocols
cable headend
CMTS
ISP
cable modem termination system
multiple 40Mbps downstream (broadcast) channels
single CMTS transmits into channels
multiple 30 Mbps upstream channels
multiple access all users contend for certain upstream channel time slots (others assigned)
Cable access network
cable modem
splitter
hellip
hellip
Internet framesTV channels control transmitted downstream at different frequencies
upstream Internet frames TV control transmitted upstream at different frequencies in time slots
Link Layer 5-39
DOCSIS data over cable service interface spec FDM over upstream downstream frequency channels
TDM upstream some slots assigned some have contention
downstream MAP frame assigns upstream slots
request for upstream slots (and data) transmitted random access (binary backoff) in selected slots
MAP frame for
Interval [t1 t2]
Residences with cable modems
Downstream channel i
Upstream channel j
t1 t2
Assigned minislots containing cable modem
upstream data frames
Minislots containing
minislots request frames
cable headend
CMTS
Cable access network
Link Layer 5-40
Summary of MAC protocols
channel partitioning by time frequency or code Time Division Frequency Division
random access (dynamic)
ALOHA S-ALOHA CSMA CSMACD
carrier sensing easy in some technologies (wire) hard in others (wireless)
CSMACD used in Ethernet
CSMACA used in 80211
taking turns
polling from central site token passing
bluetooth FDDI token ring
Link Layer 5-41
Link layer LANs outline
51 introduction services
52 error detection correction
53 multiple access protocols
54 LANs addressing ARP
Ethernet
switches
VLANS
55 link virtualization MPLS
56 data center networking
57 a day in the life of a web request
Link Layer 5-42
MAC addresses and ARP
32-bit IP address network-layer address for interface
used for layer 3 (network layer) forwarding
MAC (or LAN or physical or Ethernet) address function used lsquolocallyrdquo to get frame from one interface to
another physically-connected interface (same network in IP-addressing sense)
48 bit MAC address (for most LANs) burned in NIC ROM also sometimes software settable
eg 1A-2F-BB-76-09-AD
hexadecimal (base 16) notation
(each ldquonumberrdquo represents 4 bits)
Link Layer 5-43
LAN addresses and ARP
each adapter on LAN has unique LAN address
adapter
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN
(wired or
wireless)
Link Layer 5-44
LAN addresses (more)
MAC address allocation administered by IEEE
manufacturer buys portion of MAC address space (to assure uniqueness)
analogy MAC address like Social Security Number
IP address like postal address
MAC flat address portability can move LAN card from one LAN to another
IP hierarchical address not portable address depends on IP subnet to which node is
attached
Link Layer 5-45
ARP address resolution protocol
ARP table each IP node (host router) on LAN has table
IPMAC address mappings for some LAN nodes
lt IP address MAC address TTLgt
TTL (Time To Live) time after which address mapping will be forgotten (typically 20 min)
Question how to determine
interfacersquos MAC address
knowing its IP address
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN
137196723
137196778
137196714
137196788
Link Layer 5-46
ARP protocol same LAN
A wants to send datagram to B Brsquos MAC address not in
Arsquos ARP table
A broadcasts ARP query packet containing Bs IP address dest MAC address = FF-FF-
FF-FF-FF-FF
all nodes on LAN receive ARP query
B receives ARP packet replies to A with its (Bs) MAC address frame sent to Arsquos MAC
address (unicast)
A caches (saves) IP-to-MAC address pair in its ARP table until information becomes old (times out) soft state information that
times out (goes away) unless refreshed
ARP is ldquoplug-and-playrdquo nodes create their ARP
tables without intervention from net administrator
Link Layer 5-47
walkthrough send datagram from A to B via R
focus on addressing ndash at IP (datagram) and MAC layer (frame)
assume A knows Brsquos IP address
assume A knows IP address of first hop router R (how)
assume A knows Rrsquos MAC address (how)
Addressing routing to another LAN
R
1A-23-F9-CD-06-9B 222222222220
111111111110 E6-E9-00-17-BB-4B CC-49-DE-D0-AB-7D
111111111112
111111111111
74-29-9C-E8-FF-55
A
222222222222
49-BD-D2-C7-56-2A
222222222221 88-B2-2F-54-1A-0F
B
R
1A-23-F9-CD-06-9B 222222222220
111111111110 E6-E9-00-17-BB-4B CC-49-DE-D0-AB-7D
111111111112
111111111111
74-29-9C-E8-FF-55
A
222222222222
49-BD-D2-C7-56-2A
222222222221 88-B2-2F-54-1A-0F
B
Link Layer 5-48
Addressing routing to another LAN
IP
Eth
Phy
IP src 111111111111
IP dest 222222222222
A creates IP datagram with IP source A destination B
A creates link-layer frame with Rs MAC address as dest frame contains A-to-B IP datagram
MAC src 74-29-9C-E8-FF-55
MAC dest E6-E9-00-17-BB-4B
R
1A-23-F9-CD-06-9B 222222222220
111111111110 E6-E9-00-17-BB-4B CC-49-DE-D0-AB-7D
111111111112
111111111111
74-29-9C-E8-FF-55
A
222222222222
49-BD-D2-C7-56-2A
222222222221 88-B2-2F-54-1A-0F
B
Link Layer 5-49
Addressing routing to another LAN
IP
Eth
Phy
frame sent from A to R
IP
Eth
Phy
frame received at R datagram removed passed up to IP
MAC src 74-29-9C-E8-FF-55
MAC dest E6-E9-00-17-BB-4B
IP src 111111111111
IP dest 222222222222
IP src 111111111111
IP dest 222222222222
R
1A-23-F9-CD-06-9B 222222222220
111111111110 E6-E9-00-17-BB-4B CC-49-DE-D0-AB-7D
111111111112
111111111111
74-29-9C-E8-FF-55
A
222222222222
49-BD-D2-C7-56-2A
222222222221 88-B2-2F-54-1A-0F
B
Link Layer 5-50
Addressing routing to another LAN
IP src 111111111111
IP dest 222222222222
R forwards datagram with IP source A destination B
R creates link-layer frame with Bs MAC address as dest frame contains A-to-B IP datagram
MAC src 1A-23-F9-CD-06-9B
MAC dest 49-BD-D2-C7-56-2A
IP
Eth
Phy
IP
Eth
Phy
R
1A-23-F9-CD-06-9B 222222222220
111111111110 E6-E9-00-17-BB-4B CC-49-DE-D0-AB-7D
111111111112
111111111111
74-29-9C-E8-FF-55
A
222222222222
49-BD-D2-C7-56-2A
222222222221 88-B2-2F-54-1A-0F
B
Link Layer 5-51
Addressing routing to another LAN R forwards datagram with IP source A destination B
R creates link-layer frame with Bs MAC address as dest frame contains A-to-B IP datagram
IP src 111111111111
IP dest 222222222222
MAC src 1A-23-F9-CD-06-9B
MAC dest 49-BD-D2-C7-56-2A
IP
Eth
Phy
IP
Eth
Phy
R
1A-23-F9-CD-06-9B 222222222220
111111111110 E6-E9-00-17-BB-4B CC-49-DE-D0-AB-7D
111111111112
111111111111
74-29-9C-E8-FF-55
A
222222222222
49-BD-D2-C7-56-2A
222222222221 88-B2-2F-54-1A-0F
B
Link Layer 5-52
Addressing routing to another LAN R forwards datagram with IP source A destination B
R creates link-layer frame with Bs MAC address as dest frame contains A-to-B IP datagram
IP src 111111111111
IP dest 222222222222
MAC src 1A-23-F9-CD-06-9B
MAC dest 49-BD-D2-C7-56-2A
IP
Eth
Phy
Link Layer 5-53
Link layer LANs outline
51 introduction services
52 error detection correction
53 multiple access protocols
54 LANs addressing ARP
Ethernet
switches
VLANS
55 link virtualization MPLS
56 data center networking
57 a day in the life of a web request
Link Layer 5-54
Ethernet
ldquodominantrdquo wired LAN technology
cheap $20 for NIC
first widely used LAN technology
simpler cheaper than token LANs and ATM
kept up with speed race 10 Mbps ndash 10 Gbps
Metcalfersquos Ethernet sketch
Link Layer 5-55
Ethernet physical topology
bus popular through mid 90s all nodes in same collision domain (can collide with each
other)
star prevails today active switch in center
each ldquospokerdquo runs a (separate) Ethernet protocol (nodes
do not collide with each other)
switch
bus coaxial cable star
Link Layer 5-56
Ethernet frame structure
sending adapter encapsulates IP datagram (or other network layer protocol packet) in Ethernet frame
preamble
7 bytes with pattern 10101010 followed by one byte with pattern 10101011
used to synchronize receiver sender clock rates
dest address
source address
data (payload) CRC preamble
type
Link Layer 5-57
Ethernet frame structure (more)
addresses 6 byte source destination MAC addresses if adapter receives frame with matching destination
address or with broadcast address (eg ARP packet) it passes data in frame to network layer protocol
otherwise adapter discards frame
type indicates higher layer protocol (mostly IP but others possible eg Novell IPX AppleTalk)
CRC cyclic redundancy check at receiver error detected frame is dropped
dest address
source address
data (payload) CRC preamble
type
Link Layer 5-58
Ethernet unreliable connectionless
connectionless no handshaking between sending and receiving NICs
unreliable receiving NIC doesnt send acks or nacks to sending NIC
data in dropped frames recovered only if initial sender uses higher layer rdt (eg TCP) otherwise dropped data lost
Ethernetrsquos MAC protocol unslotted CSMACD wth binary backoff
Link Layer 5-59
8023 Ethernet standards link amp physical layers
many different Ethernet standards
common MAC protocol and frame format
different speeds 2 Mbps 10 Mbps 100 Mbps 1Gbps 10G bps
different physical layer media fiber cable
application
transport
network
link
physical
MAC protocol
and frame format
100BASE-TX
100BASE-T4
100BASE-FX 100BASE-T2
100BASE-SX 100BASE-BX
fiber physical layer copper (twister
pair) physical layer
Link Layer 5-60
Link layer LANs outline
51 introduction services
52 error detection correction
53 multiple access protocols
54 LANs addressing ARP
Ethernet
switches
VLANS
55 link virtualization MPLS
56 data center networking
57 a day in the life of a web request
Link Layer 5-61
Ethernet switch link-layer device takes an active role
store forward Ethernet frames
examine incoming framersquos MAC address selectively forward frame to one-or-more outgoing links when frame is to be forwarded on segment uses CSMACD to access segment
transparent
hosts are unaware of presence of switches
plug-and-play self-learning
switches do not need to be configured
Link Layer 5-62
Switch multiple simultaneous transmissions
hosts have dedicated direct connection to switch
switches buffer packets
Ethernet protocol used on each incoming link but no collisions full duplex
each link is its own collision domain
switching A-to-Arsquo and B-to-Brsquo can transmit simultaneously without collisions switch with six interfaces
(123456)
A
Arsquo
B
Brsquo C
Crsquo
1 2
3 4 5
6
Link Layer 5-63
Switch forwarding table
Q how does switch know Arsquo reachable via interface 4 Brsquo reachable via interface 5
switch with six interfaces
(123456)
A
Arsquo
B
Brsquo C
Crsquo
1 2
3 4 5
6 A each switch has a switch table each entry
(MAC address of host interface to
reach host time stamp)
looks like a routing table
Q how are entries created maintained in switch table
something like a routing protocol
A
Arsquo
B
Brsquo C
Crsquo
1 2
3 4 5
6
Link Layer 5-64
Switch self-learning
switch learns which hosts can be reached through which interfaces
when frame received switch ldquolearnsrdquo location of sender incoming LAN segment
records senderlocation pair in switch table
A Arsquo
Source A
Dest Arsquo
MAC addr interface TTL
Switch table
(initially empty) A 1 60
Link Layer 5-65
Switch frame filteringforwarding
when frame received at switch 1 record incoming link MAC address of sending host
2 index switch table using MAC destination address
3 if entry found for destination then
if destination on segment from which frame arrived then drop frame
else forward frame on interface indicated by entry
else flood forward on all interfaces except arriving
interface
A
Arsquo
B
Brsquo C
Crsquo
1 2
3 4 5
6
Link Layer 5-66
Self-learning forwarding example
A Arsquo
Source A
Dest Arsquo
MAC addr interface TTL
switch table
(initially empty) A 1 60
A Arsquo A Arsquo A Arsquo A Arsquo A Arsquo
frame destination Arsquo locaton unknown flood
Arsquo A
destination A location
known
Arsquo 4 60
selectively send
on just one link
Link Layer 5-67
Interconnecting switches
switches can be connected together
Q sending from A to G - how does S1 know to forward frame destined to F via S4 and S3
A self learning (works exactly the same as in single-switch case)
A
B
S1
C D
E
F
S2
S4
S3
H
I
G
Link Layer 5-68
Self-learning multi-switch example
Suppose C sends frame to I I responds to C
Q show switch tables and packet forwarding in S1 S2 S3 S4
A
B
S1
C D
E
F
S2
S4
S3
H
I
G
Link Layer 5-69
Institutional network
to external
network
router
IP subnet
mail server
web server
Link Layer 5-70
Switches vs routers
both are store-and-forward
routers network-layer devices (examine network-layer headers)
switches link-layer devices (examine link-layer headers)
both have forwarding tables
routers compute tables using routing algorithms IP addresses
switches learn forwarding table using flooding learning MAC addresses
application
transport
network
link
physical
network
link
physical
link
physical
switch
datagram
application
transport
network
link
physical
frame
frame
frame
datagram
Link Layer 5-71
VLANs motivation
consider CS user moves office to
EE but wants connect to CS switch
single broadcast domain
all layer-2 broadcast traffic (ARP DHCP unknown location of destination MAC address) must cross entire LAN
securityprivacy efficiency issues
Computer
Science Electrical
Engineering
Computer
Engineering
Link Layer 5-72
VLANs port-based VLAN switch ports
grouped (by switch management software) so that single physical switch helliphellip
switch(es) supporting
VLAN capabilities can
be configured to
define multiple virtual
LANS over single
physical LAN
infrastructure
Virtual Local
Area Network 1
8
9
16 10 2
7
hellip
Electrical Engineering
(VLAN ports 1-8)
Computer Science
(VLAN ports 9-15)
15
hellip
Electrical Engineering
(VLAN ports 1-8)
hellip
1
8 2
7 9
16 10
15
hellip
Computer Science
(VLAN ports 9-16)
hellip operates as multiple virtual switches
Link Layer 5-73
Port-based VLAN
1
8
9
16 10 2
7
hellip
Electrical Engineering
(VLAN ports 1-8)
Computer Science
(VLAN ports 9-15)
15
hellip
traffic isolation frames tofrom ports 1-8 can only reach ports 1-8 can also define VLAN based on
MAC addresses of endpoints rather than switch port
dynamic membership ports can be dynamically assigned among VLANs
router
forwarding between VLANS done via routing (just as with separate switches) in practice vendors sell combined
switches plus routers
Link Layer 5-74
VLANS spanning multiple switches
trunk port carries frames between VLANS defined over multiple physical switches frames forwarded within VLAN between switches canrsquot be vanilla
8021 frames (must carry VLAN ID info)
8021q protocol addsremoved additional header fields for frames forwarded between trunk ports
1
8
9
10 2
7
hellip
Electrical Engineering
(VLAN ports 1-8)
Computer Science
(VLAN ports 9-15)
15
hellip
2
7 3
Ports 235 belong to EE VLAN
Ports 4678 belong to CS VLAN
5
4 6 8 16
1
Link Layer 5-75
type
2-byte Tag Protocol Identifier
(value 81-00)
Tag Control Information (12 bit VLAN ID field
3 bit priority field like IP TOS)
Recomputed CRC
8021Q VLAN frame format
8021 frame
8021Q frame
dest address
source address
data (payload) CRC preamble
dest address
source address
preamble data (payload) CRC
type
Link Layer 5-76
Link layer LANs outline
51 introduction services
52 error detection correction
53 multiple access protocols
54 LANs addressing ARP
Ethernet
switches
VLANS
55 link virtualization MPLS
56 data center networking
57 a day in the life of a web request
Link Layer 5-77
Multiprotocol label switching (MPLS)
initial goal high-speed IP forwarding using fixed length label (instead of IP address) fast lookup using fixed length identifier (rather than
shortest prefix matching)
borrowing ideas from Virtual Circuit (VC) approach
but IP datagram still keeps IP address
PPP or Ethernet
header IP header remainder of link-layer frame MPLS header
label Exp S TTL
20 3 1 5
Link Layer 5-78
MPLS capable routers
aka label-switched router
forward packets to outgoing interface based only on label value (donrsquot inspect IP address) MPLS forwarding table distinct from IP forwarding tables
flexibility MPLS forwarding decisions can differ from those of IP use destination and source addresses to route flows to
same destination differently (traffic engineering)
re-route flows quickly if link fails pre-computed backup paths (useful for VoIP)
Link Layer 5-79
R2
D
R3 R5
A
R6
MPLS versus IP paths
IP router IP routing path to destination determined
by destination address alone
R4
Link Layer 5-80
R2
D
R3 R4
R5
A
R6
MPLS versus IP paths
IP-only router
IP routing path to destination determined by destination address alone
MPLS and IP router
MPLS routing path to destination can be based on source and dest address fast reroute precompute backup routes in
case of link failure
entry router (R4) can use different MPLS
routes to A based eg on source address
Link Layer 5-81
MPLS signaling
modify OSPF IS-IS link-state flooding protocols to carry info used by MPLS routing eg link bandwidth amount of ldquoreservedrdquo link bandwidth
D
R4
R5
A
R6
entry MPLS router uses RSVP-TE signaling protocol to set up MPLS forwarding at downstream routers
modified link state flooding
RSVP-TE
Link Layer 5-82
R1 R2
D
R3 R4 R5
0
1
0 0
A
R6
in out out
label label dest interface
6 - A 0
in out out
label label dest interface
10 6 A 1
12 9 D 0
in out out
label label dest interface
10 A 0
12 D 0
1
in out out
label label dest interface
8 6 A 0
0
8 A 1
MPLS forwarding tables
Link Layer 5-83
Link layer LANs outline
51 introduction services
52 error detection correction
53 multiple access protocols
54 LANs addressing ARP
Ethernet
switches
VLANS
55 link virtualization MPLS
56 data center networking
57 a day in the life of a web request
Link Layer 5-84
Data center networks
10rsquos to 100rsquos of thousands of hosts often closely coupled in close proximity e-business (eg Amazon)
content-servers (eg YouTube Akamai Apple Microsoft)
search engines data mining (eg Google)
challenges
multiple applications each serving massive numbers of clients
managingbalancing load avoiding processing networking data bottlenecks
Inside a 40-ft Microsoft container
Chicago data center
Link Layer 5-85
Server racks
TOR switches
Tier-1 switches
Tier-2 switches
Load balancer
Load balancer
B
1 2 3 4 5 6 7 8
A C
Border router
Access router
Internet
Data center networks
load balancer application-layer routing receives external client requests
directs workload within data center
returns results to external client (hiding data
center internals from client)
Server racks
TOR switches
Tier-1 switches
Tier-2 switches
1 2 3 4 5 6 7 8
Data center networks
rich interconnection among switches racks
increased throughput between racks (multiple routing
paths possible)
increased reliability via redundancy
Link Layer 5-87
Link layer LANs outline
51 introduction services
52 error detection correction
53 multiple access protocols
54 LANs addressing ARP
Ethernet
switches
VLANS
55 link virtualization MPLS
56 data center networking
57 a day in the life of a web request
Link Layer 5-88
Synthesis a day in the life of a web request
journey down protocol stack complete application transport network link
putting-it-all-together synthesis goal identify review understand protocols (at all
layers) involved in seemingly simple scenario requesting www page
scenario student attaches laptop to campus network requestsreceives wwwgooglecom
Link Layer 5-89
A day in the life scenario
Comcast network
68800013
Googlersquos network
64233160019 64233169105
web server
DNS server
school network
68802024
web page
browser
router
(runs DHCP)
Link Layer 5-90
A day in the lifehellip connecting to the Internet
connecting laptop needs to get its own IP address addr of first-hop router addr of DNS server use DHCP
DHCP
UDP
IP
Eth
Phy
DHCP
DHCP
DHCP
DHCP
DHCP
DHCP
UDP
IP
Eth
Phy
DHCP
DHCP
DHCP
DHCP DHCP
DHCP request encapsulated in UDP encapsulated in IP encapsulated in 8023 Ethernet
Ethernet frame broadcast
(dest FFFFFFFFFFFF) on LAN received at router running DHCP server
Ethernet demuxed to IP demuxed UDP demuxed to DHCP
router
(runs DHCP)
Link Layer 5-91
DHCP server formulates DHCP ACK containing clientrsquos IP address IP address of first-hop router for client name amp IP address of DNS server
DHCP
UDP
IP
Eth
Phy
DHCP
DHCP
DHCP
DHCP
DHCP
UDP
IP
Eth
Phy
DHCP
DHCP
DHCP
DHCP
DHCP
encapsulation at DHCP server frame forwarded (switch learning) through LAN demultiplexing at client
Client now has IP address knows name amp addr of DNS
server IP address of its first-hop router
DHCP client receives DHCP ACK reply
A day in the lifehellip connecting to the Internet
router
(runs DHCP)
Link Layer 5-92
A day in the lifehellip ARP (before DNS before HTTP)
before sending HTTP request need IP address of wwwgooglecom DNS
DNS
UDP
IP
Eth
Phy
DNS
DNS
DNS
DNS query created encapsulated in UDP encapsulated in IP encapsulated in Eth To send frame to router need MAC address of router interface ARP
ARP query broadcast received by
router which replies with ARP reply giving MAC address of router interface
client now knows MAC address of first hop router so can now send frame containing DNS query
ARP query
Eth
Phy
ARP
ARP
ARP reply
router
(runs DHCP)
Link Layer 5-93
DNS
UDP
IP
Eth
Phy
DNS
DNS
DNS
DNS
DNS
IP datagram containing DNS query forwarded via LAN switch from client to 1st hop router
IP datagram forwarded from campus network into comcast network routed (tables created by RIP OSPF IS-IS andor BGP routing protocols) to DNS server
demuxrsquoed to DNS server
DNS server replies to client with IP address of wwwgooglecom
Comcast network
68800013
DNS server
DNS
UDP
IP
Eth
Phy
DNS
DNS
DNS
DNS
A day in the lifehellip using DNS
router
(runs DHCP)
Link Layer 5-94
A day in the lifehellipTCP connection carrying HTTP
HTTP
TCP
IP
Eth
Phy
HTTP
to send HTTP request client first opens TCP socket to web server
TCP SYN segment (step 1 in 3-way handshake) inter-domain routed to web server
TCP connection established 64233169105
web server
SYN
SYN
SYN
SYN
TCP
IP
Eth
Phy
SYN
SYN
SYN
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
web server responds with TCP SYNACK (step 2 in 3-way handshake)
router
(runs DHCP)
Link Layer 5-95
A day in the lifehellip HTTP requestreply
HTTP
TCP
IP
Eth
Phy
HTTP
HTTP request sent into TCP socket
IP datagram containing HTTP request routed to wwwgooglecom
IP datagram containing HTTP reply routed back to client
64233169105
web server
HTTP
TCP
IP
Eth
Phy web server responds with
HTTP reply (containing web page)
HTTP
HTTP
HTTP HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
web page finally () displayed
Link Layer 5-96
Chapter 5 Summary
principles behind data link layer services error detection correction
sharing a broadcast channel multiple access
link layer addressing
instantiation and implementation of various link layer technologies Ethernet
switched LANS VLANs
virtualized networks as a link layer MPLS
synthesis a day in the life of a web request
Link Layer 5-97
Chapter 5 letrsquos take a breath
journey down protocol stack complete (except PHY)
solid understanding of networking principles practice
hellip could stop here hellip but lots of interesting topics wireless
multimedia
security
network management
Link Layer 5-35
ldquoTaking turnsrdquo MAC protocols
channel partitioning MAC protocols share channel efficiently and fairly at high load
inefficient at low load delay in channel access 1N bandwidth allocated even if only 1 active node
random access MAC protocols efficient at low load single node can fully utilize
channel
high load collision overhead
ldquotaking turnsrdquo protocols look for best of both worlds
Link Layer 5-36
polling master node ldquoinvitesrdquo
slave nodes to transmit in turn
typically used with ldquodumbrdquo slave devices
concerns
polling overhead
latency
single point of failure (master)
master
slaves
poll
data
data
ldquoTaking turnsrdquo MAC protocols
Link Layer 5-37
token passing control token passed
from one node to next sequentially
token message
concerns
token overhead
latency
single point of failure (token)
T
data
(nothing
to send)
T
ldquoTaking turnsrdquo MAC protocols
cable headend
CMTS
ISP
cable modem termination system
multiple 40Mbps downstream (broadcast) channels
single CMTS transmits into channels
multiple 30 Mbps upstream channels
multiple access all users contend for certain upstream channel time slots (others assigned)
Cable access network
cable modem
splitter
hellip
hellip
Internet framesTV channels control transmitted downstream at different frequencies
upstream Internet frames TV control transmitted upstream at different frequencies in time slots
Link Layer 5-39
DOCSIS data over cable service interface spec FDM over upstream downstream frequency channels
TDM upstream some slots assigned some have contention
downstream MAP frame assigns upstream slots
request for upstream slots (and data) transmitted random access (binary backoff) in selected slots
MAP frame for
Interval [t1 t2]
Residences with cable modems
Downstream channel i
Upstream channel j
t1 t2
Assigned minislots containing cable modem
upstream data frames
Minislots containing
minislots request frames
cable headend
CMTS
Cable access network
Link Layer 5-40
Summary of MAC protocols
channel partitioning by time frequency or code Time Division Frequency Division
random access (dynamic)
ALOHA S-ALOHA CSMA CSMACD
carrier sensing easy in some technologies (wire) hard in others (wireless)
CSMACD used in Ethernet
CSMACA used in 80211
taking turns
polling from central site token passing
bluetooth FDDI token ring
Link Layer 5-41
Link layer LANs outline
51 introduction services
52 error detection correction
53 multiple access protocols
54 LANs addressing ARP
Ethernet
switches
VLANS
55 link virtualization MPLS
56 data center networking
57 a day in the life of a web request
Link Layer 5-42
MAC addresses and ARP
32-bit IP address network-layer address for interface
used for layer 3 (network layer) forwarding
MAC (or LAN or physical or Ethernet) address function used lsquolocallyrdquo to get frame from one interface to
another physically-connected interface (same network in IP-addressing sense)
48 bit MAC address (for most LANs) burned in NIC ROM also sometimes software settable
eg 1A-2F-BB-76-09-AD
hexadecimal (base 16) notation
(each ldquonumberrdquo represents 4 bits)
Link Layer 5-43
LAN addresses and ARP
each adapter on LAN has unique LAN address
adapter
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN
(wired or
wireless)
Link Layer 5-44
LAN addresses (more)
MAC address allocation administered by IEEE
manufacturer buys portion of MAC address space (to assure uniqueness)
analogy MAC address like Social Security Number
IP address like postal address
MAC flat address portability can move LAN card from one LAN to another
IP hierarchical address not portable address depends on IP subnet to which node is
attached
Link Layer 5-45
ARP address resolution protocol
ARP table each IP node (host router) on LAN has table
IPMAC address mappings for some LAN nodes
lt IP address MAC address TTLgt
TTL (Time To Live) time after which address mapping will be forgotten (typically 20 min)
Question how to determine
interfacersquos MAC address
knowing its IP address
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN
137196723
137196778
137196714
137196788
Link Layer 5-46
ARP protocol same LAN
A wants to send datagram to B Brsquos MAC address not in
Arsquos ARP table
A broadcasts ARP query packet containing Bs IP address dest MAC address = FF-FF-
FF-FF-FF-FF
all nodes on LAN receive ARP query
B receives ARP packet replies to A with its (Bs) MAC address frame sent to Arsquos MAC
address (unicast)
A caches (saves) IP-to-MAC address pair in its ARP table until information becomes old (times out) soft state information that
times out (goes away) unless refreshed
ARP is ldquoplug-and-playrdquo nodes create their ARP
tables without intervention from net administrator
Link Layer 5-47
walkthrough send datagram from A to B via R
focus on addressing ndash at IP (datagram) and MAC layer (frame)
assume A knows Brsquos IP address
assume A knows IP address of first hop router R (how)
assume A knows Rrsquos MAC address (how)
Addressing routing to another LAN
R
1A-23-F9-CD-06-9B 222222222220
111111111110 E6-E9-00-17-BB-4B CC-49-DE-D0-AB-7D
111111111112
111111111111
74-29-9C-E8-FF-55
A
222222222222
49-BD-D2-C7-56-2A
222222222221 88-B2-2F-54-1A-0F
B
R
1A-23-F9-CD-06-9B 222222222220
111111111110 E6-E9-00-17-BB-4B CC-49-DE-D0-AB-7D
111111111112
111111111111
74-29-9C-E8-FF-55
A
222222222222
49-BD-D2-C7-56-2A
222222222221 88-B2-2F-54-1A-0F
B
Link Layer 5-48
Addressing routing to another LAN
IP
Eth
Phy
IP src 111111111111
IP dest 222222222222
A creates IP datagram with IP source A destination B
A creates link-layer frame with Rs MAC address as dest frame contains A-to-B IP datagram
MAC src 74-29-9C-E8-FF-55
MAC dest E6-E9-00-17-BB-4B
R
1A-23-F9-CD-06-9B 222222222220
111111111110 E6-E9-00-17-BB-4B CC-49-DE-D0-AB-7D
111111111112
111111111111
74-29-9C-E8-FF-55
A
222222222222
49-BD-D2-C7-56-2A
222222222221 88-B2-2F-54-1A-0F
B
Link Layer 5-49
Addressing routing to another LAN
IP
Eth
Phy
frame sent from A to R
IP
Eth
Phy
frame received at R datagram removed passed up to IP
MAC src 74-29-9C-E8-FF-55
MAC dest E6-E9-00-17-BB-4B
IP src 111111111111
IP dest 222222222222
IP src 111111111111
IP dest 222222222222
R
1A-23-F9-CD-06-9B 222222222220
111111111110 E6-E9-00-17-BB-4B CC-49-DE-D0-AB-7D
111111111112
111111111111
74-29-9C-E8-FF-55
A
222222222222
49-BD-D2-C7-56-2A
222222222221 88-B2-2F-54-1A-0F
B
Link Layer 5-50
Addressing routing to another LAN
IP src 111111111111
IP dest 222222222222
R forwards datagram with IP source A destination B
R creates link-layer frame with Bs MAC address as dest frame contains A-to-B IP datagram
MAC src 1A-23-F9-CD-06-9B
MAC dest 49-BD-D2-C7-56-2A
IP
Eth
Phy
IP
Eth
Phy
R
1A-23-F9-CD-06-9B 222222222220
111111111110 E6-E9-00-17-BB-4B CC-49-DE-D0-AB-7D
111111111112
111111111111
74-29-9C-E8-FF-55
A
222222222222
49-BD-D2-C7-56-2A
222222222221 88-B2-2F-54-1A-0F
B
Link Layer 5-51
Addressing routing to another LAN R forwards datagram with IP source A destination B
R creates link-layer frame with Bs MAC address as dest frame contains A-to-B IP datagram
IP src 111111111111
IP dest 222222222222
MAC src 1A-23-F9-CD-06-9B
MAC dest 49-BD-D2-C7-56-2A
IP
Eth
Phy
IP
Eth
Phy
R
1A-23-F9-CD-06-9B 222222222220
111111111110 E6-E9-00-17-BB-4B CC-49-DE-D0-AB-7D
111111111112
111111111111
74-29-9C-E8-FF-55
A
222222222222
49-BD-D2-C7-56-2A
222222222221 88-B2-2F-54-1A-0F
B
Link Layer 5-52
Addressing routing to another LAN R forwards datagram with IP source A destination B
R creates link-layer frame with Bs MAC address as dest frame contains A-to-B IP datagram
IP src 111111111111
IP dest 222222222222
MAC src 1A-23-F9-CD-06-9B
MAC dest 49-BD-D2-C7-56-2A
IP
Eth
Phy
Link Layer 5-53
Link layer LANs outline
51 introduction services
52 error detection correction
53 multiple access protocols
54 LANs addressing ARP
Ethernet
switches
VLANS
55 link virtualization MPLS
56 data center networking
57 a day in the life of a web request
Link Layer 5-54
Ethernet
ldquodominantrdquo wired LAN technology
cheap $20 for NIC
first widely used LAN technology
simpler cheaper than token LANs and ATM
kept up with speed race 10 Mbps ndash 10 Gbps
Metcalfersquos Ethernet sketch
Link Layer 5-55
Ethernet physical topology
bus popular through mid 90s all nodes in same collision domain (can collide with each
other)
star prevails today active switch in center
each ldquospokerdquo runs a (separate) Ethernet protocol (nodes
do not collide with each other)
switch
bus coaxial cable star
Link Layer 5-56
Ethernet frame structure
sending adapter encapsulates IP datagram (or other network layer protocol packet) in Ethernet frame
preamble
7 bytes with pattern 10101010 followed by one byte with pattern 10101011
used to synchronize receiver sender clock rates
dest address
source address
data (payload) CRC preamble
type
Link Layer 5-57
Ethernet frame structure (more)
addresses 6 byte source destination MAC addresses if adapter receives frame with matching destination
address or with broadcast address (eg ARP packet) it passes data in frame to network layer protocol
otherwise adapter discards frame
type indicates higher layer protocol (mostly IP but others possible eg Novell IPX AppleTalk)
CRC cyclic redundancy check at receiver error detected frame is dropped
dest address
source address
data (payload) CRC preamble
type
Link Layer 5-58
Ethernet unreliable connectionless
connectionless no handshaking between sending and receiving NICs
unreliable receiving NIC doesnt send acks or nacks to sending NIC
data in dropped frames recovered only if initial sender uses higher layer rdt (eg TCP) otherwise dropped data lost
Ethernetrsquos MAC protocol unslotted CSMACD wth binary backoff
Link Layer 5-59
8023 Ethernet standards link amp physical layers
many different Ethernet standards
common MAC protocol and frame format
different speeds 2 Mbps 10 Mbps 100 Mbps 1Gbps 10G bps
different physical layer media fiber cable
application
transport
network
link
physical
MAC protocol
and frame format
100BASE-TX
100BASE-T4
100BASE-FX 100BASE-T2
100BASE-SX 100BASE-BX
fiber physical layer copper (twister
pair) physical layer
Link Layer 5-60
Link layer LANs outline
51 introduction services
52 error detection correction
53 multiple access protocols
54 LANs addressing ARP
Ethernet
switches
VLANS
55 link virtualization MPLS
56 data center networking
57 a day in the life of a web request
Link Layer 5-61
Ethernet switch link-layer device takes an active role
store forward Ethernet frames
examine incoming framersquos MAC address selectively forward frame to one-or-more outgoing links when frame is to be forwarded on segment uses CSMACD to access segment
transparent
hosts are unaware of presence of switches
plug-and-play self-learning
switches do not need to be configured
Link Layer 5-62
Switch multiple simultaneous transmissions
hosts have dedicated direct connection to switch
switches buffer packets
Ethernet protocol used on each incoming link but no collisions full duplex
each link is its own collision domain
switching A-to-Arsquo and B-to-Brsquo can transmit simultaneously without collisions switch with six interfaces
(123456)
A
Arsquo
B
Brsquo C
Crsquo
1 2
3 4 5
6
Link Layer 5-63
Switch forwarding table
Q how does switch know Arsquo reachable via interface 4 Brsquo reachable via interface 5
switch with six interfaces
(123456)
A
Arsquo
B
Brsquo C
Crsquo
1 2
3 4 5
6 A each switch has a switch table each entry
(MAC address of host interface to
reach host time stamp)
looks like a routing table
Q how are entries created maintained in switch table
something like a routing protocol
A
Arsquo
B
Brsquo C
Crsquo
1 2
3 4 5
6
Link Layer 5-64
Switch self-learning
switch learns which hosts can be reached through which interfaces
when frame received switch ldquolearnsrdquo location of sender incoming LAN segment
records senderlocation pair in switch table
A Arsquo
Source A
Dest Arsquo
MAC addr interface TTL
Switch table
(initially empty) A 1 60
Link Layer 5-65
Switch frame filteringforwarding
when frame received at switch 1 record incoming link MAC address of sending host
2 index switch table using MAC destination address
3 if entry found for destination then
if destination on segment from which frame arrived then drop frame
else forward frame on interface indicated by entry
else flood forward on all interfaces except arriving
interface
A
Arsquo
B
Brsquo C
Crsquo
1 2
3 4 5
6
Link Layer 5-66
Self-learning forwarding example
A Arsquo
Source A
Dest Arsquo
MAC addr interface TTL
switch table
(initially empty) A 1 60
A Arsquo A Arsquo A Arsquo A Arsquo A Arsquo
frame destination Arsquo locaton unknown flood
Arsquo A
destination A location
known
Arsquo 4 60
selectively send
on just one link
Link Layer 5-67
Interconnecting switches
switches can be connected together
Q sending from A to G - how does S1 know to forward frame destined to F via S4 and S3
A self learning (works exactly the same as in single-switch case)
A
B
S1
C D
E
F
S2
S4
S3
H
I
G
Link Layer 5-68
Self-learning multi-switch example
Suppose C sends frame to I I responds to C
Q show switch tables and packet forwarding in S1 S2 S3 S4
A
B
S1
C D
E
F
S2
S4
S3
H
I
G
Link Layer 5-69
Institutional network
to external
network
router
IP subnet
mail server
web server
Link Layer 5-70
Switches vs routers
both are store-and-forward
routers network-layer devices (examine network-layer headers)
switches link-layer devices (examine link-layer headers)
both have forwarding tables
routers compute tables using routing algorithms IP addresses
switches learn forwarding table using flooding learning MAC addresses
application
transport
network
link
physical
network
link
physical
link
physical
switch
datagram
application
transport
network
link
physical
frame
frame
frame
datagram
Link Layer 5-71
VLANs motivation
consider CS user moves office to
EE but wants connect to CS switch
single broadcast domain
all layer-2 broadcast traffic (ARP DHCP unknown location of destination MAC address) must cross entire LAN
securityprivacy efficiency issues
Computer
Science Electrical
Engineering
Computer
Engineering
Link Layer 5-72
VLANs port-based VLAN switch ports
grouped (by switch management software) so that single physical switch helliphellip
switch(es) supporting
VLAN capabilities can
be configured to
define multiple virtual
LANS over single
physical LAN
infrastructure
Virtual Local
Area Network 1
8
9
16 10 2
7
hellip
Electrical Engineering
(VLAN ports 1-8)
Computer Science
(VLAN ports 9-15)
15
hellip
Electrical Engineering
(VLAN ports 1-8)
hellip
1
8 2
7 9
16 10
15
hellip
Computer Science
(VLAN ports 9-16)
hellip operates as multiple virtual switches
Link Layer 5-73
Port-based VLAN
1
8
9
16 10 2
7
hellip
Electrical Engineering
(VLAN ports 1-8)
Computer Science
(VLAN ports 9-15)
15
hellip
traffic isolation frames tofrom ports 1-8 can only reach ports 1-8 can also define VLAN based on
MAC addresses of endpoints rather than switch port
dynamic membership ports can be dynamically assigned among VLANs
router
forwarding between VLANS done via routing (just as with separate switches) in practice vendors sell combined
switches plus routers
Link Layer 5-74
VLANS spanning multiple switches
trunk port carries frames between VLANS defined over multiple physical switches frames forwarded within VLAN between switches canrsquot be vanilla
8021 frames (must carry VLAN ID info)
8021q protocol addsremoved additional header fields for frames forwarded between trunk ports
1
8
9
10 2
7
hellip
Electrical Engineering
(VLAN ports 1-8)
Computer Science
(VLAN ports 9-15)
15
hellip
2
7 3
Ports 235 belong to EE VLAN
Ports 4678 belong to CS VLAN
5
4 6 8 16
1
Link Layer 5-75
type
2-byte Tag Protocol Identifier
(value 81-00)
Tag Control Information (12 bit VLAN ID field
3 bit priority field like IP TOS)
Recomputed CRC
8021Q VLAN frame format
8021 frame
8021Q frame
dest address
source address
data (payload) CRC preamble
dest address
source address
preamble data (payload) CRC
type
Link Layer 5-76
Link layer LANs outline
51 introduction services
52 error detection correction
53 multiple access protocols
54 LANs addressing ARP
Ethernet
switches
VLANS
55 link virtualization MPLS
56 data center networking
57 a day in the life of a web request
Link Layer 5-77
Multiprotocol label switching (MPLS)
initial goal high-speed IP forwarding using fixed length label (instead of IP address) fast lookup using fixed length identifier (rather than
shortest prefix matching)
borrowing ideas from Virtual Circuit (VC) approach
but IP datagram still keeps IP address
PPP or Ethernet
header IP header remainder of link-layer frame MPLS header
label Exp S TTL
20 3 1 5
Link Layer 5-78
MPLS capable routers
aka label-switched router
forward packets to outgoing interface based only on label value (donrsquot inspect IP address) MPLS forwarding table distinct from IP forwarding tables
flexibility MPLS forwarding decisions can differ from those of IP use destination and source addresses to route flows to
same destination differently (traffic engineering)
re-route flows quickly if link fails pre-computed backup paths (useful for VoIP)
Link Layer 5-79
R2
D
R3 R5
A
R6
MPLS versus IP paths
IP router IP routing path to destination determined
by destination address alone
R4
Link Layer 5-80
R2
D
R3 R4
R5
A
R6
MPLS versus IP paths
IP-only router
IP routing path to destination determined by destination address alone
MPLS and IP router
MPLS routing path to destination can be based on source and dest address fast reroute precompute backup routes in
case of link failure
entry router (R4) can use different MPLS
routes to A based eg on source address
Link Layer 5-81
MPLS signaling
modify OSPF IS-IS link-state flooding protocols to carry info used by MPLS routing eg link bandwidth amount of ldquoreservedrdquo link bandwidth
D
R4
R5
A
R6
entry MPLS router uses RSVP-TE signaling protocol to set up MPLS forwarding at downstream routers
modified link state flooding
RSVP-TE
Link Layer 5-82
R1 R2
D
R3 R4 R5
0
1
0 0
A
R6
in out out
label label dest interface
6 - A 0
in out out
label label dest interface
10 6 A 1
12 9 D 0
in out out
label label dest interface
10 A 0
12 D 0
1
in out out
label label dest interface
8 6 A 0
0
8 A 1
MPLS forwarding tables
Link Layer 5-83
Link layer LANs outline
51 introduction services
52 error detection correction
53 multiple access protocols
54 LANs addressing ARP
Ethernet
switches
VLANS
55 link virtualization MPLS
56 data center networking
57 a day in the life of a web request
Link Layer 5-84
Data center networks
10rsquos to 100rsquos of thousands of hosts often closely coupled in close proximity e-business (eg Amazon)
content-servers (eg YouTube Akamai Apple Microsoft)
search engines data mining (eg Google)
challenges
multiple applications each serving massive numbers of clients
managingbalancing load avoiding processing networking data bottlenecks
Inside a 40-ft Microsoft container
Chicago data center
Link Layer 5-85
Server racks
TOR switches
Tier-1 switches
Tier-2 switches
Load balancer
Load balancer
B
1 2 3 4 5 6 7 8
A C
Border router
Access router
Internet
Data center networks
load balancer application-layer routing receives external client requests
directs workload within data center
returns results to external client (hiding data
center internals from client)
Server racks
TOR switches
Tier-1 switches
Tier-2 switches
1 2 3 4 5 6 7 8
Data center networks
rich interconnection among switches racks
increased throughput between racks (multiple routing
paths possible)
increased reliability via redundancy
Link Layer 5-87
Link layer LANs outline
51 introduction services
52 error detection correction
53 multiple access protocols
54 LANs addressing ARP
Ethernet
switches
VLANS
55 link virtualization MPLS
56 data center networking
57 a day in the life of a web request
Link Layer 5-88
Synthesis a day in the life of a web request
journey down protocol stack complete application transport network link
putting-it-all-together synthesis goal identify review understand protocols (at all
layers) involved in seemingly simple scenario requesting www page
scenario student attaches laptop to campus network requestsreceives wwwgooglecom
Link Layer 5-89
A day in the life scenario
Comcast network
68800013
Googlersquos network
64233160019 64233169105
web server
DNS server
school network
68802024
web page
browser
router
(runs DHCP)
Link Layer 5-90
A day in the lifehellip connecting to the Internet
connecting laptop needs to get its own IP address addr of first-hop router addr of DNS server use DHCP
DHCP
UDP
IP
Eth
Phy
DHCP
DHCP
DHCP
DHCP
DHCP
DHCP
UDP
IP
Eth
Phy
DHCP
DHCP
DHCP
DHCP DHCP
DHCP request encapsulated in UDP encapsulated in IP encapsulated in 8023 Ethernet
Ethernet frame broadcast
(dest FFFFFFFFFFFF) on LAN received at router running DHCP server
Ethernet demuxed to IP demuxed UDP demuxed to DHCP
router
(runs DHCP)
Link Layer 5-91
DHCP server formulates DHCP ACK containing clientrsquos IP address IP address of first-hop router for client name amp IP address of DNS server
DHCP
UDP
IP
Eth
Phy
DHCP
DHCP
DHCP
DHCP
DHCP
UDP
IP
Eth
Phy
DHCP
DHCP
DHCP
DHCP
DHCP
encapsulation at DHCP server frame forwarded (switch learning) through LAN demultiplexing at client
Client now has IP address knows name amp addr of DNS
server IP address of its first-hop router
DHCP client receives DHCP ACK reply
A day in the lifehellip connecting to the Internet
router
(runs DHCP)
Link Layer 5-92
A day in the lifehellip ARP (before DNS before HTTP)
before sending HTTP request need IP address of wwwgooglecom DNS
DNS
UDP
IP
Eth
Phy
DNS
DNS
DNS
DNS query created encapsulated in UDP encapsulated in IP encapsulated in Eth To send frame to router need MAC address of router interface ARP
ARP query broadcast received by
router which replies with ARP reply giving MAC address of router interface
client now knows MAC address of first hop router so can now send frame containing DNS query
ARP query
Eth
Phy
ARP
ARP
ARP reply
router
(runs DHCP)
Link Layer 5-93
DNS
UDP
IP
Eth
Phy
DNS
DNS
DNS
DNS
DNS
IP datagram containing DNS query forwarded via LAN switch from client to 1st hop router
IP datagram forwarded from campus network into comcast network routed (tables created by RIP OSPF IS-IS andor BGP routing protocols) to DNS server
demuxrsquoed to DNS server
DNS server replies to client with IP address of wwwgooglecom
Comcast network
68800013
DNS server
DNS
UDP
IP
Eth
Phy
DNS
DNS
DNS
DNS
A day in the lifehellip using DNS
router
(runs DHCP)
Link Layer 5-94
A day in the lifehellipTCP connection carrying HTTP
HTTP
TCP
IP
Eth
Phy
HTTP
to send HTTP request client first opens TCP socket to web server
TCP SYN segment (step 1 in 3-way handshake) inter-domain routed to web server
TCP connection established 64233169105
web server
SYN
SYN
SYN
SYN
TCP
IP
Eth
Phy
SYN
SYN
SYN
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
web server responds with TCP SYNACK (step 2 in 3-way handshake)
router
(runs DHCP)
Link Layer 5-95
A day in the lifehellip HTTP requestreply
HTTP
TCP
IP
Eth
Phy
HTTP
HTTP request sent into TCP socket
IP datagram containing HTTP request routed to wwwgooglecom
IP datagram containing HTTP reply routed back to client
64233169105
web server
HTTP
TCP
IP
Eth
Phy web server responds with
HTTP reply (containing web page)
HTTP
HTTP
HTTP HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
web page finally () displayed
Link Layer 5-96
Chapter 5 Summary
principles behind data link layer services error detection correction
sharing a broadcast channel multiple access
link layer addressing
instantiation and implementation of various link layer technologies Ethernet
switched LANS VLANs
virtualized networks as a link layer MPLS
synthesis a day in the life of a web request
Link Layer 5-97
Chapter 5 letrsquos take a breath
journey down protocol stack complete (except PHY)
solid understanding of networking principles practice
hellip could stop here hellip but lots of interesting topics wireless
multimedia
security
network management
Link Layer 5-36
polling master node ldquoinvitesrdquo
slave nodes to transmit in turn
typically used with ldquodumbrdquo slave devices
concerns
polling overhead
latency
single point of failure (master)
master
slaves
poll
data
data
ldquoTaking turnsrdquo MAC protocols
Link Layer 5-37
token passing control token passed
from one node to next sequentially
token message
concerns
token overhead
latency
single point of failure (token)
T
data
(nothing
to send)
T
ldquoTaking turnsrdquo MAC protocols
cable headend
CMTS
ISP
cable modem termination system
multiple 40Mbps downstream (broadcast) channels
single CMTS transmits into channels
multiple 30 Mbps upstream channels
multiple access all users contend for certain upstream channel time slots (others assigned)
Cable access network
cable modem
splitter
hellip
hellip
Internet framesTV channels control transmitted downstream at different frequencies
upstream Internet frames TV control transmitted upstream at different frequencies in time slots
Link Layer 5-39
DOCSIS data over cable service interface spec FDM over upstream downstream frequency channels
TDM upstream some slots assigned some have contention
downstream MAP frame assigns upstream slots
request for upstream slots (and data) transmitted random access (binary backoff) in selected slots
MAP frame for
Interval [t1 t2]
Residences with cable modems
Downstream channel i
Upstream channel j
t1 t2
Assigned minislots containing cable modem
upstream data frames
Minislots containing
minislots request frames
cable headend
CMTS
Cable access network
Link Layer 5-40
Summary of MAC protocols
channel partitioning by time frequency or code Time Division Frequency Division
random access (dynamic)
ALOHA S-ALOHA CSMA CSMACD
carrier sensing easy in some technologies (wire) hard in others (wireless)
CSMACD used in Ethernet
CSMACA used in 80211
taking turns
polling from central site token passing
bluetooth FDDI token ring
Link Layer 5-41
Link layer LANs outline
51 introduction services
52 error detection correction
53 multiple access protocols
54 LANs addressing ARP
Ethernet
switches
VLANS
55 link virtualization MPLS
56 data center networking
57 a day in the life of a web request
Link Layer 5-42
MAC addresses and ARP
32-bit IP address network-layer address for interface
used for layer 3 (network layer) forwarding
MAC (or LAN or physical or Ethernet) address function used lsquolocallyrdquo to get frame from one interface to
another physically-connected interface (same network in IP-addressing sense)
48 bit MAC address (for most LANs) burned in NIC ROM also sometimes software settable
eg 1A-2F-BB-76-09-AD
hexadecimal (base 16) notation
(each ldquonumberrdquo represents 4 bits)
Link Layer 5-43
LAN addresses and ARP
each adapter on LAN has unique LAN address
adapter
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN
(wired or
wireless)
Link Layer 5-44
LAN addresses (more)
MAC address allocation administered by IEEE
manufacturer buys portion of MAC address space (to assure uniqueness)
analogy MAC address like Social Security Number
IP address like postal address
MAC flat address portability can move LAN card from one LAN to another
IP hierarchical address not portable address depends on IP subnet to which node is
attached
Link Layer 5-45
ARP address resolution protocol
ARP table each IP node (host router) on LAN has table
IPMAC address mappings for some LAN nodes
lt IP address MAC address TTLgt
TTL (Time To Live) time after which address mapping will be forgotten (typically 20 min)
Question how to determine
interfacersquos MAC address
knowing its IP address
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN
137196723
137196778
137196714
137196788
Link Layer 5-46
ARP protocol same LAN
A wants to send datagram to B Brsquos MAC address not in
Arsquos ARP table
A broadcasts ARP query packet containing Bs IP address dest MAC address = FF-FF-
FF-FF-FF-FF
all nodes on LAN receive ARP query
B receives ARP packet replies to A with its (Bs) MAC address frame sent to Arsquos MAC
address (unicast)
A caches (saves) IP-to-MAC address pair in its ARP table until information becomes old (times out) soft state information that
times out (goes away) unless refreshed
ARP is ldquoplug-and-playrdquo nodes create their ARP
tables without intervention from net administrator
Link Layer 5-47
walkthrough send datagram from A to B via R
focus on addressing ndash at IP (datagram) and MAC layer (frame)
assume A knows Brsquos IP address
assume A knows IP address of first hop router R (how)
assume A knows Rrsquos MAC address (how)
Addressing routing to another LAN
R
1A-23-F9-CD-06-9B 222222222220
111111111110 E6-E9-00-17-BB-4B CC-49-DE-D0-AB-7D
111111111112
111111111111
74-29-9C-E8-FF-55
A
222222222222
49-BD-D2-C7-56-2A
222222222221 88-B2-2F-54-1A-0F
B
R
1A-23-F9-CD-06-9B 222222222220
111111111110 E6-E9-00-17-BB-4B CC-49-DE-D0-AB-7D
111111111112
111111111111
74-29-9C-E8-FF-55
A
222222222222
49-BD-D2-C7-56-2A
222222222221 88-B2-2F-54-1A-0F
B
Link Layer 5-48
Addressing routing to another LAN
IP
Eth
Phy
IP src 111111111111
IP dest 222222222222
A creates IP datagram with IP source A destination B
A creates link-layer frame with Rs MAC address as dest frame contains A-to-B IP datagram
MAC src 74-29-9C-E8-FF-55
MAC dest E6-E9-00-17-BB-4B
R
1A-23-F9-CD-06-9B 222222222220
111111111110 E6-E9-00-17-BB-4B CC-49-DE-D0-AB-7D
111111111112
111111111111
74-29-9C-E8-FF-55
A
222222222222
49-BD-D2-C7-56-2A
222222222221 88-B2-2F-54-1A-0F
B
Link Layer 5-49
Addressing routing to another LAN
IP
Eth
Phy
frame sent from A to R
IP
Eth
Phy
frame received at R datagram removed passed up to IP
MAC src 74-29-9C-E8-FF-55
MAC dest E6-E9-00-17-BB-4B
IP src 111111111111
IP dest 222222222222
IP src 111111111111
IP dest 222222222222
R
1A-23-F9-CD-06-9B 222222222220
111111111110 E6-E9-00-17-BB-4B CC-49-DE-D0-AB-7D
111111111112
111111111111
74-29-9C-E8-FF-55
A
222222222222
49-BD-D2-C7-56-2A
222222222221 88-B2-2F-54-1A-0F
B
Link Layer 5-50
Addressing routing to another LAN
IP src 111111111111
IP dest 222222222222
R forwards datagram with IP source A destination B
R creates link-layer frame with Bs MAC address as dest frame contains A-to-B IP datagram
MAC src 1A-23-F9-CD-06-9B
MAC dest 49-BD-D2-C7-56-2A
IP
Eth
Phy
IP
Eth
Phy
R
1A-23-F9-CD-06-9B 222222222220
111111111110 E6-E9-00-17-BB-4B CC-49-DE-D0-AB-7D
111111111112
111111111111
74-29-9C-E8-FF-55
A
222222222222
49-BD-D2-C7-56-2A
222222222221 88-B2-2F-54-1A-0F
B
Link Layer 5-51
Addressing routing to another LAN R forwards datagram with IP source A destination B
R creates link-layer frame with Bs MAC address as dest frame contains A-to-B IP datagram
IP src 111111111111
IP dest 222222222222
MAC src 1A-23-F9-CD-06-9B
MAC dest 49-BD-D2-C7-56-2A
IP
Eth
Phy
IP
Eth
Phy
R
1A-23-F9-CD-06-9B 222222222220
111111111110 E6-E9-00-17-BB-4B CC-49-DE-D0-AB-7D
111111111112
111111111111
74-29-9C-E8-FF-55
A
222222222222
49-BD-D2-C7-56-2A
222222222221 88-B2-2F-54-1A-0F
B
Link Layer 5-52
Addressing routing to another LAN R forwards datagram with IP source A destination B
R creates link-layer frame with Bs MAC address as dest frame contains A-to-B IP datagram
IP src 111111111111
IP dest 222222222222
MAC src 1A-23-F9-CD-06-9B
MAC dest 49-BD-D2-C7-56-2A
IP
Eth
Phy
Link Layer 5-53
Link layer LANs outline
51 introduction services
52 error detection correction
53 multiple access protocols
54 LANs addressing ARP
Ethernet
switches
VLANS
55 link virtualization MPLS
56 data center networking
57 a day in the life of a web request
Link Layer 5-54
Ethernet
ldquodominantrdquo wired LAN technology
cheap $20 for NIC
first widely used LAN technology
simpler cheaper than token LANs and ATM
kept up with speed race 10 Mbps ndash 10 Gbps
Metcalfersquos Ethernet sketch
Link Layer 5-55
Ethernet physical topology
bus popular through mid 90s all nodes in same collision domain (can collide with each
other)
star prevails today active switch in center
each ldquospokerdquo runs a (separate) Ethernet protocol (nodes
do not collide with each other)
switch
bus coaxial cable star
Link Layer 5-56
Ethernet frame structure
sending adapter encapsulates IP datagram (or other network layer protocol packet) in Ethernet frame
preamble
7 bytes with pattern 10101010 followed by one byte with pattern 10101011
used to synchronize receiver sender clock rates
dest address
source address
data (payload) CRC preamble
type
Link Layer 5-57
Ethernet frame structure (more)
addresses 6 byte source destination MAC addresses if adapter receives frame with matching destination
address or with broadcast address (eg ARP packet) it passes data in frame to network layer protocol
otherwise adapter discards frame
type indicates higher layer protocol (mostly IP but others possible eg Novell IPX AppleTalk)
CRC cyclic redundancy check at receiver error detected frame is dropped
dest address
source address
data (payload) CRC preamble
type
Link Layer 5-58
Ethernet unreliable connectionless
connectionless no handshaking between sending and receiving NICs
unreliable receiving NIC doesnt send acks or nacks to sending NIC
data in dropped frames recovered only if initial sender uses higher layer rdt (eg TCP) otherwise dropped data lost
Ethernetrsquos MAC protocol unslotted CSMACD wth binary backoff
Link Layer 5-59
8023 Ethernet standards link amp physical layers
many different Ethernet standards
common MAC protocol and frame format
different speeds 2 Mbps 10 Mbps 100 Mbps 1Gbps 10G bps
different physical layer media fiber cable
application
transport
network
link
physical
MAC protocol
and frame format
100BASE-TX
100BASE-T4
100BASE-FX 100BASE-T2
100BASE-SX 100BASE-BX
fiber physical layer copper (twister
pair) physical layer
Link Layer 5-60
Link layer LANs outline
51 introduction services
52 error detection correction
53 multiple access protocols
54 LANs addressing ARP
Ethernet
switches
VLANS
55 link virtualization MPLS
56 data center networking
57 a day in the life of a web request
Link Layer 5-61
Ethernet switch link-layer device takes an active role
store forward Ethernet frames
examine incoming framersquos MAC address selectively forward frame to one-or-more outgoing links when frame is to be forwarded on segment uses CSMACD to access segment
transparent
hosts are unaware of presence of switches
plug-and-play self-learning
switches do not need to be configured
Link Layer 5-62
Switch multiple simultaneous transmissions
hosts have dedicated direct connection to switch
switches buffer packets
Ethernet protocol used on each incoming link but no collisions full duplex
each link is its own collision domain
switching A-to-Arsquo and B-to-Brsquo can transmit simultaneously without collisions switch with six interfaces
(123456)
A
Arsquo
B
Brsquo C
Crsquo
1 2
3 4 5
6
Link Layer 5-63
Switch forwarding table
Q how does switch know Arsquo reachable via interface 4 Brsquo reachable via interface 5
switch with six interfaces
(123456)
A
Arsquo
B
Brsquo C
Crsquo
1 2
3 4 5
6 A each switch has a switch table each entry
(MAC address of host interface to
reach host time stamp)
looks like a routing table
Q how are entries created maintained in switch table
something like a routing protocol
A
Arsquo
B
Brsquo C
Crsquo
1 2
3 4 5
6
Link Layer 5-64
Switch self-learning
switch learns which hosts can be reached through which interfaces
when frame received switch ldquolearnsrdquo location of sender incoming LAN segment
records senderlocation pair in switch table
A Arsquo
Source A
Dest Arsquo
MAC addr interface TTL
Switch table
(initially empty) A 1 60
Link Layer 5-65
Switch frame filteringforwarding
when frame received at switch 1 record incoming link MAC address of sending host
2 index switch table using MAC destination address
3 if entry found for destination then
if destination on segment from which frame arrived then drop frame
else forward frame on interface indicated by entry
else flood forward on all interfaces except arriving
interface
A
Arsquo
B
Brsquo C
Crsquo
1 2
3 4 5
6
Link Layer 5-66
Self-learning forwarding example
A Arsquo
Source A
Dest Arsquo
MAC addr interface TTL
switch table
(initially empty) A 1 60
A Arsquo A Arsquo A Arsquo A Arsquo A Arsquo
frame destination Arsquo locaton unknown flood
Arsquo A
destination A location
known
Arsquo 4 60
selectively send
on just one link
Link Layer 5-67
Interconnecting switches
switches can be connected together
Q sending from A to G - how does S1 know to forward frame destined to F via S4 and S3
A self learning (works exactly the same as in single-switch case)
A
B
S1
C D
E
F
S2
S4
S3
H
I
G
Link Layer 5-68
Self-learning multi-switch example
Suppose C sends frame to I I responds to C
Q show switch tables and packet forwarding in S1 S2 S3 S4
A
B
S1
C D
E
F
S2
S4
S3
H
I
G
Link Layer 5-69
Institutional network
to external
network
router
IP subnet
mail server
web server
Link Layer 5-70
Switches vs routers
both are store-and-forward
routers network-layer devices (examine network-layer headers)
switches link-layer devices (examine link-layer headers)
both have forwarding tables
routers compute tables using routing algorithms IP addresses
switches learn forwarding table using flooding learning MAC addresses
application
transport
network
link
physical
network
link
physical
link
physical
switch
datagram
application
transport
network
link
physical
frame
frame
frame
datagram
Link Layer 5-71
VLANs motivation
consider CS user moves office to
EE but wants connect to CS switch
single broadcast domain
all layer-2 broadcast traffic (ARP DHCP unknown location of destination MAC address) must cross entire LAN
securityprivacy efficiency issues
Computer
Science Electrical
Engineering
Computer
Engineering
Link Layer 5-72
VLANs port-based VLAN switch ports
grouped (by switch management software) so that single physical switch helliphellip
switch(es) supporting
VLAN capabilities can
be configured to
define multiple virtual
LANS over single
physical LAN
infrastructure
Virtual Local
Area Network 1
8
9
16 10 2
7
hellip
Electrical Engineering
(VLAN ports 1-8)
Computer Science
(VLAN ports 9-15)
15
hellip
Electrical Engineering
(VLAN ports 1-8)
hellip
1
8 2
7 9
16 10
15
hellip
Computer Science
(VLAN ports 9-16)
hellip operates as multiple virtual switches
Link Layer 5-73
Port-based VLAN
1
8
9
16 10 2
7
hellip
Electrical Engineering
(VLAN ports 1-8)
Computer Science
(VLAN ports 9-15)
15
hellip
traffic isolation frames tofrom ports 1-8 can only reach ports 1-8 can also define VLAN based on
MAC addresses of endpoints rather than switch port
dynamic membership ports can be dynamically assigned among VLANs
router
forwarding between VLANS done via routing (just as with separate switches) in practice vendors sell combined
switches plus routers
Link Layer 5-74
VLANS spanning multiple switches
trunk port carries frames between VLANS defined over multiple physical switches frames forwarded within VLAN between switches canrsquot be vanilla
8021 frames (must carry VLAN ID info)
8021q protocol addsremoved additional header fields for frames forwarded between trunk ports
1
8
9
10 2
7
hellip
Electrical Engineering
(VLAN ports 1-8)
Computer Science
(VLAN ports 9-15)
15
hellip
2
7 3
Ports 235 belong to EE VLAN
Ports 4678 belong to CS VLAN
5
4 6 8 16
1
Link Layer 5-75
type
2-byte Tag Protocol Identifier
(value 81-00)
Tag Control Information (12 bit VLAN ID field
3 bit priority field like IP TOS)
Recomputed CRC
8021Q VLAN frame format
8021 frame
8021Q frame
dest address
source address
data (payload) CRC preamble
dest address
source address
preamble data (payload) CRC
type
Link Layer 5-76
Link layer LANs outline
51 introduction services
52 error detection correction
53 multiple access protocols
54 LANs addressing ARP
Ethernet
switches
VLANS
55 link virtualization MPLS
56 data center networking
57 a day in the life of a web request
Link Layer 5-77
Multiprotocol label switching (MPLS)
initial goal high-speed IP forwarding using fixed length label (instead of IP address) fast lookup using fixed length identifier (rather than
shortest prefix matching)
borrowing ideas from Virtual Circuit (VC) approach
but IP datagram still keeps IP address
PPP or Ethernet
header IP header remainder of link-layer frame MPLS header
label Exp S TTL
20 3 1 5
Link Layer 5-78
MPLS capable routers
aka label-switched router
forward packets to outgoing interface based only on label value (donrsquot inspect IP address) MPLS forwarding table distinct from IP forwarding tables
flexibility MPLS forwarding decisions can differ from those of IP use destination and source addresses to route flows to
same destination differently (traffic engineering)
re-route flows quickly if link fails pre-computed backup paths (useful for VoIP)
Link Layer 5-79
R2
D
R3 R5
A
R6
MPLS versus IP paths
IP router IP routing path to destination determined
by destination address alone
R4
Link Layer 5-80
R2
D
R3 R4
R5
A
R6
MPLS versus IP paths
IP-only router
IP routing path to destination determined by destination address alone
MPLS and IP router
MPLS routing path to destination can be based on source and dest address fast reroute precompute backup routes in
case of link failure
entry router (R4) can use different MPLS
routes to A based eg on source address
Link Layer 5-81
MPLS signaling
modify OSPF IS-IS link-state flooding protocols to carry info used by MPLS routing eg link bandwidth amount of ldquoreservedrdquo link bandwidth
D
R4
R5
A
R6
entry MPLS router uses RSVP-TE signaling protocol to set up MPLS forwarding at downstream routers
modified link state flooding
RSVP-TE
Link Layer 5-82
R1 R2
D
R3 R4 R5
0
1
0 0
A
R6
in out out
label label dest interface
6 - A 0
in out out
label label dest interface
10 6 A 1
12 9 D 0
in out out
label label dest interface
10 A 0
12 D 0
1
in out out
label label dest interface
8 6 A 0
0
8 A 1
MPLS forwarding tables
Link Layer 5-83
Link layer LANs outline
51 introduction services
52 error detection correction
53 multiple access protocols
54 LANs addressing ARP
Ethernet
switches
VLANS
55 link virtualization MPLS
56 data center networking
57 a day in the life of a web request
Link Layer 5-84
Data center networks
10rsquos to 100rsquos of thousands of hosts often closely coupled in close proximity e-business (eg Amazon)
content-servers (eg YouTube Akamai Apple Microsoft)
search engines data mining (eg Google)
challenges
multiple applications each serving massive numbers of clients
managingbalancing load avoiding processing networking data bottlenecks
Inside a 40-ft Microsoft container
Chicago data center
Link Layer 5-85
Server racks
TOR switches
Tier-1 switches
Tier-2 switches
Load balancer
Load balancer
B
1 2 3 4 5 6 7 8
A C
Border router
Access router
Internet
Data center networks
load balancer application-layer routing receives external client requests
directs workload within data center
returns results to external client (hiding data
center internals from client)
Server racks
TOR switches
Tier-1 switches
Tier-2 switches
1 2 3 4 5 6 7 8
Data center networks
rich interconnection among switches racks
increased throughput between racks (multiple routing
paths possible)
increased reliability via redundancy
Link Layer 5-87
Link layer LANs outline
51 introduction services
52 error detection correction
53 multiple access protocols
54 LANs addressing ARP
Ethernet
switches
VLANS
55 link virtualization MPLS
56 data center networking
57 a day in the life of a web request
Link Layer 5-88
Synthesis a day in the life of a web request
journey down protocol stack complete application transport network link
putting-it-all-together synthesis goal identify review understand protocols (at all
layers) involved in seemingly simple scenario requesting www page
scenario student attaches laptop to campus network requestsreceives wwwgooglecom
Link Layer 5-89
A day in the life scenario
Comcast network
68800013
Googlersquos network
64233160019 64233169105
web server
DNS server
school network
68802024
web page
browser
router
(runs DHCP)
Link Layer 5-90
A day in the lifehellip connecting to the Internet
connecting laptop needs to get its own IP address addr of first-hop router addr of DNS server use DHCP
DHCP
UDP
IP
Eth
Phy
DHCP
DHCP
DHCP
DHCP
DHCP
DHCP
UDP
IP
Eth
Phy
DHCP
DHCP
DHCP
DHCP DHCP
DHCP request encapsulated in UDP encapsulated in IP encapsulated in 8023 Ethernet
Ethernet frame broadcast
(dest FFFFFFFFFFFF) on LAN received at router running DHCP server
Ethernet demuxed to IP demuxed UDP demuxed to DHCP
router
(runs DHCP)
Link Layer 5-91
DHCP server formulates DHCP ACK containing clientrsquos IP address IP address of first-hop router for client name amp IP address of DNS server
DHCP
UDP
IP
Eth
Phy
DHCP
DHCP
DHCP
DHCP
DHCP
UDP
IP
Eth
Phy
DHCP
DHCP
DHCP
DHCP
DHCP
encapsulation at DHCP server frame forwarded (switch learning) through LAN demultiplexing at client
Client now has IP address knows name amp addr of DNS
server IP address of its first-hop router
DHCP client receives DHCP ACK reply
A day in the lifehellip connecting to the Internet
router
(runs DHCP)
Link Layer 5-92
A day in the lifehellip ARP (before DNS before HTTP)
before sending HTTP request need IP address of wwwgooglecom DNS
DNS
UDP
IP
Eth
Phy
DNS
DNS
DNS
DNS query created encapsulated in UDP encapsulated in IP encapsulated in Eth To send frame to router need MAC address of router interface ARP
ARP query broadcast received by
router which replies with ARP reply giving MAC address of router interface
client now knows MAC address of first hop router so can now send frame containing DNS query
ARP query
Eth
Phy
ARP
ARP
ARP reply
router
(runs DHCP)
Link Layer 5-93
DNS
UDP
IP
Eth
Phy
DNS
DNS
DNS
DNS
DNS
IP datagram containing DNS query forwarded via LAN switch from client to 1st hop router
IP datagram forwarded from campus network into comcast network routed (tables created by RIP OSPF IS-IS andor BGP routing protocols) to DNS server
demuxrsquoed to DNS server
DNS server replies to client with IP address of wwwgooglecom
Comcast network
68800013
DNS server
DNS
UDP
IP
Eth
Phy
DNS
DNS
DNS
DNS
A day in the lifehellip using DNS
router
(runs DHCP)
Link Layer 5-94
A day in the lifehellipTCP connection carrying HTTP
HTTP
TCP
IP
Eth
Phy
HTTP
to send HTTP request client first opens TCP socket to web server
TCP SYN segment (step 1 in 3-way handshake) inter-domain routed to web server
TCP connection established 64233169105
web server
SYN
SYN
SYN
SYN
TCP
IP
Eth
Phy
SYN
SYN
SYN
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
web server responds with TCP SYNACK (step 2 in 3-way handshake)
router
(runs DHCP)
Link Layer 5-95
A day in the lifehellip HTTP requestreply
HTTP
TCP
IP
Eth
Phy
HTTP
HTTP request sent into TCP socket
IP datagram containing HTTP request routed to wwwgooglecom
IP datagram containing HTTP reply routed back to client
64233169105
web server
HTTP
TCP
IP
Eth
Phy web server responds with
HTTP reply (containing web page)
HTTP
HTTP
HTTP HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
web page finally () displayed
Link Layer 5-96
Chapter 5 Summary
principles behind data link layer services error detection correction
sharing a broadcast channel multiple access
link layer addressing
instantiation and implementation of various link layer technologies Ethernet
switched LANS VLANs
virtualized networks as a link layer MPLS
synthesis a day in the life of a web request
Link Layer 5-97
Chapter 5 letrsquos take a breath
journey down protocol stack complete (except PHY)
solid understanding of networking principles practice
hellip could stop here hellip but lots of interesting topics wireless
multimedia
security
network management
Link Layer 5-37
token passing control token passed
from one node to next sequentially
token message
concerns
token overhead
latency
single point of failure (token)
T
data
(nothing
to send)
T
ldquoTaking turnsrdquo MAC protocols
cable headend
CMTS
ISP
cable modem termination system
multiple 40Mbps downstream (broadcast) channels
single CMTS transmits into channels
multiple 30 Mbps upstream channels
multiple access all users contend for certain upstream channel time slots (others assigned)
Cable access network
cable modem
splitter
hellip
hellip
Internet framesTV channels control transmitted downstream at different frequencies
upstream Internet frames TV control transmitted upstream at different frequencies in time slots
Link Layer 5-39
DOCSIS data over cable service interface spec FDM over upstream downstream frequency channels
TDM upstream some slots assigned some have contention
downstream MAP frame assigns upstream slots
request for upstream slots (and data) transmitted random access (binary backoff) in selected slots
MAP frame for
Interval [t1 t2]
Residences with cable modems
Downstream channel i
Upstream channel j
t1 t2
Assigned minislots containing cable modem
upstream data frames
Minislots containing
minislots request frames
cable headend
CMTS
Cable access network
Link Layer 5-40
Summary of MAC protocols
channel partitioning by time frequency or code Time Division Frequency Division
random access (dynamic)
ALOHA S-ALOHA CSMA CSMACD
carrier sensing easy in some technologies (wire) hard in others (wireless)
CSMACD used in Ethernet
CSMACA used in 80211
taking turns
polling from central site token passing
bluetooth FDDI token ring
Link Layer 5-41
Link layer LANs outline
51 introduction services
52 error detection correction
53 multiple access protocols
54 LANs addressing ARP
Ethernet
switches
VLANS
55 link virtualization MPLS
56 data center networking
57 a day in the life of a web request
Link Layer 5-42
MAC addresses and ARP
32-bit IP address network-layer address for interface
used for layer 3 (network layer) forwarding
MAC (or LAN or physical or Ethernet) address function used lsquolocallyrdquo to get frame from one interface to
another physically-connected interface (same network in IP-addressing sense)
48 bit MAC address (for most LANs) burned in NIC ROM also sometimes software settable
eg 1A-2F-BB-76-09-AD
hexadecimal (base 16) notation
(each ldquonumberrdquo represents 4 bits)
Link Layer 5-43
LAN addresses and ARP
each adapter on LAN has unique LAN address
adapter
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN
(wired or
wireless)
Link Layer 5-44
LAN addresses (more)
MAC address allocation administered by IEEE
manufacturer buys portion of MAC address space (to assure uniqueness)
analogy MAC address like Social Security Number
IP address like postal address
MAC flat address portability can move LAN card from one LAN to another
IP hierarchical address not portable address depends on IP subnet to which node is
attached
Link Layer 5-45
ARP address resolution protocol
ARP table each IP node (host router) on LAN has table
IPMAC address mappings for some LAN nodes
lt IP address MAC address TTLgt
TTL (Time To Live) time after which address mapping will be forgotten (typically 20 min)
Question how to determine
interfacersquos MAC address
knowing its IP address
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN
137196723
137196778
137196714
137196788
Link Layer 5-46
ARP protocol same LAN
A wants to send datagram to B Brsquos MAC address not in
Arsquos ARP table
A broadcasts ARP query packet containing Bs IP address dest MAC address = FF-FF-
FF-FF-FF-FF
all nodes on LAN receive ARP query
B receives ARP packet replies to A with its (Bs) MAC address frame sent to Arsquos MAC
address (unicast)
A caches (saves) IP-to-MAC address pair in its ARP table until information becomes old (times out) soft state information that
times out (goes away) unless refreshed
ARP is ldquoplug-and-playrdquo nodes create their ARP
tables without intervention from net administrator
Link Layer 5-47
walkthrough send datagram from A to B via R
focus on addressing ndash at IP (datagram) and MAC layer (frame)
assume A knows Brsquos IP address
assume A knows IP address of first hop router R (how)
assume A knows Rrsquos MAC address (how)
Addressing routing to another LAN
R
1A-23-F9-CD-06-9B 222222222220
111111111110 E6-E9-00-17-BB-4B CC-49-DE-D0-AB-7D
111111111112
111111111111
74-29-9C-E8-FF-55
A
222222222222
49-BD-D2-C7-56-2A
222222222221 88-B2-2F-54-1A-0F
B
R
1A-23-F9-CD-06-9B 222222222220
111111111110 E6-E9-00-17-BB-4B CC-49-DE-D0-AB-7D
111111111112
111111111111
74-29-9C-E8-FF-55
A
222222222222
49-BD-D2-C7-56-2A
222222222221 88-B2-2F-54-1A-0F
B
Link Layer 5-48
Addressing routing to another LAN
IP
Eth
Phy
IP src 111111111111
IP dest 222222222222
A creates IP datagram with IP source A destination B
A creates link-layer frame with Rs MAC address as dest frame contains A-to-B IP datagram
MAC src 74-29-9C-E8-FF-55
MAC dest E6-E9-00-17-BB-4B
R
1A-23-F9-CD-06-9B 222222222220
111111111110 E6-E9-00-17-BB-4B CC-49-DE-D0-AB-7D
111111111112
111111111111
74-29-9C-E8-FF-55
A
222222222222
49-BD-D2-C7-56-2A
222222222221 88-B2-2F-54-1A-0F
B
Link Layer 5-49
Addressing routing to another LAN
IP
Eth
Phy
frame sent from A to R
IP
Eth
Phy
frame received at R datagram removed passed up to IP
MAC src 74-29-9C-E8-FF-55
MAC dest E6-E9-00-17-BB-4B
IP src 111111111111
IP dest 222222222222
IP src 111111111111
IP dest 222222222222
R
1A-23-F9-CD-06-9B 222222222220
111111111110 E6-E9-00-17-BB-4B CC-49-DE-D0-AB-7D
111111111112
111111111111
74-29-9C-E8-FF-55
A
222222222222
49-BD-D2-C7-56-2A
222222222221 88-B2-2F-54-1A-0F
B
Link Layer 5-50
Addressing routing to another LAN
IP src 111111111111
IP dest 222222222222
R forwards datagram with IP source A destination B
R creates link-layer frame with Bs MAC address as dest frame contains A-to-B IP datagram
MAC src 1A-23-F9-CD-06-9B
MAC dest 49-BD-D2-C7-56-2A
IP
Eth
Phy
IP
Eth
Phy
R
1A-23-F9-CD-06-9B 222222222220
111111111110 E6-E9-00-17-BB-4B CC-49-DE-D0-AB-7D
111111111112
111111111111
74-29-9C-E8-FF-55
A
222222222222
49-BD-D2-C7-56-2A
222222222221 88-B2-2F-54-1A-0F
B
Link Layer 5-51
Addressing routing to another LAN R forwards datagram with IP source A destination B
R creates link-layer frame with Bs MAC address as dest frame contains A-to-B IP datagram
IP src 111111111111
IP dest 222222222222
MAC src 1A-23-F9-CD-06-9B
MAC dest 49-BD-D2-C7-56-2A
IP
Eth
Phy
IP
Eth
Phy
R
1A-23-F9-CD-06-9B 222222222220
111111111110 E6-E9-00-17-BB-4B CC-49-DE-D0-AB-7D
111111111112
111111111111
74-29-9C-E8-FF-55
A
222222222222
49-BD-D2-C7-56-2A
222222222221 88-B2-2F-54-1A-0F
B
Link Layer 5-52
Addressing routing to another LAN R forwards datagram with IP source A destination B
R creates link-layer frame with Bs MAC address as dest frame contains A-to-B IP datagram
IP src 111111111111
IP dest 222222222222
MAC src 1A-23-F9-CD-06-9B
MAC dest 49-BD-D2-C7-56-2A
IP
Eth
Phy
Link Layer 5-53
Link layer LANs outline
51 introduction services
52 error detection correction
53 multiple access protocols
54 LANs addressing ARP
Ethernet
switches
VLANS
55 link virtualization MPLS
56 data center networking
57 a day in the life of a web request
Link Layer 5-54
Ethernet
ldquodominantrdquo wired LAN technology
cheap $20 for NIC
first widely used LAN technology
simpler cheaper than token LANs and ATM
kept up with speed race 10 Mbps ndash 10 Gbps
Metcalfersquos Ethernet sketch
Link Layer 5-55
Ethernet physical topology
bus popular through mid 90s all nodes in same collision domain (can collide with each
other)
star prevails today active switch in center
each ldquospokerdquo runs a (separate) Ethernet protocol (nodes
do not collide with each other)
switch
bus coaxial cable star
Link Layer 5-56
Ethernet frame structure
sending adapter encapsulates IP datagram (or other network layer protocol packet) in Ethernet frame
preamble
7 bytes with pattern 10101010 followed by one byte with pattern 10101011
used to synchronize receiver sender clock rates
dest address
source address
data (payload) CRC preamble
type
Link Layer 5-57
Ethernet frame structure (more)
addresses 6 byte source destination MAC addresses if adapter receives frame with matching destination
address or with broadcast address (eg ARP packet) it passes data in frame to network layer protocol
otherwise adapter discards frame
type indicates higher layer protocol (mostly IP but others possible eg Novell IPX AppleTalk)
CRC cyclic redundancy check at receiver error detected frame is dropped
dest address
source address
data (payload) CRC preamble
type
Link Layer 5-58
Ethernet unreliable connectionless
connectionless no handshaking between sending and receiving NICs
unreliable receiving NIC doesnt send acks or nacks to sending NIC
data in dropped frames recovered only if initial sender uses higher layer rdt (eg TCP) otherwise dropped data lost
Ethernetrsquos MAC protocol unslotted CSMACD wth binary backoff
Link Layer 5-59
8023 Ethernet standards link amp physical layers
many different Ethernet standards
common MAC protocol and frame format
different speeds 2 Mbps 10 Mbps 100 Mbps 1Gbps 10G bps
different physical layer media fiber cable
application
transport
network
link
physical
MAC protocol
and frame format
100BASE-TX
100BASE-T4
100BASE-FX 100BASE-T2
100BASE-SX 100BASE-BX
fiber physical layer copper (twister
pair) physical layer
Link Layer 5-60
Link layer LANs outline
51 introduction services
52 error detection correction
53 multiple access protocols
54 LANs addressing ARP
Ethernet
switches
VLANS
55 link virtualization MPLS
56 data center networking
57 a day in the life of a web request
Link Layer 5-61
Ethernet switch link-layer device takes an active role
store forward Ethernet frames
examine incoming framersquos MAC address selectively forward frame to one-or-more outgoing links when frame is to be forwarded on segment uses CSMACD to access segment
transparent
hosts are unaware of presence of switches
plug-and-play self-learning
switches do not need to be configured
Link Layer 5-62
Switch multiple simultaneous transmissions
hosts have dedicated direct connection to switch
switches buffer packets
Ethernet protocol used on each incoming link but no collisions full duplex
each link is its own collision domain
switching A-to-Arsquo and B-to-Brsquo can transmit simultaneously without collisions switch with six interfaces
(123456)
A
Arsquo
B
Brsquo C
Crsquo
1 2
3 4 5
6
Link Layer 5-63
Switch forwarding table
Q how does switch know Arsquo reachable via interface 4 Brsquo reachable via interface 5
switch with six interfaces
(123456)
A
Arsquo
B
Brsquo C
Crsquo
1 2
3 4 5
6 A each switch has a switch table each entry
(MAC address of host interface to
reach host time stamp)
looks like a routing table
Q how are entries created maintained in switch table
something like a routing protocol
A
Arsquo
B
Brsquo C
Crsquo
1 2
3 4 5
6
Link Layer 5-64
Switch self-learning
switch learns which hosts can be reached through which interfaces
when frame received switch ldquolearnsrdquo location of sender incoming LAN segment
records senderlocation pair in switch table
A Arsquo
Source A
Dest Arsquo
MAC addr interface TTL
Switch table
(initially empty) A 1 60
Link Layer 5-65
Switch frame filteringforwarding
when frame received at switch 1 record incoming link MAC address of sending host
2 index switch table using MAC destination address
3 if entry found for destination then
if destination on segment from which frame arrived then drop frame
else forward frame on interface indicated by entry
else flood forward on all interfaces except arriving
interface
A
Arsquo
B
Brsquo C
Crsquo
1 2
3 4 5
6
Link Layer 5-66
Self-learning forwarding example
A Arsquo
Source A
Dest Arsquo
MAC addr interface TTL
switch table
(initially empty) A 1 60
A Arsquo A Arsquo A Arsquo A Arsquo A Arsquo
frame destination Arsquo locaton unknown flood
Arsquo A
destination A location
known
Arsquo 4 60
selectively send
on just one link
Link Layer 5-67
Interconnecting switches
switches can be connected together
Q sending from A to G - how does S1 know to forward frame destined to F via S4 and S3
A self learning (works exactly the same as in single-switch case)
A
B
S1
C D
E
F
S2
S4
S3
H
I
G
Link Layer 5-68
Self-learning multi-switch example
Suppose C sends frame to I I responds to C
Q show switch tables and packet forwarding in S1 S2 S3 S4
A
B
S1
C D
E
F
S2
S4
S3
H
I
G
Link Layer 5-69
Institutional network
to external
network
router
IP subnet
mail server
web server
Link Layer 5-70
Switches vs routers
both are store-and-forward
routers network-layer devices (examine network-layer headers)
switches link-layer devices (examine link-layer headers)
both have forwarding tables
routers compute tables using routing algorithms IP addresses
switches learn forwarding table using flooding learning MAC addresses
application
transport
network
link
physical
network
link
physical
link
physical
switch
datagram
application
transport
network
link
physical
frame
frame
frame
datagram
Link Layer 5-71
VLANs motivation
consider CS user moves office to
EE but wants connect to CS switch
single broadcast domain
all layer-2 broadcast traffic (ARP DHCP unknown location of destination MAC address) must cross entire LAN
securityprivacy efficiency issues
Computer
Science Electrical
Engineering
Computer
Engineering
Link Layer 5-72
VLANs port-based VLAN switch ports
grouped (by switch management software) so that single physical switch helliphellip
switch(es) supporting
VLAN capabilities can
be configured to
define multiple virtual
LANS over single
physical LAN
infrastructure
Virtual Local
Area Network 1
8
9
16 10 2
7
hellip
Electrical Engineering
(VLAN ports 1-8)
Computer Science
(VLAN ports 9-15)
15
hellip
Electrical Engineering
(VLAN ports 1-8)
hellip
1
8 2
7 9
16 10
15
hellip
Computer Science
(VLAN ports 9-16)
hellip operates as multiple virtual switches
Link Layer 5-73
Port-based VLAN
1
8
9
16 10 2
7
hellip
Electrical Engineering
(VLAN ports 1-8)
Computer Science
(VLAN ports 9-15)
15
hellip
traffic isolation frames tofrom ports 1-8 can only reach ports 1-8 can also define VLAN based on
MAC addresses of endpoints rather than switch port
dynamic membership ports can be dynamically assigned among VLANs
router
forwarding between VLANS done via routing (just as with separate switches) in practice vendors sell combined
switches plus routers
Link Layer 5-74
VLANS spanning multiple switches
trunk port carries frames between VLANS defined over multiple physical switches frames forwarded within VLAN between switches canrsquot be vanilla
8021 frames (must carry VLAN ID info)
8021q protocol addsremoved additional header fields for frames forwarded between trunk ports
1
8
9
10 2
7
hellip
Electrical Engineering
(VLAN ports 1-8)
Computer Science
(VLAN ports 9-15)
15
hellip
2
7 3
Ports 235 belong to EE VLAN
Ports 4678 belong to CS VLAN
5
4 6 8 16
1
Link Layer 5-75
type
2-byte Tag Protocol Identifier
(value 81-00)
Tag Control Information (12 bit VLAN ID field
3 bit priority field like IP TOS)
Recomputed CRC
8021Q VLAN frame format
8021 frame
8021Q frame
dest address
source address
data (payload) CRC preamble
dest address
source address
preamble data (payload) CRC
type
Link Layer 5-76
Link layer LANs outline
51 introduction services
52 error detection correction
53 multiple access protocols
54 LANs addressing ARP
Ethernet
switches
VLANS
55 link virtualization MPLS
56 data center networking
57 a day in the life of a web request
Link Layer 5-77
Multiprotocol label switching (MPLS)
initial goal high-speed IP forwarding using fixed length label (instead of IP address) fast lookup using fixed length identifier (rather than
shortest prefix matching)
borrowing ideas from Virtual Circuit (VC) approach
but IP datagram still keeps IP address
PPP or Ethernet
header IP header remainder of link-layer frame MPLS header
label Exp S TTL
20 3 1 5
Link Layer 5-78
MPLS capable routers
aka label-switched router
forward packets to outgoing interface based only on label value (donrsquot inspect IP address) MPLS forwarding table distinct from IP forwarding tables
flexibility MPLS forwarding decisions can differ from those of IP use destination and source addresses to route flows to
same destination differently (traffic engineering)
re-route flows quickly if link fails pre-computed backup paths (useful for VoIP)
Link Layer 5-79
R2
D
R3 R5
A
R6
MPLS versus IP paths
IP router IP routing path to destination determined
by destination address alone
R4
Link Layer 5-80
R2
D
R3 R4
R5
A
R6
MPLS versus IP paths
IP-only router
IP routing path to destination determined by destination address alone
MPLS and IP router
MPLS routing path to destination can be based on source and dest address fast reroute precompute backup routes in
case of link failure
entry router (R4) can use different MPLS
routes to A based eg on source address
Link Layer 5-81
MPLS signaling
modify OSPF IS-IS link-state flooding protocols to carry info used by MPLS routing eg link bandwidth amount of ldquoreservedrdquo link bandwidth
D
R4
R5
A
R6
entry MPLS router uses RSVP-TE signaling protocol to set up MPLS forwarding at downstream routers
modified link state flooding
RSVP-TE
Link Layer 5-82
R1 R2
D
R3 R4 R5
0
1
0 0
A
R6
in out out
label label dest interface
6 - A 0
in out out
label label dest interface
10 6 A 1
12 9 D 0
in out out
label label dest interface
10 A 0
12 D 0
1
in out out
label label dest interface
8 6 A 0
0
8 A 1
MPLS forwarding tables
Link Layer 5-83
Link layer LANs outline
51 introduction services
52 error detection correction
53 multiple access protocols
54 LANs addressing ARP
Ethernet
switches
VLANS
55 link virtualization MPLS
56 data center networking
57 a day in the life of a web request
Link Layer 5-84
Data center networks
10rsquos to 100rsquos of thousands of hosts often closely coupled in close proximity e-business (eg Amazon)
content-servers (eg YouTube Akamai Apple Microsoft)
search engines data mining (eg Google)
challenges
multiple applications each serving massive numbers of clients
managingbalancing load avoiding processing networking data bottlenecks
Inside a 40-ft Microsoft container
Chicago data center
Link Layer 5-85
Server racks
TOR switches
Tier-1 switches
Tier-2 switches
Load balancer
Load balancer
B
1 2 3 4 5 6 7 8
A C
Border router
Access router
Internet
Data center networks
load balancer application-layer routing receives external client requests
directs workload within data center
returns results to external client (hiding data
center internals from client)
Server racks
TOR switches
Tier-1 switches
Tier-2 switches
1 2 3 4 5 6 7 8
Data center networks
rich interconnection among switches racks
increased throughput between racks (multiple routing
paths possible)
increased reliability via redundancy
Link Layer 5-87
Link layer LANs outline
51 introduction services
52 error detection correction
53 multiple access protocols
54 LANs addressing ARP
Ethernet
switches
VLANS
55 link virtualization MPLS
56 data center networking
57 a day in the life of a web request
Link Layer 5-88
Synthesis a day in the life of a web request
journey down protocol stack complete application transport network link
putting-it-all-together synthesis goal identify review understand protocols (at all
layers) involved in seemingly simple scenario requesting www page
scenario student attaches laptop to campus network requestsreceives wwwgooglecom
Link Layer 5-89
A day in the life scenario
Comcast network
68800013
Googlersquos network
64233160019 64233169105
web server
DNS server
school network
68802024
web page
browser
router
(runs DHCP)
Link Layer 5-90
A day in the lifehellip connecting to the Internet
connecting laptop needs to get its own IP address addr of first-hop router addr of DNS server use DHCP
DHCP
UDP
IP
Eth
Phy
DHCP
DHCP
DHCP
DHCP
DHCP
DHCP
UDP
IP
Eth
Phy
DHCP
DHCP
DHCP
DHCP DHCP
DHCP request encapsulated in UDP encapsulated in IP encapsulated in 8023 Ethernet
Ethernet frame broadcast
(dest FFFFFFFFFFFF) on LAN received at router running DHCP server
Ethernet demuxed to IP demuxed UDP demuxed to DHCP
router
(runs DHCP)
Link Layer 5-91
DHCP server formulates DHCP ACK containing clientrsquos IP address IP address of first-hop router for client name amp IP address of DNS server
DHCP
UDP
IP
Eth
Phy
DHCP
DHCP
DHCP
DHCP
DHCP
UDP
IP
Eth
Phy
DHCP
DHCP
DHCP
DHCP
DHCP
encapsulation at DHCP server frame forwarded (switch learning) through LAN demultiplexing at client
Client now has IP address knows name amp addr of DNS
server IP address of its first-hop router
DHCP client receives DHCP ACK reply
A day in the lifehellip connecting to the Internet
router
(runs DHCP)
Link Layer 5-92
A day in the lifehellip ARP (before DNS before HTTP)
before sending HTTP request need IP address of wwwgooglecom DNS
DNS
UDP
IP
Eth
Phy
DNS
DNS
DNS
DNS query created encapsulated in UDP encapsulated in IP encapsulated in Eth To send frame to router need MAC address of router interface ARP
ARP query broadcast received by
router which replies with ARP reply giving MAC address of router interface
client now knows MAC address of first hop router so can now send frame containing DNS query
ARP query
Eth
Phy
ARP
ARP
ARP reply
router
(runs DHCP)
Link Layer 5-93
DNS
UDP
IP
Eth
Phy
DNS
DNS
DNS
DNS
DNS
IP datagram containing DNS query forwarded via LAN switch from client to 1st hop router
IP datagram forwarded from campus network into comcast network routed (tables created by RIP OSPF IS-IS andor BGP routing protocols) to DNS server
demuxrsquoed to DNS server
DNS server replies to client with IP address of wwwgooglecom
Comcast network
68800013
DNS server
DNS
UDP
IP
Eth
Phy
DNS
DNS
DNS
DNS
A day in the lifehellip using DNS
router
(runs DHCP)
Link Layer 5-94
A day in the lifehellipTCP connection carrying HTTP
HTTP
TCP
IP
Eth
Phy
HTTP
to send HTTP request client first opens TCP socket to web server
TCP SYN segment (step 1 in 3-way handshake) inter-domain routed to web server
TCP connection established 64233169105
web server
SYN
SYN
SYN
SYN
TCP
IP
Eth
Phy
SYN
SYN
SYN
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
web server responds with TCP SYNACK (step 2 in 3-way handshake)
router
(runs DHCP)
Link Layer 5-95
A day in the lifehellip HTTP requestreply
HTTP
TCP
IP
Eth
Phy
HTTP
HTTP request sent into TCP socket
IP datagram containing HTTP request routed to wwwgooglecom
IP datagram containing HTTP reply routed back to client
64233169105
web server
HTTP
TCP
IP
Eth
Phy web server responds with
HTTP reply (containing web page)
HTTP
HTTP
HTTP HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
web page finally () displayed
Link Layer 5-96
Chapter 5 Summary
principles behind data link layer services error detection correction
sharing a broadcast channel multiple access
link layer addressing
instantiation and implementation of various link layer technologies Ethernet
switched LANS VLANs
virtualized networks as a link layer MPLS
synthesis a day in the life of a web request
Link Layer 5-97
Chapter 5 letrsquos take a breath
journey down protocol stack complete (except PHY)
solid understanding of networking principles practice
hellip could stop here hellip but lots of interesting topics wireless
multimedia
security
network management
cable headend
CMTS
ISP
cable modem termination system
multiple 40Mbps downstream (broadcast) channels
single CMTS transmits into channels
multiple 30 Mbps upstream channels
multiple access all users contend for certain upstream channel time slots (others assigned)
Cable access network
cable modem
splitter
hellip
hellip
Internet framesTV channels control transmitted downstream at different frequencies
upstream Internet frames TV control transmitted upstream at different frequencies in time slots
Link Layer 5-39
DOCSIS data over cable service interface spec FDM over upstream downstream frequency channels
TDM upstream some slots assigned some have contention
downstream MAP frame assigns upstream slots
request for upstream slots (and data) transmitted random access (binary backoff) in selected slots
MAP frame for
Interval [t1 t2]
Residences with cable modems
Downstream channel i
Upstream channel j
t1 t2
Assigned minislots containing cable modem
upstream data frames
Minislots containing
minislots request frames
cable headend
CMTS
Cable access network
Link Layer 5-40
Summary of MAC protocols
channel partitioning by time frequency or code Time Division Frequency Division
random access (dynamic)
ALOHA S-ALOHA CSMA CSMACD
carrier sensing easy in some technologies (wire) hard in others (wireless)
CSMACD used in Ethernet
CSMACA used in 80211
taking turns
polling from central site token passing
bluetooth FDDI token ring
Link Layer 5-41
Link layer LANs outline
51 introduction services
52 error detection correction
53 multiple access protocols
54 LANs addressing ARP
Ethernet
switches
VLANS
55 link virtualization MPLS
56 data center networking
57 a day in the life of a web request
Link Layer 5-42
MAC addresses and ARP
32-bit IP address network-layer address for interface
used for layer 3 (network layer) forwarding
MAC (or LAN or physical or Ethernet) address function used lsquolocallyrdquo to get frame from one interface to
another physically-connected interface (same network in IP-addressing sense)
48 bit MAC address (for most LANs) burned in NIC ROM also sometimes software settable
eg 1A-2F-BB-76-09-AD
hexadecimal (base 16) notation
(each ldquonumberrdquo represents 4 bits)
Link Layer 5-43
LAN addresses and ARP
each adapter on LAN has unique LAN address
adapter
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN
(wired or
wireless)
Link Layer 5-44
LAN addresses (more)
MAC address allocation administered by IEEE
manufacturer buys portion of MAC address space (to assure uniqueness)
analogy MAC address like Social Security Number
IP address like postal address
MAC flat address portability can move LAN card from one LAN to another
IP hierarchical address not portable address depends on IP subnet to which node is
attached
Link Layer 5-45
ARP address resolution protocol
ARP table each IP node (host router) on LAN has table
IPMAC address mappings for some LAN nodes
lt IP address MAC address TTLgt
TTL (Time To Live) time after which address mapping will be forgotten (typically 20 min)
Question how to determine
interfacersquos MAC address
knowing its IP address
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN
137196723
137196778
137196714
137196788
Link Layer 5-46
ARP protocol same LAN
A wants to send datagram to B Brsquos MAC address not in
Arsquos ARP table
A broadcasts ARP query packet containing Bs IP address dest MAC address = FF-FF-
FF-FF-FF-FF
all nodes on LAN receive ARP query
B receives ARP packet replies to A with its (Bs) MAC address frame sent to Arsquos MAC
address (unicast)
A caches (saves) IP-to-MAC address pair in its ARP table until information becomes old (times out) soft state information that
times out (goes away) unless refreshed
ARP is ldquoplug-and-playrdquo nodes create their ARP
tables without intervention from net administrator
Link Layer 5-47
walkthrough send datagram from A to B via R
focus on addressing ndash at IP (datagram) and MAC layer (frame)
assume A knows Brsquos IP address
assume A knows IP address of first hop router R (how)
assume A knows Rrsquos MAC address (how)
Addressing routing to another LAN
R
1A-23-F9-CD-06-9B 222222222220
111111111110 E6-E9-00-17-BB-4B CC-49-DE-D0-AB-7D
111111111112
111111111111
74-29-9C-E8-FF-55
A
222222222222
49-BD-D2-C7-56-2A
222222222221 88-B2-2F-54-1A-0F
B
R
1A-23-F9-CD-06-9B 222222222220
111111111110 E6-E9-00-17-BB-4B CC-49-DE-D0-AB-7D
111111111112
111111111111
74-29-9C-E8-FF-55
A
222222222222
49-BD-D2-C7-56-2A
222222222221 88-B2-2F-54-1A-0F
B
Link Layer 5-48
Addressing routing to another LAN
IP
Eth
Phy
IP src 111111111111
IP dest 222222222222
A creates IP datagram with IP source A destination B
A creates link-layer frame with Rs MAC address as dest frame contains A-to-B IP datagram
MAC src 74-29-9C-E8-FF-55
MAC dest E6-E9-00-17-BB-4B
R
1A-23-F9-CD-06-9B 222222222220
111111111110 E6-E9-00-17-BB-4B CC-49-DE-D0-AB-7D
111111111112
111111111111
74-29-9C-E8-FF-55
A
222222222222
49-BD-D2-C7-56-2A
222222222221 88-B2-2F-54-1A-0F
B
Link Layer 5-49
Addressing routing to another LAN
IP
Eth
Phy
frame sent from A to R
IP
Eth
Phy
frame received at R datagram removed passed up to IP
MAC src 74-29-9C-E8-FF-55
MAC dest E6-E9-00-17-BB-4B
IP src 111111111111
IP dest 222222222222
IP src 111111111111
IP dest 222222222222
R
1A-23-F9-CD-06-9B 222222222220
111111111110 E6-E9-00-17-BB-4B CC-49-DE-D0-AB-7D
111111111112
111111111111
74-29-9C-E8-FF-55
A
222222222222
49-BD-D2-C7-56-2A
222222222221 88-B2-2F-54-1A-0F
B
Link Layer 5-50
Addressing routing to another LAN
IP src 111111111111
IP dest 222222222222
R forwards datagram with IP source A destination B
R creates link-layer frame with Bs MAC address as dest frame contains A-to-B IP datagram
MAC src 1A-23-F9-CD-06-9B
MAC dest 49-BD-D2-C7-56-2A
IP
Eth
Phy
IP
Eth
Phy
R
1A-23-F9-CD-06-9B 222222222220
111111111110 E6-E9-00-17-BB-4B CC-49-DE-D0-AB-7D
111111111112
111111111111
74-29-9C-E8-FF-55
A
222222222222
49-BD-D2-C7-56-2A
222222222221 88-B2-2F-54-1A-0F
B
Link Layer 5-51
Addressing routing to another LAN R forwards datagram with IP source A destination B
R creates link-layer frame with Bs MAC address as dest frame contains A-to-B IP datagram
IP src 111111111111
IP dest 222222222222
MAC src 1A-23-F9-CD-06-9B
MAC dest 49-BD-D2-C7-56-2A
IP
Eth
Phy
IP
Eth
Phy
R
1A-23-F9-CD-06-9B 222222222220
111111111110 E6-E9-00-17-BB-4B CC-49-DE-D0-AB-7D
111111111112
111111111111
74-29-9C-E8-FF-55
A
222222222222
49-BD-D2-C7-56-2A
222222222221 88-B2-2F-54-1A-0F
B
Link Layer 5-52
Addressing routing to another LAN R forwards datagram with IP source A destination B
R creates link-layer frame with Bs MAC address as dest frame contains A-to-B IP datagram
IP src 111111111111
IP dest 222222222222
MAC src 1A-23-F9-CD-06-9B
MAC dest 49-BD-D2-C7-56-2A
IP
Eth
Phy
Link Layer 5-53
Link layer LANs outline
51 introduction services
52 error detection correction
53 multiple access protocols
54 LANs addressing ARP
Ethernet
switches
VLANS
55 link virtualization MPLS
56 data center networking
57 a day in the life of a web request
Link Layer 5-54
Ethernet
ldquodominantrdquo wired LAN technology
cheap $20 for NIC
first widely used LAN technology
simpler cheaper than token LANs and ATM
kept up with speed race 10 Mbps ndash 10 Gbps
Metcalfersquos Ethernet sketch
Link Layer 5-55
Ethernet physical topology
bus popular through mid 90s all nodes in same collision domain (can collide with each
other)
star prevails today active switch in center
each ldquospokerdquo runs a (separate) Ethernet protocol (nodes
do not collide with each other)
switch
bus coaxial cable star
Link Layer 5-56
Ethernet frame structure
sending adapter encapsulates IP datagram (or other network layer protocol packet) in Ethernet frame
preamble
7 bytes with pattern 10101010 followed by one byte with pattern 10101011
used to synchronize receiver sender clock rates
dest address
source address
data (payload) CRC preamble
type
Link Layer 5-57
Ethernet frame structure (more)
addresses 6 byte source destination MAC addresses if adapter receives frame with matching destination
address or with broadcast address (eg ARP packet) it passes data in frame to network layer protocol
otherwise adapter discards frame
type indicates higher layer protocol (mostly IP but others possible eg Novell IPX AppleTalk)
CRC cyclic redundancy check at receiver error detected frame is dropped
dest address
source address
data (payload) CRC preamble
type
Link Layer 5-58
Ethernet unreliable connectionless
connectionless no handshaking between sending and receiving NICs
unreliable receiving NIC doesnt send acks or nacks to sending NIC
data in dropped frames recovered only if initial sender uses higher layer rdt (eg TCP) otherwise dropped data lost
Ethernetrsquos MAC protocol unslotted CSMACD wth binary backoff
Link Layer 5-59
8023 Ethernet standards link amp physical layers
many different Ethernet standards
common MAC protocol and frame format
different speeds 2 Mbps 10 Mbps 100 Mbps 1Gbps 10G bps
different physical layer media fiber cable
application
transport
network
link
physical
MAC protocol
and frame format
100BASE-TX
100BASE-T4
100BASE-FX 100BASE-T2
100BASE-SX 100BASE-BX
fiber physical layer copper (twister
pair) physical layer
Link Layer 5-60
Link layer LANs outline
51 introduction services
52 error detection correction
53 multiple access protocols
54 LANs addressing ARP
Ethernet
switches
VLANS
55 link virtualization MPLS
56 data center networking
57 a day in the life of a web request
Link Layer 5-61
Ethernet switch link-layer device takes an active role
store forward Ethernet frames
examine incoming framersquos MAC address selectively forward frame to one-or-more outgoing links when frame is to be forwarded on segment uses CSMACD to access segment
transparent
hosts are unaware of presence of switches
plug-and-play self-learning
switches do not need to be configured
Link Layer 5-62
Switch multiple simultaneous transmissions
hosts have dedicated direct connection to switch
switches buffer packets
Ethernet protocol used on each incoming link but no collisions full duplex
each link is its own collision domain
switching A-to-Arsquo and B-to-Brsquo can transmit simultaneously without collisions switch with six interfaces
(123456)
A
Arsquo
B
Brsquo C
Crsquo
1 2
3 4 5
6
Link Layer 5-63
Switch forwarding table
Q how does switch know Arsquo reachable via interface 4 Brsquo reachable via interface 5
switch with six interfaces
(123456)
A
Arsquo
B
Brsquo C
Crsquo
1 2
3 4 5
6 A each switch has a switch table each entry
(MAC address of host interface to
reach host time stamp)
looks like a routing table
Q how are entries created maintained in switch table
something like a routing protocol
A
Arsquo
B
Brsquo C
Crsquo
1 2
3 4 5
6
Link Layer 5-64
Switch self-learning
switch learns which hosts can be reached through which interfaces
when frame received switch ldquolearnsrdquo location of sender incoming LAN segment
records senderlocation pair in switch table
A Arsquo
Source A
Dest Arsquo
MAC addr interface TTL
Switch table
(initially empty) A 1 60
Link Layer 5-65
Switch frame filteringforwarding
when frame received at switch 1 record incoming link MAC address of sending host
2 index switch table using MAC destination address
3 if entry found for destination then
if destination on segment from which frame arrived then drop frame
else forward frame on interface indicated by entry
else flood forward on all interfaces except arriving
interface
A
Arsquo
B
Brsquo C
Crsquo
1 2
3 4 5
6
Link Layer 5-66
Self-learning forwarding example
A Arsquo
Source A
Dest Arsquo
MAC addr interface TTL
switch table
(initially empty) A 1 60
A Arsquo A Arsquo A Arsquo A Arsquo A Arsquo
frame destination Arsquo locaton unknown flood
Arsquo A
destination A location
known
Arsquo 4 60
selectively send
on just one link
Link Layer 5-67
Interconnecting switches
switches can be connected together
Q sending from A to G - how does S1 know to forward frame destined to F via S4 and S3
A self learning (works exactly the same as in single-switch case)
A
B
S1
C D
E
F
S2
S4
S3
H
I
G
Link Layer 5-68
Self-learning multi-switch example
Suppose C sends frame to I I responds to C
Q show switch tables and packet forwarding in S1 S2 S3 S4
A
B
S1
C D
E
F
S2
S4
S3
H
I
G
Link Layer 5-69
Institutional network
to external
network
router
IP subnet
mail server
web server
Link Layer 5-70
Switches vs routers
both are store-and-forward
routers network-layer devices (examine network-layer headers)
switches link-layer devices (examine link-layer headers)
both have forwarding tables
routers compute tables using routing algorithms IP addresses
switches learn forwarding table using flooding learning MAC addresses
application
transport
network
link
physical
network
link
physical
link
physical
switch
datagram
application
transport
network
link
physical
frame
frame
frame
datagram
Link Layer 5-71
VLANs motivation
consider CS user moves office to
EE but wants connect to CS switch
single broadcast domain
all layer-2 broadcast traffic (ARP DHCP unknown location of destination MAC address) must cross entire LAN
securityprivacy efficiency issues
Computer
Science Electrical
Engineering
Computer
Engineering
Link Layer 5-72
VLANs port-based VLAN switch ports
grouped (by switch management software) so that single physical switch helliphellip
switch(es) supporting
VLAN capabilities can
be configured to
define multiple virtual
LANS over single
physical LAN
infrastructure
Virtual Local
Area Network 1
8
9
16 10 2
7
hellip
Electrical Engineering
(VLAN ports 1-8)
Computer Science
(VLAN ports 9-15)
15
hellip
Electrical Engineering
(VLAN ports 1-8)
hellip
1
8 2
7 9
16 10
15
hellip
Computer Science
(VLAN ports 9-16)
hellip operates as multiple virtual switches
Link Layer 5-73
Port-based VLAN
1
8
9
16 10 2
7
hellip
Electrical Engineering
(VLAN ports 1-8)
Computer Science
(VLAN ports 9-15)
15
hellip
traffic isolation frames tofrom ports 1-8 can only reach ports 1-8 can also define VLAN based on
MAC addresses of endpoints rather than switch port
dynamic membership ports can be dynamically assigned among VLANs
router
forwarding between VLANS done via routing (just as with separate switches) in practice vendors sell combined
switches plus routers
Link Layer 5-74
VLANS spanning multiple switches
trunk port carries frames between VLANS defined over multiple physical switches frames forwarded within VLAN between switches canrsquot be vanilla
8021 frames (must carry VLAN ID info)
8021q protocol addsremoved additional header fields for frames forwarded between trunk ports
1
8
9
10 2
7
hellip
Electrical Engineering
(VLAN ports 1-8)
Computer Science
(VLAN ports 9-15)
15
hellip
2
7 3
Ports 235 belong to EE VLAN
Ports 4678 belong to CS VLAN
5
4 6 8 16
1
Link Layer 5-75
type
2-byte Tag Protocol Identifier
(value 81-00)
Tag Control Information (12 bit VLAN ID field
3 bit priority field like IP TOS)
Recomputed CRC
8021Q VLAN frame format
8021 frame
8021Q frame
dest address
source address
data (payload) CRC preamble
dest address
source address
preamble data (payload) CRC
type
Link Layer 5-76
Link layer LANs outline
51 introduction services
52 error detection correction
53 multiple access protocols
54 LANs addressing ARP
Ethernet
switches
VLANS
55 link virtualization MPLS
56 data center networking
57 a day in the life of a web request
Link Layer 5-77
Multiprotocol label switching (MPLS)
initial goal high-speed IP forwarding using fixed length label (instead of IP address) fast lookup using fixed length identifier (rather than
shortest prefix matching)
borrowing ideas from Virtual Circuit (VC) approach
but IP datagram still keeps IP address
PPP or Ethernet
header IP header remainder of link-layer frame MPLS header
label Exp S TTL
20 3 1 5
Link Layer 5-78
MPLS capable routers
aka label-switched router
forward packets to outgoing interface based only on label value (donrsquot inspect IP address) MPLS forwarding table distinct from IP forwarding tables
flexibility MPLS forwarding decisions can differ from those of IP use destination and source addresses to route flows to
same destination differently (traffic engineering)
re-route flows quickly if link fails pre-computed backup paths (useful for VoIP)
Link Layer 5-79
R2
D
R3 R5
A
R6
MPLS versus IP paths
IP router IP routing path to destination determined
by destination address alone
R4
Link Layer 5-80
R2
D
R3 R4
R5
A
R6
MPLS versus IP paths
IP-only router
IP routing path to destination determined by destination address alone
MPLS and IP router
MPLS routing path to destination can be based on source and dest address fast reroute precompute backup routes in
case of link failure
entry router (R4) can use different MPLS
routes to A based eg on source address
Link Layer 5-81
MPLS signaling
modify OSPF IS-IS link-state flooding protocols to carry info used by MPLS routing eg link bandwidth amount of ldquoreservedrdquo link bandwidth
D
R4
R5
A
R6
entry MPLS router uses RSVP-TE signaling protocol to set up MPLS forwarding at downstream routers
modified link state flooding
RSVP-TE
Link Layer 5-82
R1 R2
D
R3 R4 R5
0
1
0 0
A
R6
in out out
label label dest interface
6 - A 0
in out out
label label dest interface
10 6 A 1
12 9 D 0
in out out
label label dest interface
10 A 0
12 D 0
1
in out out
label label dest interface
8 6 A 0
0
8 A 1
MPLS forwarding tables
Link Layer 5-83
Link layer LANs outline
51 introduction services
52 error detection correction
53 multiple access protocols
54 LANs addressing ARP
Ethernet
switches
VLANS
55 link virtualization MPLS
56 data center networking
57 a day in the life of a web request
Link Layer 5-84
Data center networks
10rsquos to 100rsquos of thousands of hosts often closely coupled in close proximity e-business (eg Amazon)
content-servers (eg YouTube Akamai Apple Microsoft)
search engines data mining (eg Google)
challenges
multiple applications each serving massive numbers of clients
managingbalancing load avoiding processing networking data bottlenecks
Inside a 40-ft Microsoft container
Chicago data center
Link Layer 5-85
Server racks
TOR switches
Tier-1 switches
Tier-2 switches
Load balancer
Load balancer
B
1 2 3 4 5 6 7 8
A C
Border router
Access router
Internet
Data center networks
load balancer application-layer routing receives external client requests
directs workload within data center
returns results to external client (hiding data
center internals from client)
Server racks
TOR switches
Tier-1 switches
Tier-2 switches
1 2 3 4 5 6 7 8
Data center networks
rich interconnection among switches racks
increased throughput between racks (multiple routing
paths possible)
increased reliability via redundancy
Link Layer 5-87
Link layer LANs outline
51 introduction services
52 error detection correction
53 multiple access protocols
54 LANs addressing ARP
Ethernet
switches
VLANS
55 link virtualization MPLS
56 data center networking
57 a day in the life of a web request
Link Layer 5-88
Synthesis a day in the life of a web request
journey down protocol stack complete application transport network link
putting-it-all-together synthesis goal identify review understand protocols (at all
layers) involved in seemingly simple scenario requesting www page
scenario student attaches laptop to campus network requestsreceives wwwgooglecom
Link Layer 5-89
A day in the life scenario
Comcast network
68800013
Googlersquos network
64233160019 64233169105
web server
DNS server
school network
68802024
web page
browser
router
(runs DHCP)
Link Layer 5-90
A day in the lifehellip connecting to the Internet
connecting laptop needs to get its own IP address addr of first-hop router addr of DNS server use DHCP
DHCP
UDP
IP
Eth
Phy
DHCP
DHCP
DHCP
DHCP
DHCP
DHCP
UDP
IP
Eth
Phy
DHCP
DHCP
DHCP
DHCP DHCP
DHCP request encapsulated in UDP encapsulated in IP encapsulated in 8023 Ethernet
Ethernet frame broadcast
(dest FFFFFFFFFFFF) on LAN received at router running DHCP server
Ethernet demuxed to IP demuxed UDP demuxed to DHCP
router
(runs DHCP)
Link Layer 5-91
DHCP server formulates DHCP ACK containing clientrsquos IP address IP address of first-hop router for client name amp IP address of DNS server
DHCP
UDP
IP
Eth
Phy
DHCP
DHCP
DHCP
DHCP
DHCP
UDP
IP
Eth
Phy
DHCP
DHCP
DHCP
DHCP
DHCP
encapsulation at DHCP server frame forwarded (switch learning) through LAN demultiplexing at client
Client now has IP address knows name amp addr of DNS
server IP address of its first-hop router
DHCP client receives DHCP ACK reply
A day in the lifehellip connecting to the Internet
router
(runs DHCP)
Link Layer 5-92
A day in the lifehellip ARP (before DNS before HTTP)
before sending HTTP request need IP address of wwwgooglecom DNS
DNS
UDP
IP
Eth
Phy
DNS
DNS
DNS
DNS query created encapsulated in UDP encapsulated in IP encapsulated in Eth To send frame to router need MAC address of router interface ARP
ARP query broadcast received by
router which replies with ARP reply giving MAC address of router interface
client now knows MAC address of first hop router so can now send frame containing DNS query
ARP query
Eth
Phy
ARP
ARP
ARP reply
router
(runs DHCP)
Link Layer 5-93
DNS
UDP
IP
Eth
Phy
DNS
DNS
DNS
DNS
DNS
IP datagram containing DNS query forwarded via LAN switch from client to 1st hop router
IP datagram forwarded from campus network into comcast network routed (tables created by RIP OSPF IS-IS andor BGP routing protocols) to DNS server
demuxrsquoed to DNS server
DNS server replies to client with IP address of wwwgooglecom
Comcast network
68800013
DNS server
DNS
UDP
IP
Eth
Phy
DNS
DNS
DNS
DNS
A day in the lifehellip using DNS
router
(runs DHCP)
Link Layer 5-94
A day in the lifehellipTCP connection carrying HTTP
HTTP
TCP
IP
Eth
Phy
HTTP
to send HTTP request client first opens TCP socket to web server
TCP SYN segment (step 1 in 3-way handshake) inter-domain routed to web server
TCP connection established 64233169105
web server
SYN
SYN
SYN
SYN
TCP
IP
Eth
Phy
SYN
SYN
SYN
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
web server responds with TCP SYNACK (step 2 in 3-way handshake)
router
(runs DHCP)
Link Layer 5-95
A day in the lifehellip HTTP requestreply
HTTP
TCP
IP
Eth
Phy
HTTP
HTTP request sent into TCP socket
IP datagram containing HTTP request routed to wwwgooglecom
IP datagram containing HTTP reply routed back to client
64233169105
web server
HTTP
TCP
IP
Eth
Phy web server responds with
HTTP reply (containing web page)
HTTP
HTTP
HTTP HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
web page finally () displayed
Link Layer 5-96
Chapter 5 Summary
principles behind data link layer services error detection correction
sharing a broadcast channel multiple access
link layer addressing
instantiation and implementation of various link layer technologies Ethernet
switched LANS VLANs
virtualized networks as a link layer MPLS
synthesis a day in the life of a web request
Link Layer 5-97
Chapter 5 letrsquos take a breath
journey down protocol stack complete (except PHY)
solid understanding of networking principles practice
hellip could stop here hellip but lots of interesting topics wireless
multimedia
security
network management
Link Layer 5-39
DOCSIS data over cable service interface spec FDM over upstream downstream frequency channels
TDM upstream some slots assigned some have contention
downstream MAP frame assigns upstream slots
request for upstream slots (and data) transmitted random access (binary backoff) in selected slots
MAP frame for
Interval [t1 t2]
Residences with cable modems
Downstream channel i
Upstream channel j
t1 t2
Assigned minislots containing cable modem
upstream data frames
Minislots containing
minislots request frames
cable headend
CMTS
Cable access network
Link Layer 5-40
Summary of MAC protocols
channel partitioning by time frequency or code Time Division Frequency Division
random access (dynamic)
ALOHA S-ALOHA CSMA CSMACD
carrier sensing easy in some technologies (wire) hard in others (wireless)
CSMACD used in Ethernet
CSMACA used in 80211
taking turns
polling from central site token passing
bluetooth FDDI token ring
Link Layer 5-41
Link layer LANs outline
51 introduction services
52 error detection correction
53 multiple access protocols
54 LANs addressing ARP
Ethernet
switches
VLANS
55 link virtualization MPLS
56 data center networking
57 a day in the life of a web request
Link Layer 5-42
MAC addresses and ARP
32-bit IP address network-layer address for interface
used for layer 3 (network layer) forwarding
MAC (or LAN or physical or Ethernet) address function used lsquolocallyrdquo to get frame from one interface to
another physically-connected interface (same network in IP-addressing sense)
48 bit MAC address (for most LANs) burned in NIC ROM also sometimes software settable
eg 1A-2F-BB-76-09-AD
hexadecimal (base 16) notation
(each ldquonumberrdquo represents 4 bits)
Link Layer 5-43
LAN addresses and ARP
each adapter on LAN has unique LAN address
adapter
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN
(wired or
wireless)
Link Layer 5-44
LAN addresses (more)
MAC address allocation administered by IEEE
manufacturer buys portion of MAC address space (to assure uniqueness)
analogy MAC address like Social Security Number
IP address like postal address
MAC flat address portability can move LAN card from one LAN to another
IP hierarchical address not portable address depends on IP subnet to which node is
attached
Link Layer 5-45
ARP address resolution protocol
ARP table each IP node (host router) on LAN has table
IPMAC address mappings for some LAN nodes
lt IP address MAC address TTLgt
TTL (Time To Live) time after which address mapping will be forgotten (typically 20 min)
Question how to determine
interfacersquos MAC address
knowing its IP address
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN
137196723
137196778
137196714
137196788
Link Layer 5-46
ARP protocol same LAN
A wants to send datagram to B Brsquos MAC address not in
Arsquos ARP table
A broadcasts ARP query packet containing Bs IP address dest MAC address = FF-FF-
FF-FF-FF-FF
all nodes on LAN receive ARP query
B receives ARP packet replies to A with its (Bs) MAC address frame sent to Arsquos MAC
address (unicast)
A caches (saves) IP-to-MAC address pair in its ARP table until information becomes old (times out) soft state information that
times out (goes away) unless refreshed
ARP is ldquoplug-and-playrdquo nodes create their ARP
tables without intervention from net administrator
Link Layer 5-47
walkthrough send datagram from A to B via R
focus on addressing ndash at IP (datagram) and MAC layer (frame)
assume A knows Brsquos IP address
assume A knows IP address of first hop router R (how)
assume A knows Rrsquos MAC address (how)
Addressing routing to another LAN
R
1A-23-F9-CD-06-9B 222222222220
111111111110 E6-E9-00-17-BB-4B CC-49-DE-D0-AB-7D
111111111112
111111111111
74-29-9C-E8-FF-55
A
222222222222
49-BD-D2-C7-56-2A
222222222221 88-B2-2F-54-1A-0F
B
R
1A-23-F9-CD-06-9B 222222222220
111111111110 E6-E9-00-17-BB-4B CC-49-DE-D0-AB-7D
111111111112
111111111111
74-29-9C-E8-FF-55
A
222222222222
49-BD-D2-C7-56-2A
222222222221 88-B2-2F-54-1A-0F
B
Link Layer 5-48
Addressing routing to another LAN
IP
Eth
Phy
IP src 111111111111
IP dest 222222222222
A creates IP datagram with IP source A destination B
A creates link-layer frame with Rs MAC address as dest frame contains A-to-B IP datagram
MAC src 74-29-9C-E8-FF-55
MAC dest E6-E9-00-17-BB-4B
R
1A-23-F9-CD-06-9B 222222222220
111111111110 E6-E9-00-17-BB-4B CC-49-DE-D0-AB-7D
111111111112
111111111111
74-29-9C-E8-FF-55
A
222222222222
49-BD-D2-C7-56-2A
222222222221 88-B2-2F-54-1A-0F
B
Link Layer 5-49
Addressing routing to another LAN
IP
Eth
Phy
frame sent from A to R
IP
Eth
Phy
frame received at R datagram removed passed up to IP
MAC src 74-29-9C-E8-FF-55
MAC dest E6-E9-00-17-BB-4B
IP src 111111111111
IP dest 222222222222
IP src 111111111111
IP dest 222222222222
R
1A-23-F9-CD-06-9B 222222222220
111111111110 E6-E9-00-17-BB-4B CC-49-DE-D0-AB-7D
111111111112
111111111111
74-29-9C-E8-FF-55
A
222222222222
49-BD-D2-C7-56-2A
222222222221 88-B2-2F-54-1A-0F
B
Link Layer 5-50
Addressing routing to another LAN
IP src 111111111111
IP dest 222222222222
R forwards datagram with IP source A destination B
R creates link-layer frame with Bs MAC address as dest frame contains A-to-B IP datagram
MAC src 1A-23-F9-CD-06-9B
MAC dest 49-BD-D2-C7-56-2A
IP
Eth
Phy
IP
Eth
Phy
R
1A-23-F9-CD-06-9B 222222222220
111111111110 E6-E9-00-17-BB-4B CC-49-DE-D0-AB-7D
111111111112
111111111111
74-29-9C-E8-FF-55
A
222222222222
49-BD-D2-C7-56-2A
222222222221 88-B2-2F-54-1A-0F
B
Link Layer 5-51
Addressing routing to another LAN R forwards datagram with IP source A destination B
R creates link-layer frame with Bs MAC address as dest frame contains A-to-B IP datagram
IP src 111111111111
IP dest 222222222222
MAC src 1A-23-F9-CD-06-9B
MAC dest 49-BD-D2-C7-56-2A
IP
Eth
Phy
IP
Eth
Phy
R
1A-23-F9-CD-06-9B 222222222220
111111111110 E6-E9-00-17-BB-4B CC-49-DE-D0-AB-7D
111111111112
111111111111
74-29-9C-E8-FF-55
A
222222222222
49-BD-D2-C7-56-2A
222222222221 88-B2-2F-54-1A-0F
B
Link Layer 5-52
Addressing routing to another LAN R forwards datagram with IP source A destination B
R creates link-layer frame with Bs MAC address as dest frame contains A-to-B IP datagram
IP src 111111111111
IP dest 222222222222
MAC src 1A-23-F9-CD-06-9B
MAC dest 49-BD-D2-C7-56-2A
IP
Eth
Phy
Link Layer 5-53
Link layer LANs outline
51 introduction services
52 error detection correction
53 multiple access protocols
54 LANs addressing ARP
Ethernet
switches
VLANS
55 link virtualization MPLS
56 data center networking
57 a day in the life of a web request
Link Layer 5-54
Ethernet
ldquodominantrdquo wired LAN technology
cheap $20 for NIC
first widely used LAN technology
simpler cheaper than token LANs and ATM
kept up with speed race 10 Mbps ndash 10 Gbps
Metcalfersquos Ethernet sketch
Link Layer 5-55
Ethernet physical topology
bus popular through mid 90s all nodes in same collision domain (can collide with each
other)
star prevails today active switch in center
each ldquospokerdquo runs a (separate) Ethernet protocol (nodes
do not collide with each other)
switch
bus coaxial cable star
Link Layer 5-56
Ethernet frame structure
sending adapter encapsulates IP datagram (or other network layer protocol packet) in Ethernet frame
preamble
7 bytes with pattern 10101010 followed by one byte with pattern 10101011
used to synchronize receiver sender clock rates
dest address
source address
data (payload) CRC preamble
type
Link Layer 5-57
Ethernet frame structure (more)
addresses 6 byte source destination MAC addresses if adapter receives frame with matching destination
address or with broadcast address (eg ARP packet) it passes data in frame to network layer protocol
otherwise adapter discards frame
type indicates higher layer protocol (mostly IP but others possible eg Novell IPX AppleTalk)
CRC cyclic redundancy check at receiver error detected frame is dropped
dest address
source address
data (payload) CRC preamble
type
Link Layer 5-58
Ethernet unreliable connectionless
connectionless no handshaking between sending and receiving NICs
unreliable receiving NIC doesnt send acks or nacks to sending NIC
data in dropped frames recovered only if initial sender uses higher layer rdt (eg TCP) otherwise dropped data lost
Ethernetrsquos MAC protocol unslotted CSMACD wth binary backoff
Link Layer 5-59
8023 Ethernet standards link amp physical layers
many different Ethernet standards
common MAC protocol and frame format
different speeds 2 Mbps 10 Mbps 100 Mbps 1Gbps 10G bps
different physical layer media fiber cable
application
transport
network
link
physical
MAC protocol
and frame format
100BASE-TX
100BASE-T4
100BASE-FX 100BASE-T2
100BASE-SX 100BASE-BX
fiber physical layer copper (twister
pair) physical layer
Link Layer 5-60
Link layer LANs outline
51 introduction services
52 error detection correction
53 multiple access protocols
54 LANs addressing ARP
Ethernet
switches
VLANS
55 link virtualization MPLS
56 data center networking
57 a day in the life of a web request
Link Layer 5-61
Ethernet switch link-layer device takes an active role
store forward Ethernet frames
examine incoming framersquos MAC address selectively forward frame to one-or-more outgoing links when frame is to be forwarded on segment uses CSMACD to access segment
transparent
hosts are unaware of presence of switches
plug-and-play self-learning
switches do not need to be configured
Link Layer 5-62
Switch multiple simultaneous transmissions
hosts have dedicated direct connection to switch
switches buffer packets
Ethernet protocol used on each incoming link but no collisions full duplex
each link is its own collision domain
switching A-to-Arsquo and B-to-Brsquo can transmit simultaneously without collisions switch with six interfaces
(123456)
A
Arsquo
B
Brsquo C
Crsquo
1 2
3 4 5
6
Link Layer 5-63
Switch forwarding table
Q how does switch know Arsquo reachable via interface 4 Brsquo reachable via interface 5
switch with six interfaces
(123456)
A
Arsquo
B
Brsquo C
Crsquo
1 2
3 4 5
6 A each switch has a switch table each entry
(MAC address of host interface to
reach host time stamp)
looks like a routing table
Q how are entries created maintained in switch table
something like a routing protocol
A
Arsquo
B
Brsquo C
Crsquo
1 2
3 4 5
6
Link Layer 5-64
Switch self-learning
switch learns which hosts can be reached through which interfaces
when frame received switch ldquolearnsrdquo location of sender incoming LAN segment
records senderlocation pair in switch table
A Arsquo
Source A
Dest Arsquo
MAC addr interface TTL
Switch table
(initially empty) A 1 60
Link Layer 5-65
Switch frame filteringforwarding
when frame received at switch 1 record incoming link MAC address of sending host
2 index switch table using MAC destination address
3 if entry found for destination then
if destination on segment from which frame arrived then drop frame
else forward frame on interface indicated by entry
else flood forward on all interfaces except arriving
interface
A
Arsquo
B
Brsquo C
Crsquo
1 2
3 4 5
6
Link Layer 5-66
Self-learning forwarding example
A Arsquo
Source A
Dest Arsquo
MAC addr interface TTL
switch table
(initially empty) A 1 60
A Arsquo A Arsquo A Arsquo A Arsquo A Arsquo
frame destination Arsquo locaton unknown flood
Arsquo A
destination A location
known
Arsquo 4 60
selectively send
on just one link
Link Layer 5-67
Interconnecting switches
switches can be connected together
Q sending from A to G - how does S1 know to forward frame destined to F via S4 and S3
A self learning (works exactly the same as in single-switch case)
A
B
S1
C D
E
F
S2
S4
S3
H
I
G
Link Layer 5-68
Self-learning multi-switch example
Suppose C sends frame to I I responds to C
Q show switch tables and packet forwarding in S1 S2 S3 S4
A
B
S1
C D
E
F
S2
S4
S3
H
I
G
Link Layer 5-69
Institutional network
to external
network
router
IP subnet
mail server
web server
Link Layer 5-70
Switches vs routers
both are store-and-forward
routers network-layer devices (examine network-layer headers)
switches link-layer devices (examine link-layer headers)
both have forwarding tables
routers compute tables using routing algorithms IP addresses
switches learn forwarding table using flooding learning MAC addresses
application
transport
network
link
physical
network
link
physical
link
physical
switch
datagram
application
transport
network
link
physical
frame
frame
frame
datagram
Link Layer 5-71
VLANs motivation
consider CS user moves office to
EE but wants connect to CS switch
single broadcast domain
all layer-2 broadcast traffic (ARP DHCP unknown location of destination MAC address) must cross entire LAN
securityprivacy efficiency issues
Computer
Science Electrical
Engineering
Computer
Engineering
Link Layer 5-72
VLANs port-based VLAN switch ports
grouped (by switch management software) so that single physical switch helliphellip
switch(es) supporting
VLAN capabilities can
be configured to
define multiple virtual
LANS over single
physical LAN
infrastructure
Virtual Local
Area Network 1
8
9
16 10 2
7
hellip
Electrical Engineering
(VLAN ports 1-8)
Computer Science
(VLAN ports 9-15)
15
hellip
Electrical Engineering
(VLAN ports 1-8)
hellip
1
8 2
7 9
16 10
15
hellip
Computer Science
(VLAN ports 9-16)
hellip operates as multiple virtual switches
Link Layer 5-73
Port-based VLAN
1
8
9
16 10 2
7
hellip
Electrical Engineering
(VLAN ports 1-8)
Computer Science
(VLAN ports 9-15)
15
hellip
traffic isolation frames tofrom ports 1-8 can only reach ports 1-8 can also define VLAN based on
MAC addresses of endpoints rather than switch port
dynamic membership ports can be dynamically assigned among VLANs
router
forwarding between VLANS done via routing (just as with separate switches) in practice vendors sell combined
switches plus routers
Link Layer 5-74
VLANS spanning multiple switches
trunk port carries frames between VLANS defined over multiple physical switches frames forwarded within VLAN between switches canrsquot be vanilla
8021 frames (must carry VLAN ID info)
8021q protocol addsremoved additional header fields for frames forwarded between trunk ports
1
8
9
10 2
7
hellip
Electrical Engineering
(VLAN ports 1-8)
Computer Science
(VLAN ports 9-15)
15
hellip
2
7 3
Ports 235 belong to EE VLAN
Ports 4678 belong to CS VLAN
5
4 6 8 16
1
Link Layer 5-75
type
2-byte Tag Protocol Identifier
(value 81-00)
Tag Control Information (12 bit VLAN ID field
3 bit priority field like IP TOS)
Recomputed CRC
8021Q VLAN frame format
8021 frame
8021Q frame
dest address
source address
data (payload) CRC preamble
dest address
source address
preamble data (payload) CRC
type
Link Layer 5-76
Link layer LANs outline
51 introduction services
52 error detection correction
53 multiple access protocols
54 LANs addressing ARP
Ethernet
switches
VLANS
55 link virtualization MPLS
56 data center networking
57 a day in the life of a web request
Link Layer 5-77
Multiprotocol label switching (MPLS)
initial goal high-speed IP forwarding using fixed length label (instead of IP address) fast lookup using fixed length identifier (rather than
shortest prefix matching)
borrowing ideas from Virtual Circuit (VC) approach
but IP datagram still keeps IP address
PPP or Ethernet
header IP header remainder of link-layer frame MPLS header
label Exp S TTL
20 3 1 5
Link Layer 5-78
MPLS capable routers
aka label-switched router
forward packets to outgoing interface based only on label value (donrsquot inspect IP address) MPLS forwarding table distinct from IP forwarding tables
flexibility MPLS forwarding decisions can differ from those of IP use destination and source addresses to route flows to
same destination differently (traffic engineering)
re-route flows quickly if link fails pre-computed backup paths (useful for VoIP)
Link Layer 5-79
R2
D
R3 R5
A
R6
MPLS versus IP paths
IP router IP routing path to destination determined
by destination address alone
R4
Link Layer 5-80
R2
D
R3 R4
R5
A
R6
MPLS versus IP paths
IP-only router
IP routing path to destination determined by destination address alone
MPLS and IP router
MPLS routing path to destination can be based on source and dest address fast reroute precompute backup routes in
case of link failure
entry router (R4) can use different MPLS
routes to A based eg on source address
Link Layer 5-81
MPLS signaling
modify OSPF IS-IS link-state flooding protocols to carry info used by MPLS routing eg link bandwidth amount of ldquoreservedrdquo link bandwidth
D
R4
R5
A
R6
entry MPLS router uses RSVP-TE signaling protocol to set up MPLS forwarding at downstream routers
modified link state flooding
RSVP-TE
Link Layer 5-82
R1 R2
D
R3 R4 R5
0
1
0 0
A
R6
in out out
label label dest interface
6 - A 0
in out out
label label dest interface
10 6 A 1
12 9 D 0
in out out
label label dest interface
10 A 0
12 D 0
1
in out out
label label dest interface
8 6 A 0
0
8 A 1
MPLS forwarding tables
Link Layer 5-83
Link layer LANs outline
51 introduction services
52 error detection correction
53 multiple access protocols
54 LANs addressing ARP
Ethernet
switches
VLANS
55 link virtualization MPLS
56 data center networking
57 a day in the life of a web request
Link Layer 5-84
Data center networks
10rsquos to 100rsquos of thousands of hosts often closely coupled in close proximity e-business (eg Amazon)
content-servers (eg YouTube Akamai Apple Microsoft)
search engines data mining (eg Google)
challenges
multiple applications each serving massive numbers of clients
managingbalancing load avoiding processing networking data bottlenecks
Inside a 40-ft Microsoft container
Chicago data center
Link Layer 5-85
Server racks
TOR switches
Tier-1 switches
Tier-2 switches
Load balancer
Load balancer
B
1 2 3 4 5 6 7 8
A C
Border router
Access router
Internet
Data center networks
load balancer application-layer routing receives external client requests
directs workload within data center
returns results to external client (hiding data
center internals from client)
Server racks
TOR switches
Tier-1 switches
Tier-2 switches
1 2 3 4 5 6 7 8
Data center networks
rich interconnection among switches racks
increased throughput between racks (multiple routing
paths possible)
increased reliability via redundancy
Link Layer 5-87
Link layer LANs outline
51 introduction services
52 error detection correction
53 multiple access protocols
54 LANs addressing ARP
Ethernet
switches
VLANS
55 link virtualization MPLS
56 data center networking
57 a day in the life of a web request
Link Layer 5-88
Synthesis a day in the life of a web request
journey down protocol stack complete application transport network link
putting-it-all-together synthesis goal identify review understand protocols (at all
layers) involved in seemingly simple scenario requesting www page
scenario student attaches laptop to campus network requestsreceives wwwgooglecom
Link Layer 5-89
A day in the life scenario
Comcast network
68800013
Googlersquos network
64233160019 64233169105
web server
DNS server
school network
68802024
web page
browser
router
(runs DHCP)
Link Layer 5-90
A day in the lifehellip connecting to the Internet
connecting laptop needs to get its own IP address addr of first-hop router addr of DNS server use DHCP
DHCP
UDP
IP
Eth
Phy
DHCP
DHCP
DHCP
DHCP
DHCP
DHCP
UDP
IP
Eth
Phy
DHCP
DHCP
DHCP
DHCP DHCP
DHCP request encapsulated in UDP encapsulated in IP encapsulated in 8023 Ethernet
Ethernet frame broadcast
(dest FFFFFFFFFFFF) on LAN received at router running DHCP server
Ethernet demuxed to IP demuxed UDP demuxed to DHCP
router
(runs DHCP)
Link Layer 5-91
DHCP server formulates DHCP ACK containing clientrsquos IP address IP address of first-hop router for client name amp IP address of DNS server
DHCP
UDP
IP
Eth
Phy
DHCP
DHCP
DHCP
DHCP
DHCP
UDP
IP
Eth
Phy
DHCP
DHCP
DHCP
DHCP
DHCP
encapsulation at DHCP server frame forwarded (switch learning) through LAN demultiplexing at client
Client now has IP address knows name amp addr of DNS
server IP address of its first-hop router
DHCP client receives DHCP ACK reply
A day in the lifehellip connecting to the Internet
router
(runs DHCP)
Link Layer 5-92
A day in the lifehellip ARP (before DNS before HTTP)
before sending HTTP request need IP address of wwwgooglecom DNS
DNS
UDP
IP
Eth
Phy
DNS
DNS
DNS
DNS query created encapsulated in UDP encapsulated in IP encapsulated in Eth To send frame to router need MAC address of router interface ARP
ARP query broadcast received by
router which replies with ARP reply giving MAC address of router interface
client now knows MAC address of first hop router so can now send frame containing DNS query
ARP query
Eth
Phy
ARP
ARP
ARP reply
router
(runs DHCP)
Link Layer 5-93
DNS
UDP
IP
Eth
Phy
DNS
DNS
DNS
DNS
DNS
IP datagram containing DNS query forwarded via LAN switch from client to 1st hop router
IP datagram forwarded from campus network into comcast network routed (tables created by RIP OSPF IS-IS andor BGP routing protocols) to DNS server
demuxrsquoed to DNS server
DNS server replies to client with IP address of wwwgooglecom
Comcast network
68800013
DNS server
DNS
UDP
IP
Eth
Phy
DNS
DNS
DNS
DNS
A day in the lifehellip using DNS
router
(runs DHCP)
Link Layer 5-94
A day in the lifehellipTCP connection carrying HTTP
HTTP
TCP
IP
Eth
Phy
HTTP
to send HTTP request client first opens TCP socket to web server
TCP SYN segment (step 1 in 3-way handshake) inter-domain routed to web server
TCP connection established 64233169105
web server
SYN
SYN
SYN
SYN
TCP
IP
Eth
Phy
SYN
SYN
SYN
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
web server responds with TCP SYNACK (step 2 in 3-way handshake)
router
(runs DHCP)
Link Layer 5-95
A day in the lifehellip HTTP requestreply
HTTP
TCP
IP
Eth
Phy
HTTP
HTTP request sent into TCP socket
IP datagram containing HTTP request routed to wwwgooglecom
IP datagram containing HTTP reply routed back to client
64233169105
web server
HTTP
TCP
IP
Eth
Phy web server responds with
HTTP reply (containing web page)
HTTP
HTTP
HTTP HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
web page finally () displayed
Link Layer 5-96
Chapter 5 Summary
principles behind data link layer services error detection correction
sharing a broadcast channel multiple access
link layer addressing
instantiation and implementation of various link layer technologies Ethernet
switched LANS VLANs
virtualized networks as a link layer MPLS
synthesis a day in the life of a web request
Link Layer 5-97
Chapter 5 letrsquos take a breath
journey down protocol stack complete (except PHY)
solid understanding of networking principles practice
hellip could stop here hellip but lots of interesting topics wireless
multimedia
security
network management
Link Layer 5-40
Summary of MAC protocols
channel partitioning by time frequency or code Time Division Frequency Division
random access (dynamic)
ALOHA S-ALOHA CSMA CSMACD
carrier sensing easy in some technologies (wire) hard in others (wireless)
CSMACD used in Ethernet
CSMACA used in 80211
taking turns
polling from central site token passing
bluetooth FDDI token ring
Link Layer 5-41
Link layer LANs outline
51 introduction services
52 error detection correction
53 multiple access protocols
54 LANs addressing ARP
Ethernet
switches
VLANS
55 link virtualization MPLS
56 data center networking
57 a day in the life of a web request
Link Layer 5-42
MAC addresses and ARP
32-bit IP address network-layer address for interface
used for layer 3 (network layer) forwarding
MAC (or LAN or physical or Ethernet) address function used lsquolocallyrdquo to get frame from one interface to
another physically-connected interface (same network in IP-addressing sense)
48 bit MAC address (for most LANs) burned in NIC ROM also sometimes software settable
eg 1A-2F-BB-76-09-AD
hexadecimal (base 16) notation
(each ldquonumberrdquo represents 4 bits)
Link Layer 5-43
LAN addresses and ARP
each adapter on LAN has unique LAN address
adapter
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN
(wired or
wireless)
Link Layer 5-44
LAN addresses (more)
MAC address allocation administered by IEEE
manufacturer buys portion of MAC address space (to assure uniqueness)
analogy MAC address like Social Security Number
IP address like postal address
MAC flat address portability can move LAN card from one LAN to another
IP hierarchical address not portable address depends on IP subnet to which node is
attached
Link Layer 5-45
ARP address resolution protocol
ARP table each IP node (host router) on LAN has table
IPMAC address mappings for some LAN nodes
lt IP address MAC address TTLgt
TTL (Time To Live) time after which address mapping will be forgotten (typically 20 min)
Question how to determine
interfacersquos MAC address
knowing its IP address
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN
137196723
137196778
137196714
137196788
Link Layer 5-46
ARP protocol same LAN
A wants to send datagram to B Brsquos MAC address not in
Arsquos ARP table
A broadcasts ARP query packet containing Bs IP address dest MAC address = FF-FF-
FF-FF-FF-FF
all nodes on LAN receive ARP query
B receives ARP packet replies to A with its (Bs) MAC address frame sent to Arsquos MAC
address (unicast)
A caches (saves) IP-to-MAC address pair in its ARP table until information becomes old (times out) soft state information that
times out (goes away) unless refreshed
ARP is ldquoplug-and-playrdquo nodes create their ARP
tables without intervention from net administrator
Link Layer 5-47
walkthrough send datagram from A to B via R
focus on addressing ndash at IP (datagram) and MAC layer (frame)
assume A knows Brsquos IP address
assume A knows IP address of first hop router R (how)
assume A knows Rrsquos MAC address (how)
Addressing routing to another LAN
R
1A-23-F9-CD-06-9B 222222222220
111111111110 E6-E9-00-17-BB-4B CC-49-DE-D0-AB-7D
111111111112
111111111111
74-29-9C-E8-FF-55
A
222222222222
49-BD-D2-C7-56-2A
222222222221 88-B2-2F-54-1A-0F
B
R
1A-23-F9-CD-06-9B 222222222220
111111111110 E6-E9-00-17-BB-4B CC-49-DE-D0-AB-7D
111111111112
111111111111
74-29-9C-E8-FF-55
A
222222222222
49-BD-D2-C7-56-2A
222222222221 88-B2-2F-54-1A-0F
B
Link Layer 5-48
Addressing routing to another LAN
IP
Eth
Phy
IP src 111111111111
IP dest 222222222222
A creates IP datagram with IP source A destination B
A creates link-layer frame with Rs MAC address as dest frame contains A-to-B IP datagram
MAC src 74-29-9C-E8-FF-55
MAC dest E6-E9-00-17-BB-4B
R
1A-23-F9-CD-06-9B 222222222220
111111111110 E6-E9-00-17-BB-4B CC-49-DE-D0-AB-7D
111111111112
111111111111
74-29-9C-E8-FF-55
A
222222222222
49-BD-D2-C7-56-2A
222222222221 88-B2-2F-54-1A-0F
B
Link Layer 5-49
Addressing routing to another LAN
IP
Eth
Phy
frame sent from A to R
IP
Eth
Phy
frame received at R datagram removed passed up to IP
MAC src 74-29-9C-E8-FF-55
MAC dest E6-E9-00-17-BB-4B
IP src 111111111111
IP dest 222222222222
IP src 111111111111
IP dest 222222222222
R
1A-23-F9-CD-06-9B 222222222220
111111111110 E6-E9-00-17-BB-4B CC-49-DE-D0-AB-7D
111111111112
111111111111
74-29-9C-E8-FF-55
A
222222222222
49-BD-D2-C7-56-2A
222222222221 88-B2-2F-54-1A-0F
B
Link Layer 5-50
Addressing routing to another LAN
IP src 111111111111
IP dest 222222222222
R forwards datagram with IP source A destination B
R creates link-layer frame with Bs MAC address as dest frame contains A-to-B IP datagram
MAC src 1A-23-F9-CD-06-9B
MAC dest 49-BD-D2-C7-56-2A
IP
Eth
Phy
IP
Eth
Phy
R
1A-23-F9-CD-06-9B 222222222220
111111111110 E6-E9-00-17-BB-4B CC-49-DE-D0-AB-7D
111111111112
111111111111
74-29-9C-E8-FF-55
A
222222222222
49-BD-D2-C7-56-2A
222222222221 88-B2-2F-54-1A-0F
B
Link Layer 5-51
Addressing routing to another LAN R forwards datagram with IP source A destination B
R creates link-layer frame with Bs MAC address as dest frame contains A-to-B IP datagram
IP src 111111111111
IP dest 222222222222
MAC src 1A-23-F9-CD-06-9B
MAC dest 49-BD-D2-C7-56-2A
IP
Eth
Phy
IP
Eth
Phy
R
1A-23-F9-CD-06-9B 222222222220
111111111110 E6-E9-00-17-BB-4B CC-49-DE-D0-AB-7D
111111111112
111111111111
74-29-9C-E8-FF-55
A
222222222222
49-BD-D2-C7-56-2A
222222222221 88-B2-2F-54-1A-0F
B
Link Layer 5-52
Addressing routing to another LAN R forwards datagram with IP source A destination B
R creates link-layer frame with Bs MAC address as dest frame contains A-to-B IP datagram
IP src 111111111111
IP dest 222222222222
MAC src 1A-23-F9-CD-06-9B
MAC dest 49-BD-D2-C7-56-2A
IP
Eth
Phy
Link Layer 5-53
Link layer LANs outline
51 introduction services
52 error detection correction
53 multiple access protocols
54 LANs addressing ARP
Ethernet
switches
VLANS
55 link virtualization MPLS
56 data center networking
57 a day in the life of a web request
Link Layer 5-54
Ethernet
ldquodominantrdquo wired LAN technology
cheap $20 for NIC
first widely used LAN technology
simpler cheaper than token LANs and ATM
kept up with speed race 10 Mbps ndash 10 Gbps
Metcalfersquos Ethernet sketch
Link Layer 5-55
Ethernet physical topology
bus popular through mid 90s all nodes in same collision domain (can collide with each
other)
star prevails today active switch in center
each ldquospokerdquo runs a (separate) Ethernet protocol (nodes
do not collide with each other)
switch
bus coaxial cable star
Link Layer 5-56
Ethernet frame structure
sending adapter encapsulates IP datagram (or other network layer protocol packet) in Ethernet frame
preamble
7 bytes with pattern 10101010 followed by one byte with pattern 10101011
used to synchronize receiver sender clock rates
dest address
source address
data (payload) CRC preamble
type
Link Layer 5-57
Ethernet frame structure (more)
addresses 6 byte source destination MAC addresses if adapter receives frame with matching destination
address or with broadcast address (eg ARP packet) it passes data in frame to network layer protocol
otherwise adapter discards frame
type indicates higher layer protocol (mostly IP but others possible eg Novell IPX AppleTalk)
CRC cyclic redundancy check at receiver error detected frame is dropped
dest address
source address
data (payload) CRC preamble
type
Link Layer 5-58
Ethernet unreliable connectionless
connectionless no handshaking between sending and receiving NICs
unreliable receiving NIC doesnt send acks or nacks to sending NIC
data in dropped frames recovered only if initial sender uses higher layer rdt (eg TCP) otherwise dropped data lost
Ethernetrsquos MAC protocol unslotted CSMACD wth binary backoff
Link Layer 5-59
8023 Ethernet standards link amp physical layers
many different Ethernet standards
common MAC protocol and frame format
different speeds 2 Mbps 10 Mbps 100 Mbps 1Gbps 10G bps
different physical layer media fiber cable
application
transport
network
link
physical
MAC protocol
and frame format
100BASE-TX
100BASE-T4
100BASE-FX 100BASE-T2
100BASE-SX 100BASE-BX
fiber physical layer copper (twister
pair) physical layer
Link Layer 5-60
Link layer LANs outline
51 introduction services
52 error detection correction
53 multiple access protocols
54 LANs addressing ARP
Ethernet
switches
VLANS
55 link virtualization MPLS
56 data center networking
57 a day in the life of a web request
Link Layer 5-61
Ethernet switch link-layer device takes an active role
store forward Ethernet frames
examine incoming framersquos MAC address selectively forward frame to one-or-more outgoing links when frame is to be forwarded on segment uses CSMACD to access segment
transparent
hosts are unaware of presence of switches
plug-and-play self-learning
switches do not need to be configured
Link Layer 5-62
Switch multiple simultaneous transmissions
hosts have dedicated direct connection to switch
switches buffer packets
Ethernet protocol used on each incoming link but no collisions full duplex
each link is its own collision domain
switching A-to-Arsquo and B-to-Brsquo can transmit simultaneously without collisions switch with six interfaces
(123456)
A
Arsquo
B
Brsquo C
Crsquo
1 2
3 4 5
6
Link Layer 5-63
Switch forwarding table
Q how does switch know Arsquo reachable via interface 4 Brsquo reachable via interface 5
switch with six interfaces
(123456)
A
Arsquo
B
Brsquo C
Crsquo
1 2
3 4 5
6 A each switch has a switch table each entry
(MAC address of host interface to
reach host time stamp)
looks like a routing table
Q how are entries created maintained in switch table
something like a routing protocol
A
Arsquo
B
Brsquo C
Crsquo
1 2
3 4 5
6
Link Layer 5-64
Switch self-learning
switch learns which hosts can be reached through which interfaces
when frame received switch ldquolearnsrdquo location of sender incoming LAN segment
records senderlocation pair in switch table
A Arsquo
Source A
Dest Arsquo
MAC addr interface TTL
Switch table
(initially empty) A 1 60
Link Layer 5-65
Switch frame filteringforwarding
when frame received at switch 1 record incoming link MAC address of sending host
2 index switch table using MAC destination address
3 if entry found for destination then
if destination on segment from which frame arrived then drop frame
else forward frame on interface indicated by entry
else flood forward on all interfaces except arriving
interface
A
Arsquo
B
Brsquo C
Crsquo
1 2
3 4 5
6
Link Layer 5-66
Self-learning forwarding example
A Arsquo
Source A
Dest Arsquo
MAC addr interface TTL
switch table
(initially empty) A 1 60
A Arsquo A Arsquo A Arsquo A Arsquo A Arsquo
frame destination Arsquo locaton unknown flood
Arsquo A
destination A location
known
Arsquo 4 60
selectively send
on just one link
Link Layer 5-67
Interconnecting switches
switches can be connected together
Q sending from A to G - how does S1 know to forward frame destined to F via S4 and S3
A self learning (works exactly the same as in single-switch case)
A
B
S1
C D
E
F
S2
S4
S3
H
I
G
Link Layer 5-68
Self-learning multi-switch example
Suppose C sends frame to I I responds to C
Q show switch tables and packet forwarding in S1 S2 S3 S4
A
B
S1
C D
E
F
S2
S4
S3
H
I
G
Link Layer 5-69
Institutional network
to external
network
router
IP subnet
mail server
web server
Link Layer 5-70
Switches vs routers
both are store-and-forward
routers network-layer devices (examine network-layer headers)
switches link-layer devices (examine link-layer headers)
both have forwarding tables
routers compute tables using routing algorithms IP addresses
switches learn forwarding table using flooding learning MAC addresses
application
transport
network
link
physical
network
link
physical
link
physical
switch
datagram
application
transport
network
link
physical
frame
frame
frame
datagram
Link Layer 5-71
VLANs motivation
consider CS user moves office to
EE but wants connect to CS switch
single broadcast domain
all layer-2 broadcast traffic (ARP DHCP unknown location of destination MAC address) must cross entire LAN
securityprivacy efficiency issues
Computer
Science Electrical
Engineering
Computer
Engineering
Link Layer 5-72
VLANs port-based VLAN switch ports
grouped (by switch management software) so that single physical switch helliphellip
switch(es) supporting
VLAN capabilities can
be configured to
define multiple virtual
LANS over single
physical LAN
infrastructure
Virtual Local
Area Network 1
8
9
16 10 2
7
hellip
Electrical Engineering
(VLAN ports 1-8)
Computer Science
(VLAN ports 9-15)
15
hellip
Electrical Engineering
(VLAN ports 1-8)
hellip
1
8 2
7 9
16 10
15
hellip
Computer Science
(VLAN ports 9-16)
hellip operates as multiple virtual switches
Link Layer 5-73
Port-based VLAN
1
8
9
16 10 2
7
hellip
Electrical Engineering
(VLAN ports 1-8)
Computer Science
(VLAN ports 9-15)
15
hellip
traffic isolation frames tofrom ports 1-8 can only reach ports 1-8 can also define VLAN based on
MAC addresses of endpoints rather than switch port
dynamic membership ports can be dynamically assigned among VLANs
router
forwarding between VLANS done via routing (just as with separate switches) in practice vendors sell combined
switches plus routers
Link Layer 5-74
VLANS spanning multiple switches
trunk port carries frames between VLANS defined over multiple physical switches frames forwarded within VLAN between switches canrsquot be vanilla
8021 frames (must carry VLAN ID info)
8021q protocol addsremoved additional header fields for frames forwarded between trunk ports
1
8
9
10 2
7
hellip
Electrical Engineering
(VLAN ports 1-8)
Computer Science
(VLAN ports 9-15)
15
hellip
2
7 3
Ports 235 belong to EE VLAN
Ports 4678 belong to CS VLAN
5
4 6 8 16
1
Link Layer 5-75
type
2-byte Tag Protocol Identifier
(value 81-00)
Tag Control Information (12 bit VLAN ID field
3 bit priority field like IP TOS)
Recomputed CRC
8021Q VLAN frame format
8021 frame
8021Q frame
dest address
source address
data (payload) CRC preamble
dest address
source address
preamble data (payload) CRC
type
Link Layer 5-76
Link layer LANs outline
51 introduction services
52 error detection correction
53 multiple access protocols
54 LANs addressing ARP
Ethernet
switches
VLANS
55 link virtualization MPLS
56 data center networking
57 a day in the life of a web request
Link Layer 5-77
Multiprotocol label switching (MPLS)
initial goal high-speed IP forwarding using fixed length label (instead of IP address) fast lookup using fixed length identifier (rather than
shortest prefix matching)
borrowing ideas from Virtual Circuit (VC) approach
but IP datagram still keeps IP address
PPP or Ethernet
header IP header remainder of link-layer frame MPLS header
label Exp S TTL
20 3 1 5
Link Layer 5-78
MPLS capable routers
aka label-switched router
forward packets to outgoing interface based only on label value (donrsquot inspect IP address) MPLS forwarding table distinct from IP forwarding tables
flexibility MPLS forwarding decisions can differ from those of IP use destination and source addresses to route flows to
same destination differently (traffic engineering)
re-route flows quickly if link fails pre-computed backup paths (useful for VoIP)
Link Layer 5-79
R2
D
R3 R5
A
R6
MPLS versus IP paths
IP router IP routing path to destination determined
by destination address alone
R4
Link Layer 5-80
R2
D
R3 R4
R5
A
R6
MPLS versus IP paths
IP-only router
IP routing path to destination determined by destination address alone
MPLS and IP router
MPLS routing path to destination can be based on source and dest address fast reroute precompute backup routes in
case of link failure
entry router (R4) can use different MPLS
routes to A based eg on source address
Link Layer 5-81
MPLS signaling
modify OSPF IS-IS link-state flooding protocols to carry info used by MPLS routing eg link bandwidth amount of ldquoreservedrdquo link bandwidth
D
R4
R5
A
R6
entry MPLS router uses RSVP-TE signaling protocol to set up MPLS forwarding at downstream routers
modified link state flooding
RSVP-TE
Link Layer 5-82
R1 R2
D
R3 R4 R5
0
1
0 0
A
R6
in out out
label label dest interface
6 - A 0
in out out
label label dest interface
10 6 A 1
12 9 D 0
in out out
label label dest interface
10 A 0
12 D 0
1
in out out
label label dest interface
8 6 A 0
0
8 A 1
MPLS forwarding tables
Link Layer 5-83
Link layer LANs outline
51 introduction services
52 error detection correction
53 multiple access protocols
54 LANs addressing ARP
Ethernet
switches
VLANS
55 link virtualization MPLS
56 data center networking
57 a day in the life of a web request
Link Layer 5-84
Data center networks
10rsquos to 100rsquos of thousands of hosts often closely coupled in close proximity e-business (eg Amazon)
content-servers (eg YouTube Akamai Apple Microsoft)
search engines data mining (eg Google)
challenges
multiple applications each serving massive numbers of clients
managingbalancing load avoiding processing networking data bottlenecks
Inside a 40-ft Microsoft container
Chicago data center
Link Layer 5-85
Server racks
TOR switches
Tier-1 switches
Tier-2 switches
Load balancer
Load balancer
B
1 2 3 4 5 6 7 8
A C
Border router
Access router
Internet
Data center networks
load balancer application-layer routing receives external client requests
directs workload within data center
returns results to external client (hiding data
center internals from client)
Server racks
TOR switches
Tier-1 switches
Tier-2 switches
1 2 3 4 5 6 7 8
Data center networks
rich interconnection among switches racks
increased throughput between racks (multiple routing
paths possible)
increased reliability via redundancy
Link Layer 5-87
Link layer LANs outline
51 introduction services
52 error detection correction
53 multiple access protocols
54 LANs addressing ARP
Ethernet
switches
VLANS
55 link virtualization MPLS
56 data center networking
57 a day in the life of a web request
Link Layer 5-88
Synthesis a day in the life of a web request
journey down protocol stack complete application transport network link
putting-it-all-together synthesis goal identify review understand protocols (at all
layers) involved in seemingly simple scenario requesting www page
scenario student attaches laptop to campus network requestsreceives wwwgooglecom
Link Layer 5-89
A day in the life scenario
Comcast network
68800013
Googlersquos network
64233160019 64233169105
web server
DNS server
school network
68802024
web page
browser
router
(runs DHCP)
Link Layer 5-90
A day in the lifehellip connecting to the Internet
connecting laptop needs to get its own IP address addr of first-hop router addr of DNS server use DHCP
DHCP
UDP
IP
Eth
Phy
DHCP
DHCP
DHCP
DHCP
DHCP
DHCP
UDP
IP
Eth
Phy
DHCP
DHCP
DHCP
DHCP DHCP
DHCP request encapsulated in UDP encapsulated in IP encapsulated in 8023 Ethernet
Ethernet frame broadcast
(dest FFFFFFFFFFFF) on LAN received at router running DHCP server
Ethernet demuxed to IP demuxed UDP demuxed to DHCP
router
(runs DHCP)
Link Layer 5-91
DHCP server formulates DHCP ACK containing clientrsquos IP address IP address of first-hop router for client name amp IP address of DNS server
DHCP
UDP
IP
Eth
Phy
DHCP
DHCP
DHCP
DHCP
DHCP
UDP
IP
Eth
Phy
DHCP
DHCP
DHCP
DHCP
DHCP
encapsulation at DHCP server frame forwarded (switch learning) through LAN demultiplexing at client
Client now has IP address knows name amp addr of DNS
server IP address of its first-hop router
DHCP client receives DHCP ACK reply
A day in the lifehellip connecting to the Internet
router
(runs DHCP)
Link Layer 5-92
A day in the lifehellip ARP (before DNS before HTTP)
before sending HTTP request need IP address of wwwgooglecom DNS
DNS
UDP
IP
Eth
Phy
DNS
DNS
DNS
DNS query created encapsulated in UDP encapsulated in IP encapsulated in Eth To send frame to router need MAC address of router interface ARP
ARP query broadcast received by
router which replies with ARP reply giving MAC address of router interface
client now knows MAC address of first hop router so can now send frame containing DNS query
ARP query
Eth
Phy
ARP
ARP
ARP reply
router
(runs DHCP)
Link Layer 5-93
DNS
UDP
IP
Eth
Phy
DNS
DNS
DNS
DNS
DNS
IP datagram containing DNS query forwarded via LAN switch from client to 1st hop router
IP datagram forwarded from campus network into comcast network routed (tables created by RIP OSPF IS-IS andor BGP routing protocols) to DNS server
demuxrsquoed to DNS server
DNS server replies to client with IP address of wwwgooglecom
Comcast network
68800013
DNS server
DNS
UDP
IP
Eth
Phy
DNS
DNS
DNS
DNS
A day in the lifehellip using DNS
router
(runs DHCP)
Link Layer 5-94
A day in the lifehellipTCP connection carrying HTTP
HTTP
TCP
IP
Eth
Phy
HTTP
to send HTTP request client first opens TCP socket to web server
TCP SYN segment (step 1 in 3-way handshake) inter-domain routed to web server
TCP connection established 64233169105
web server
SYN
SYN
SYN
SYN
TCP
IP
Eth
Phy
SYN
SYN
SYN
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
web server responds with TCP SYNACK (step 2 in 3-way handshake)
router
(runs DHCP)
Link Layer 5-95
A day in the lifehellip HTTP requestreply
HTTP
TCP
IP
Eth
Phy
HTTP
HTTP request sent into TCP socket
IP datagram containing HTTP request routed to wwwgooglecom
IP datagram containing HTTP reply routed back to client
64233169105
web server
HTTP
TCP
IP
Eth
Phy web server responds with
HTTP reply (containing web page)
HTTP
HTTP
HTTP HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
web page finally () displayed
Link Layer 5-96
Chapter 5 Summary
principles behind data link layer services error detection correction
sharing a broadcast channel multiple access
link layer addressing
instantiation and implementation of various link layer technologies Ethernet
switched LANS VLANs
virtualized networks as a link layer MPLS
synthesis a day in the life of a web request
Link Layer 5-97
Chapter 5 letrsquos take a breath
journey down protocol stack complete (except PHY)
solid understanding of networking principles practice
hellip could stop here hellip but lots of interesting topics wireless
multimedia
security
network management
Link Layer 5-41
Link layer LANs outline
51 introduction services
52 error detection correction
53 multiple access protocols
54 LANs addressing ARP
Ethernet
switches
VLANS
55 link virtualization MPLS
56 data center networking
57 a day in the life of a web request
Link Layer 5-42
MAC addresses and ARP
32-bit IP address network-layer address for interface
used for layer 3 (network layer) forwarding
MAC (or LAN or physical or Ethernet) address function used lsquolocallyrdquo to get frame from one interface to
another physically-connected interface (same network in IP-addressing sense)
48 bit MAC address (for most LANs) burned in NIC ROM also sometimes software settable
eg 1A-2F-BB-76-09-AD
hexadecimal (base 16) notation
(each ldquonumberrdquo represents 4 bits)
Link Layer 5-43
LAN addresses and ARP
each adapter on LAN has unique LAN address
adapter
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN
(wired or
wireless)
Link Layer 5-44
LAN addresses (more)
MAC address allocation administered by IEEE
manufacturer buys portion of MAC address space (to assure uniqueness)
analogy MAC address like Social Security Number
IP address like postal address
MAC flat address portability can move LAN card from one LAN to another
IP hierarchical address not portable address depends on IP subnet to which node is
attached
Link Layer 5-45
ARP address resolution protocol
ARP table each IP node (host router) on LAN has table
IPMAC address mappings for some LAN nodes
lt IP address MAC address TTLgt
TTL (Time To Live) time after which address mapping will be forgotten (typically 20 min)
Question how to determine
interfacersquos MAC address
knowing its IP address
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN
137196723
137196778
137196714
137196788
Link Layer 5-46
ARP protocol same LAN
A wants to send datagram to B Brsquos MAC address not in
Arsquos ARP table
A broadcasts ARP query packet containing Bs IP address dest MAC address = FF-FF-
FF-FF-FF-FF
all nodes on LAN receive ARP query
B receives ARP packet replies to A with its (Bs) MAC address frame sent to Arsquos MAC
address (unicast)
A caches (saves) IP-to-MAC address pair in its ARP table until information becomes old (times out) soft state information that
times out (goes away) unless refreshed
ARP is ldquoplug-and-playrdquo nodes create their ARP
tables without intervention from net administrator
Link Layer 5-47
walkthrough send datagram from A to B via R
focus on addressing ndash at IP (datagram) and MAC layer (frame)
assume A knows Brsquos IP address
assume A knows IP address of first hop router R (how)
assume A knows Rrsquos MAC address (how)
Addressing routing to another LAN
R
1A-23-F9-CD-06-9B 222222222220
111111111110 E6-E9-00-17-BB-4B CC-49-DE-D0-AB-7D
111111111112
111111111111
74-29-9C-E8-FF-55
A
222222222222
49-BD-D2-C7-56-2A
222222222221 88-B2-2F-54-1A-0F
B
R
1A-23-F9-CD-06-9B 222222222220
111111111110 E6-E9-00-17-BB-4B CC-49-DE-D0-AB-7D
111111111112
111111111111
74-29-9C-E8-FF-55
A
222222222222
49-BD-D2-C7-56-2A
222222222221 88-B2-2F-54-1A-0F
B
Link Layer 5-48
Addressing routing to another LAN
IP
Eth
Phy
IP src 111111111111
IP dest 222222222222
A creates IP datagram with IP source A destination B
A creates link-layer frame with Rs MAC address as dest frame contains A-to-B IP datagram
MAC src 74-29-9C-E8-FF-55
MAC dest E6-E9-00-17-BB-4B
R
1A-23-F9-CD-06-9B 222222222220
111111111110 E6-E9-00-17-BB-4B CC-49-DE-D0-AB-7D
111111111112
111111111111
74-29-9C-E8-FF-55
A
222222222222
49-BD-D2-C7-56-2A
222222222221 88-B2-2F-54-1A-0F
B
Link Layer 5-49
Addressing routing to another LAN
IP
Eth
Phy
frame sent from A to R
IP
Eth
Phy
frame received at R datagram removed passed up to IP
MAC src 74-29-9C-E8-FF-55
MAC dest E6-E9-00-17-BB-4B
IP src 111111111111
IP dest 222222222222
IP src 111111111111
IP dest 222222222222
R
1A-23-F9-CD-06-9B 222222222220
111111111110 E6-E9-00-17-BB-4B CC-49-DE-D0-AB-7D
111111111112
111111111111
74-29-9C-E8-FF-55
A
222222222222
49-BD-D2-C7-56-2A
222222222221 88-B2-2F-54-1A-0F
B
Link Layer 5-50
Addressing routing to another LAN
IP src 111111111111
IP dest 222222222222
R forwards datagram with IP source A destination B
R creates link-layer frame with Bs MAC address as dest frame contains A-to-B IP datagram
MAC src 1A-23-F9-CD-06-9B
MAC dest 49-BD-D2-C7-56-2A
IP
Eth
Phy
IP
Eth
Phy
R
1A-23-F9-CD-06-9B 222222222220
111111111110 E6-E9-00-17-BB-4B CC-49-DE-D0-AB-7D
111111111112
111111111111
74-29-9C-E8-FF-55
A
222222222222
49-BD-D2-C7-56-2A
222222222221 88-B2-2F-54-1A-0F
B
Link Layer 5-51
Addressing routing to another LAN R forwards datagram with IP source A destination B
R creates link-layer frame with Bs MAC address as dest frame contains A-to-B IP datagram
IP src 111111111111
IP dest 222222222222
MAC src 1A-23-F9-CD-06-9B
MAC dest 49-BD-D2-C7-56-2A
IP
Eth
Phy
IP
Eth
Phy
R
1A-23-F9-CD-06-9B 222222222220
111111111110 E6-E9-00-17-BB-4B CC-49-DE-D0-AB-7D
111111111112
111111111111
74-29-9C-E8-FF-55
A
222222222222
49-BD-D2-C7-56-2A
222222222221 88-B2-2F-54-1A-0F
B
Link Layer 5-52
Addressing routing to another LAN R forwards datagram with IP source A destination B
R creates link-layer frame with Bs MAC address as dest frame contains A-to-B IP datagram
IP src 111111111111
IP dest 222222222222
MAC src 1A-23-F9-CD-06-9B
MAC dest 49-BD-D2-C7-56-2A
IP
Eth
Phy
Link Layer 5-53
Link layer LANs outline
51 introduction services
52 error detection correction
53 multiple access protocols
54 LANs addressing ARP
Ethernet
switches
VLANS
55 link virtualization MPLS
56 data center networking
57 a day in the life of a web request
Link Layer 5-54
Ethernet
ldquodominantrdquo wired LAN technology
cheap $20 for NIC
first widely used LAN technology
simpler cheaper than token LANs and ATM
kept up with speed race 10 Mbps ndash 10 Gbps
Metcalfersquos Ethernet sketch
Link Layer 5-55
Ethernet physical topology
bus popular through mid 90s all nodes in same collision domain (can collide with each
other)
star prevails today active switch in center
each ldquospokerdquo runs a (separate) Ethernet protocol (nodes
do not collide with each other)
switch
bus coaxial cable star
Link Layer 5-56
Ethernet frame structure
sending adapter encapsulates IP datagram (or other network layer protocol packet) in Ethernet frame
preamble
7 bytes with pattern 10101010 followed by one byte with pattern 10101011
used to synchronize receiver sender clock rates
dest address
source address
data (payload) CRC preamble
type
Link Layer 5-57
Ethernet frame structure (more)
addresses 6 byte source destination MAC addresses if adapter receives frame with matching destination
address or with broadcast address (eg ARP packet) it passes data in frame to network layer protocol
otherwise adapter discards frame
type indicates higher layer protocol (mostly IP but others possible eg Novell IPX AppleTalk)
CRC cyclic redundancy check at receiver error detected frame is dropped
dest address
source address
data (payload) CRC preamble
type
Link Layer 5-58
Ethernet unreliable connectionless
connectionless no handshaking between sending and receiving NICs
unreliable receiving NIC doesnt send acks or nacks to sending NIC
data in dropped frames recovered only if initial sender uses higher layer rdt (eg TCP) otherwise dropped data lost
Ethernetrsquos MAC protocol unslotted CSMACD wth binary backoff
Link Layer 5-59
8023 Ethernet standards link amp physical layers
many different Ethernet standards
common MAC protocol and frame format
different speeds 2 Mbps 10 Mbps 100 Mbps 1Gbps 10G bps
different physical layer media fiber cable
application
transport
network
link
physical
MAC protocol
and frame format
100BASE-TX
100BASE-T4
100BASE-FX 100BASE-T2
100BASE-SX 100BASE-BX
fiber physical layer copper (twister
pair) physical layer
Link Layer 5-60
Link layer LANs outline
51 introduction services
52 error detection correction
53 multiple access protocols
54 LANs addressing ARP
Ethernet
switches
VLANS
55 link virtualization MPLS
56 data center networking
57 a day in the life of a web request
Link Layer 5-61
Ethernet switch link-layer device takes an active role
store forward Ethernet frames
examine incoming framersquos MAC address selectively forward frame to one-or-more outgoing links when frame is to be forwarded on segment uses CSMACD to access segment
transparent
hosts are unaware of presence of switches
plug-and-play self-learning
switches do not need to be configured
Link Layer 5-62
Switch multiple simultaneous transmissions
hosts have dedicated direct connection to switch
switches buffer packets
Ethernet protocol used on each incoming link but no collisions full duplex
each link is its own collision domain
switching A-to-Arsquo and B-to-Brsquo can transmit simultaneously without collisions switch with six interfaces
(123456)
A
Arsquo
B
Brsquo C
Crsquo
1 2
3 4 5
6
Link Layer 5-63
Switch forwarding table
Q how does switch know Arsquo reachable via interface 4 Brsquo reachable via interface 5
switch with six interfaces
(123456)
A
Arsquo
B
Brsquo C
Crsquo
1 2
3 4 5
6 A each switch has a switch table each entry
(MAC address of host interface to
reach host time stamp)
looks like a routing table
Q how are entries created maintained in switch table
something like a routing protocol
A
Arsquo
B
Brsquo C
Crsquo
1 2
3 4 5
6
Link Layer 5-64
Switch self-learning
switch learns which hosts can be reached through which interfaces
when frame received switch ldquolearnsrdquo location of sender incoming LAN segment
records senderlocation pair in switch table
A Arsquo
Source A
Dest Arsquo
MAC addr interface TTL
Switch table
(initially empty) A 1 60
Link Layer 5-65
Switch frame filteringforwarding
when frame received at switch 1 record incoming link MAC address of sending host
2 index switch table using MAC destination address
3 if entry found for destination then
if destination on segment from which frame arrived then drop frame
else forward frame on interface indicated by entry
else flood forward on all interfaces except arriving
interface
A
Arsquo
B
Brsquo C
Crsquo
1 2
3 4 5
6
Link Layer 5-66
Self-learning forwarding example
A Arsquo
Source A
Dest Arsquo
MAC addr interface TTL
switch table
(initially empty) A 1 60
A Arsquo A Arsquo A Arsquo A Arsquo A Arsquo
frame destination Arsquo locaton unknown flood
Arsquo A
destination A location
known
Arsquo 4 60
selectively send
on just one link
Link Layer 5-67
Interconnecting switches
switches can be connected together
Q sending from A to G - how does S1 know to forward frame destined to F via S4 and S3
A self learning (works exactly the same as in single-switch case)
A
B
S1
C D
E
F
S2
S4
S3
H
I
G
Link Layer 5-68
Self-learning multi-switch example
Suppose C sends frame to I I responds to C
Q show switch tables and packet forwarding in S1 S2 S3 S4
A
B
S1
C D
E
F
S2
S4
S3
H
I
G
Link Layer 5-69
Institutional network
to external
network
router
IP subnet
mail server
web server
Link Layer 5-70
Switches vs routers
both are store-and-forward
routers network-layer devices (examine network-layer headers)
switches link-layer devices (examine link-layer headers)
both have forwarding tables
routers compute tables using routing algorithms IP addresses
switches learn forwarding table using flooding learning MAC addresses
application
transport
network
link
physical
network
link
physical
link
physical
switch
datagram
application
transport
network
link
physical
frame
frame
frame
datagram
Link Layer 5-71
VLANs motivation
consider CS user moves office to
EE but wants connect to CS switch
single broadcast domain
all layer-2 broadcast traffic (ARP DHCP unknown location of destination MAC address) must cross entire LAN
securityprivacy efficiency issues
Computer
Science Electrical
Engineering
Computer
Engineering
Link Layer 5-72
VLANs port-based VLAN switch ports
grouped (by switch management software) so that single physical switch helliphellip
switch(es) supporting
VLAN capabilities can
be configured to
define multiple virtual
LANS over single
physical LAN
infrastructure
Virtual Local
Area Network 1
8
9
16 10 2
7
hellip
Electrical Engineering
(VLAN ports 1-8)
Computer Science
(VLAN ports 9-15)
15
hellip
Electrical Engineering
(VLAN ports 1-8)
hellip
1
8 2
7 9
16 10
15
hellip
Computer Science
(VLAN ports 9-16)
hellip operates as multiple virtual switches
Link Layer 5-73
Port-based VLAN
1
8
9
16 10 2
7
hellip
Electrical Engineering
(VLAN ports 1-8)
Computer Science
(VLAN ports 9-15)
15
hellip
traffic isolation frames tofrom ports 1-8 can only reach ports 1-8 can also define VLAN based on
MAC addresses of endpoints rather than switch port
dynamic membership ports can be dynamically assigned among VLANs
router
forwarding between VLANS done via routing (just as with separate switches) in practice vendors sell combined
switches plus routers
Link Layer 5-74
VLANS spanning multiple switches
trunk port carries frames between VLANS defined over multiple physical switches frames forwarded within VLAN between switches canrsquot be vanilla
8021 frames (must carry VLAN ID info)
8021q protocol addsremoved additional header fields for frames forwarded between trunk ports
1
8
9
10 2
7
hellip
Electrical Engineering
(VLAN ports 1-8)
Computer Science
(VLAN ports 9-15)
15
hellip
2
7 3
Ports 235 belong to EE VLAN
Ports 4678 belong to CS VLAN
5
4 6 8 16
1
Link Layer 5-75
type
2-byte Tag Protocol Identifier
(value 81-00)
Tag Control Information (12 bit VLAN ID field
3 bit priority field like IP TOS)
Recomputed CRC
8021Q VLAN frame format
8021 frame
8021Q frame
dest address
source address
data (payload) CRC preamble
dest address
source address
preamble data (payload) CRC
type
Link Layer 5-76
Link layer LANs outline
51 introduction services
52 error detection correction
53 multiple access protocols
54 LANs addressing ARP
Ethernet
switches
VLANS
55 link virtualization MPLS
56 data center networking
57 a day in the life of a web request
Link Layer 5-77
Multiprotocol label switching (MPLS)
initial goal high-speed IP forwarding using fixed length label (instead of IP address) fast lookup using fixed length identifier (rather than
shortest prefix matching)
borrowing ideas from Virtual Circuit (VC) approach
but IP datagram still keeps IP address
PPP or Ethernet
header IP header remainder of link-layer frame MPLS header
label Exp S TTL
20 3 1 5
Link Layer 5-78
MPLS capable routers
aka label-switched router
forward packets to outgoing interface based only on label value (donrsquot inspect IP address) MPLS forwarding table distinct from IP forwarding tables
flexibility MPLS forwarding decisions can differ from those of IP use destination and source addresses to route flows to
same destination differently (traffic engineering)
re-route flows quickly if link fails pre-computed backup paths (useful for VoIP)
Link Layer 5-79
R2
D
R3 R5
A
R6
MPLS versus IP paths
IP router IP routing path to destination determined
by destination address alone
R4
Link Layer 5-80
R2
D
R3 R4
R5
A
R6
MPLS versus IP paths
IP-only router
IP routing path to destination determined by destination address alone
MPLS and IP router
MPLS routing path to destination can be based on source and dest address fast reroute precompute backup routes in
case of link failure
entry router (R4) can use different MPLS
routes to A based eg on source address
Link Layer 5-81
MPLS signaling
modify OSPF IS-IS link-state flooding protocols to carry info used by MPLS routing eg link bandwidth amount of ldquoreservedrdquo link bandwidth
D
R4
R5
A
R6
entry MPLS router uses RSVP-TE signaling protocol to set up MPLS forwarding at downstream routers
modified link state flooding
RSVP-TE
Link Layer 5-82
R1 R2
D
R3 R4 R5
0
1
0 0
A
R6
in out out
label label dest interface
6 - A 0
in out out
label label dest interface
10 6 A 1
12 9 D 0
in out out
label label dest interface
10 A 0
12 D 0
1
in out out
label label dest interface
8 6 A 0
0
8 A 1
MPLS forwarding tables
Link Layer 5-83
Link layer LANs outline
51 introduction services
52 error detection correction
53 multiple access protocols
54 LANs addressing ARP
Ethernet
switches
VLANS
55 link virtualization MPLS
56 data center networking
57 a day in the life of a web request
Link Layer 5-84
Data center networks
10rsquos to 100rsquos of thousands of hosts often closely coupled in close proximity e-business (eg Amazon)
content-servers (eg YouTube Akamai Apple Microsoft)
search engines data mining (eg Google)
challenges
multiple applications each serving massive numbers of clients
managingbalancing load avoiding processing networking data bottlenecks
Inside a 40-ft Microsoft container
Chicago data center
Link Layer 5-85
Server racks
TOR switches
Tier-1 switches
Tier-2 switches
Load balancer
Load balancer
B
1 2 3 4 5 6 7 8
A C
Border router
Access router
Internet
Data center networks
load balancer application-layer routing receives external client requests
directs workload within data center
returns results to external client (hiding data
center internals from client)
Server racks
TOR switches
Tier-1 switches
Tier-2 switches
1 2 3 4 5 6 7 8
Data center networks
rich interconnection among switches racks
increased throughput between racks (multiple routing
paths possible)
increased reliability via redundancy
Link Layer 5-87
Link layer LANs outline
51 introduction services
52 error detection correction
53 multiple access protocols
54 LANs addressing ARP
Ethernet
switches
VLANS
55 link virtualization MPLS
56 data center networking
57 a day in the life of a web request
Link Layer 5-88
Synthesis a day in the life of a web request
journey down protocol stack complete application transport network link
putting-it-all-together synthesis goal identify review understand protocols (at all
layers) involved in seemingly simple scenario requesting www page
scenario student attaches laptop to campus network requestsreceives wwwgooglecom
Link Layer 5-89
A day in the life scenario
Comcast network
68800013
Googlersquos network
64233160019 64233169105
web server
DNS server
school network
68802024
web page
browser
router
(runs DHCP)
Link Layer 5-90
A day in the lifehellip connecting to the Internet
connecting laptop needs to get its own IP address addr of first-hop router addr of DNS server use DHCP
DHCP
UDP
IP
Eth
Phy
DHCP
DHCP
DHCP
DHCP
DHCP
DHCP
UDP
IP
Eth
Phy
DHCP
DHCP
DHCP
DHCP DHCP
DHCP request encapsulated in UDP encapsulated in IP encapsulated in 8023 Ethernet
Ethernet frame broadcast
(dest FFFFFFFFFFFF) on LAN received at router running DHCP server
Ethernet demuxed to IP demuxed UDP demuxed to DHCP
router
(runs DHCP)
Link Layer 5-91
DHCP server formulates DHCP ACK containing clientrsquos IP address IP address of first-hop router for client name amp IP address of DNS server
DHCP
UDP
IP
Eth
Phy
DHCP
DHCP
DHCP
DHCP
DHCP
UDP
IP
Eth
Phy
DHCP
DHCP
DHCP
DHCP
DHCP
encapsulation at DHCP server frame forwarded (switch learning) through LAN demultiplexing at client
Client now has IP address knows name amp addr of DNS
server IP address of its first-hop router
DHCP client receives DHCP ACK reply
A day in the lifehellip connecting to the Internet
router
(runs DHCP)
Link Layer 5-92
A day in the lifehellip ARP (before DNS before HTTP)
before sending HTTP request need IP address of wwwgooglecom DNS
DNS
UDP
IP
Eth
Phy
DNS
DNS
DNS
DNS query created encapsulated in UDP encapsulated in IP encapsulated in Eth To send frame to router need MAC address of router interface ARP
ARP query broadcast received by
router which replies with ARP reply giving MAC address of router interface
client now knows MAC address of first hop router so can now send frame containing DNS query
ARP query
Eth
Phy
ARP
ARP
ARP reply
router
(runs DHCP)
Link Layer 5-93
DNS
UDP
IP
Eth
Phy
DNS
DNS
DNS
DNS
DNS
IP datagram containing DNS query forwarded via LAN switch from client to 1st hop router
IP datagram forwarded from campus network into comcast network routed (tables created by RIP OSPF IS-IS andor BGP routing protocols) to DNS server
demuxrsquoed to DNS server
DNS server replies to client with IP address of wwwgooglecom
Comcast network
68800013
DNS server
DNS
UDP
IP
Eth
Phy
DNS
DNS
DNS
DNS
A day in the lifehellip using DNS
router
(runs DHCP)
Link Layer 5-94
A day in the lifehellipTCP connection carrying HTTP
HTTP
TCP
IP
Eth
Phy
HTTP
to send HTTP request client first opens TCP socket to web server
TCP SYN segment (step 1 in 3-way handshake) inter-domain routed to web server
TCP connection established 64233169105
web server
SYN
SYN
SYN
SYN
TCP
IP
Eth
Phy
SYN
SYN
SYN
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
web server responds with TCP SYNACK (step 2 in 3-way handshake)
router
(runs DHCP)
Link Layer 5-95
A day in the lifehellip HTTP requestreply
HTTP
TCP
IP
Eth
Phy
HTTP
HTTP request sent into TCP socket
IP datagram containing HTTP request routed to wwwgooglecom
IP datagram containing HTTP reply routed back to client
64233169105
web server
HTTP
TCP
IP
Eth
Phy web server responds with
HTTP reply (containing web page)
HTTP
HTTP
HTTP HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
web page finally () displayed
Link Layer 5-96
Chapter 5 Summary
principles behind data link layer services error detection correction
sharing a broadcast channel multiple access
link layer addressing
instantiation and implementation of various link layer technologies Ethernet
switched LANS VLANs
virtualized networks as a link layer MPLS
synthesis a day in the life of a web request
Link Layer 5-97
Chapter 5 letrsquos take a breath
journey down protocol stack complete (except PHY)
solid understanding of networking principles practice
hellip could stop here hellip but lots of interesting topics wireless
multimedia
security
network management
Link Layer 5-42
MAC addresses and ARP
32-bit IP address network-layer address for interface
used for layer 3 (network layer) forwarding
MAC (or LAN or physical or Ethernet) address function used lsquolocallyrdquo to get frame from one interface to
another physically-connected interface (same network in IP-addressing sense)
48 bit MAC address (for most LANs) burned in NIC ROM also sometimes software settable
eg 1A-2F-BB-76-09-AD
hexadecimal (base 16) notation
(each ldquonumberrdquo represents 4 bits)
Link Layer 5-43
LAN addresses and ARP
each adapter on LAN has unique LAN address
adapter
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN
(wired or
wireless)
Link Layer 5-44
LAN addresses (more)
MAC address allocation administered by IEEE
manufacturer buys portion of MAC address space (to assure uniqueness)
analogy MAC address like Social Security Number
IP address like postal address
MAC flat address portability can move LAN card from one LAN to another
IP hierarchical address not portable address depends on IP subnet to which node is
attached
Link Layer 5-45
ARP address resolution protocol
ARP table each IP node (host router) on LAN has table
IPMAC address mappings for some LAN nodes
lt IP address MAC address TTLgt
TTL (Time To Live) time after which address mapping will be forgotten (typically 20 min)
Question how to determine
interfacersquos MAC address
knowing its IP address
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN
137196723
137196778
137196714
137196788
Link Layer 5-46
ARP protocol same LAN
A wants to send datagram to B Brsquos MAC address not in
Arsquos ARP table
A broadcasts ARP query packet containing Bs IP address dest MAC address = FF-FF-
FF-FF-FF-FF
all nodes on LAN receive ARP query
B receives ARP packet replies to A with its (Bs) MAC address frame sent to Arsquos MAC
address (unicast)
A caches (saves) IP-to-MAC address pair in its ARP table until information becomes old (times out) soft state information that
times out (goes away) unless refreshed
ARP is ldquoplug-and-playrdquo nodes create their ARP
tables without intervention from net administrator
Link Layer 5-47
walkthrough send datagram from A to B via R
focus on addressing ndash at IP (datagram) and MAC layer (frame)
assume A knows Brsquos IP address
assume A knows IP address of first hop router R (how)
assume A knows Rrsquos MAC address (how)
Addressing routing to another LAN
R
1A-23-F9-CD-06-9B 222222222220
111111111110 E6-E9-00-17-BB-4B CC-49-DE-D0-AB-7D
111111111112
111111111111
74-29-9C-E8-FF-55
A
222222222222
49-BD-D2-C7-56-2A
222222222221 88-B2-2F-54-1A-0F
B
R
1A-23-F9-CD-06-9B 222222222220
111111111110 E6-E9-00-17-BB-4B CC-49-DE-D0-AB-7D
111111111112
111111111111
74-29-9C-E8-FF-55
A
222222222222
49-BD-D2-C7-56-2A
222222222221 88-B2-2F-54-1A-0F
B
Link Layer 5-48
Addressing routing to another LAN
IP
Eth
Phy
IP src 111111111111
IP dest 222222222222
A creates IP datagram with IP source A destination B
A creates link-layer frame with Rs MAC address as dest frame contains A-to-B IP datagram
MAC src 74-29-9C-E8-FF-55
MAC dest E6-E9-00-17-BB-4B
R
1A-23-F9-CD-06-9B 222222222220
111111111110 E6-E9-00-17-BB-4B CC-49-DE-D0-AB-7D
111111111112
111111111111
74-29-9C-E8-FF-55
A
222222222222
49-BD-D2-C7-56-2A
222222222221 88-B2-2F-54-1A-0F
B
Link Layer 5-49
Addressing routing to another LAN
IP
Eth
Phy
frame sent from A to R
IP
Eth
Phy
frame received at R datagram removed passed up to IP
MAC src 74-29-9C-E8-FF-55
MAC dest E6-E9-00-17-BB-4B
IP src 111111111111
IP dest 222222222222
IP src 111111111111
IP dest 222222222222
R
1A-23-F9-CD-06-9B 222222222220
111111111110 E6-E9-00-17-BB-4B CC-49-DE-D0-AB-7D
111111111112
111111111111
74-29-9C-E8-FF-55
A
222222222222
49-BD-D2-C7-56-2A
222222222221 88-B2-2F-54-1A-0F
B
Link Layer 5-50
Addressing routing to another LAN
IP src 111111111111
IP dest 222222222222
R forwards datagram with IP source A destination B
R creates link-layer frame with Bs MAC address as dest frame contains A-to-B IP datagram
MAC src 1A-23-F9-CD-06-9B
MAC dest 49-BD-D2-C7-56-2A
IP
Eth
Phy
IP
Eth
Phy
R
1A-23-F9-CD-06-9B 222222222220
111111111110 E6-E9-00-17-BB-4B CC-49-DE-D0-AB-7D
111111111112
111111111111
74-29-9C-E8-FF-55
A
222222222222
49-BD-D2-C7-56-2A
222222222221 88-B2-2F-54-1A-0F
B
Link Layer 5-51
Addressing routing to another LAN R forwards datagram with IP source A destination B
R creates link-layer frame with Bs MAC address as dest frame contains A-to-B IP datagram
IP src 111111111111
IP dest 222222222222
MAC src 1A-23-F9-CD-06-9B
MAC dest 49-BD-D2-C7-56-2A
IP
Eth
Phy
IP
Eth
Phy
R
1A-23-F9-CD-06-9B 222222222220
111111111110 E6-E9-00-17-BB-4B CC-49-DE-D0-AB-7D
111111111112
111111111111
74-29-9C-E8-FF-55
A
222222222222
49-BD-D2-C7-56-2A
222222222221 88-B2-2F-54-1A-0F
B
Link Layer 5-52
Addressing routing to another LAN R forwards datagram with IP source A destination B
R creates link-layer frame with Bs MAC address as dest frame contains A-to-B IP datagram
IP src 111111111111
IP dest 222222222222
MAC src 1A-23-F9-CD-06-9B
MAC dest 49-BD-D2-C7-56-2A
IP
Eth
Phy
Link Layer 5-53
Link layer LANs outline
51 introduction services
52 error detection correction
53 multiple access protocols
54 LANs addressing ARP
Ethernet
switches
VLANS
55 link virtualization MPLS
56 data center networking
57 a day in the life of a web request
Link Layer 5-54
Ethernet
ldquodominantrdquo wired LAN technology
cheap $20 for NIC
first widely used LAN technology
simpler cheaper than token LANs and ATM
kept up with speed race 10 Mbps ndash 10 Gbps
Metcalfersquos Ethernet sketch
Link Layer 5-55
Ethernet physical topology
bus popular through mid 90s all nodes in same collision domain (can collide with each
other)
star prevails today active switch in center
each ldquospokerdquo runs a (separate) Ethernet protocol (nodes
do not collide with each other)
switch
bus coaxial cable star
Link Layer 5-56
Ethernet frame structure
sending adapter encapsulates IP datagram (or other network layer protocol packet) in Ethernet frame
preamble
7 bytes with pattern 10101010 followed by one byte with pattern 10101011
used to synchronize receiver sender clock rates
dest address
source address
data (payload) CRC preamble
type
Link Layer 5-57
Ethernet frame structure (more)
addresses 6 byte source destination MAC addresses if adapter receives frame with matching destination
address or with broadcast address (eg ARP packet) it passes data in frame to network layer protocol
otherwise adapter discards frame
type indicates higher layer protocol (mostly IP but others possible eg Novell IPX AppleTalk)
CRC cyclic redundancy check at receiver error detected frame is dropped
dest address
source address
data (payload) CRC preamble
type
Link Layer 5-58
Ethernet unreliable connectionless
connectionless no handshaking between sending and receiving NICs
unreliable receiving NIC doesnt send acks or nacks to sending NIC
data in dropped frames recovered only if initial sender uses higher layer rdt (eg TCP) otherwise dropped data lost
Ethernetrsquos MAC protocol unslotted CSMACD wth binary backoff
Link Layer 5-59
8023 Ethernet standards link amp physical layers
many different Ethernet standards
common MAC protocol and frame format
different speeds 2 Mbps 10 Mbps 100 Mbps 1Gbps 10G bps
different physical layer media fiber cable
application
transport
network
link
physical
MAC protocol
and frame format
100BASE-TX
100BASE-T4
100BASE-FX 100BASE-T2
100BASE-SX 100BASE-BX
fiber physical layer copper (twister
pair) physical layer
Link Layer 5-60
Link layer LANs outline
51 introduction services
52 error detection correction
53 multiple access protocols
54 LANs addressing ARP
Ethernet
switches
VLANS
55 link virtualization MPLS
56 data center networking
57 a day in the life of a web request
Link Layer 5-61
Ethernet switch link-layer device takes an active role
store forward Ethernet frames
examine incoming framersquos MAC address selectively forward frame to one-or-more outgoing links when frame is to be forwarded on segment uses CSMACD to access segment
transparent
hosts are unaware of presence of switches
plug-and-play self-learning
switches do not need to be configured
Link Layer 5-62
Switch multiple simultaneous transmissions
hosts have dedicated direct connection to switch
switches buffer packets
Ethernet protocol used on each incoming link but no collisions full duplex
each link is its own collision domain
switching A-to-Arsquo and B-to-Brsquo can transmit simultaneously without collisions switch with six interfaces
(123456)
A
Arsquo
B
Brsquo C
Crsquo
1 2
3 4 5
6
Link Layer 5-63
Switch forwarding table
Q how does switch know Arsquo reachable via interface 4 Brsquo reachable via interface 5
switch with six interfaces
(123456)
A
Arsquo
B
Brsquo C
Crsquo
1 2
3 4 5
6 A each switch has a switch table each entry
(MAC address of host interface to
reach host time stamp)
looks like a routing table
Q how are entries created maintained in switch table
something like a routing protocol
A
Arsquo
B
Brsquo C
Crsquo
1 2
3 4 5
6
Link Layer 5-64
Switch self-learning
switch learns which hosts can be reached through which interfaces
when frame received switch ldquolearnsrdquo location of sender incoming LAN segment
records senderlocation pair in switch table
A Arsquo
Source A
Dest Arsquo
MAC addr interface TTL
Switch table
(initially empty) A 1 60
Link Layer 5-65
Switch frame filteringforwarding
when frame received at switch 1 record incoming link MAC address of sending host
2 index switch table using MAC destination address
3 if entry found for destination then
if destination on segment from which frame arrived then drop frame
else forward frame on interface indicated by entry
else flood forward on all interfaces except arriving
interface
A
Arsquo
B
Brsquo C
Crsquo
1 2
3 4 5
6
Link Layer 5-66
Self-learning forwarding example
A Arsquo
Source A
Dest Arsquo
MAC addr interface TTL
switch table
(initially empty) A 1 60
A Arsquo A Arsquo A Arsquo A Arsquo A Arsquo
frame destination Arsquo locaton unknown flood
Arsquo A
destination A location
known
Arsquo 4 60
selectively send
on just one link
Link Layer 5-67
Interconnecting switches
switches can be connected together
Q sending from A to G - how does S1 know to forward frame destined to F via S4 and S3
A self learning (works exactly the same as in single-switch case)
A
B
S1
C D
E
F
S2
S4
S3
H
I
G
Link Layer 5-68
Self-learning multi-switch example
Suppose C sends frame to I I responds to C
Q show switch tables and packet forwarding in S1 S2 S3 S4
A
B
S1
C D
E
F
S2
S4
S3
H
I
G
Link Layer 5-69
Institutional network
to external
network
router
IP subnet
mail server
web server
Link Layer 5-70
Switches vs routers
both are store-and-forward
routers network-layer devices (examine network-layer headers)
switches link-layer devices (examine link-layer headers)
both have forwarding tables
routers compute tables using routing algorithms IP addresses
switches learn forwarding table using flooding learning MAC addresses
application
transport
network
link
physical
network
link
physical
link
physical
switch
datagram
application
transport
network
link
physical
frame
frame
frame
datagram
Link Layer 5-71
VLANs motivation
consider CS user moves office to
EE but wants connect to CS switch
single broadcast domain
all layer-2 broadcast traffic (ARP DHCP unknown location of destination MAC address) must cross entire LAN
securityprivacy efficiency issues
Computer
Science Electrical
Engineering
Computer
Engineering
Link Layer 5-72
VLANs port-based VLAN switch ports
grouped (by switch management software) so that single physical switch helliphellip
switch(es) supporting
VLAN capabilities can
be configured to
define multiple virtual
LANS over single
physical LAN
infrastructure
Virtual Local
Area Network 1
8
9
16 10 2
7
hellip
Electrical Engineering
(VLAN ports 1-8)
Computer Science
(VLAN ports 9-15)
15
hellip
Electrical Engineering
(VLAN ports 1-8)
hellip
1
8 2
7 9
16 10
15
hellip
Computer Science
(VLAN ports 9-16)
hellip operates as multiple virtual switches
Link Layer 5-73
Port-based VLAN
1
8
9
16 10 2
7
hellip
Electrical Engineering
(VLAN ports 1-8)
Computer Science
(VLAN ports 9-15)
15
hellip
traffic isolation frames tofrom ports 1-8 can only reach ports 1-8 can also define VLAN based on
MAC addresses of endpoints rather than switch port
dynamic membership ports can be dynamically assigned among VLANs
router
forwarding between VLANS done via routing (just as with separate switches) in practice vendors sell combined
switches plus routers
Link Layer 5-74
VLANS spanning multiple switches
trunk port carries frames between VLANS defined over multiple physical switches frames forwarded within VLAN between switches canrsquot be vanilla
8021 frames (must carry VLAN ID info)
8021q protocol addsremoved additional header fields for frames forwarded between trunk ports
1
8
9
10 2
7
hellip
Electrical Engineering
(VLAN ports 1-8)
Computer Science
(VLAN ports 9-15)
15
hellip
2
7 3
Ports 235 belong to EE VLAN
Ports 4678 belong to CS VLAN
5
4 6 8 16
1
Link Layer 5-75
type
2-byte Tag Protocol Identifier
(value 81-00)
Tag Control Information (12 bit VLAN ID field
3 bit priority field like IP TOS)
Recomputed CRC
8021Q VLAN frame format
8021 frame
8021Q frame
dest address
source address
data (payload) CRC preamble
dest address
source address
preamble data (payload) CRC
type
Link Layer 5-76
Link layer LANs outline
51 introduction services
52 error detection correction
53 multiple access protocols
54 LANs addressing ARP
Ethernet
switches
VLANS
55 link virtualization MPLS
56 data center networking
57 a day in the life of a web request
Link Layer 5-77
Multiprotocol label switching (MPLS)
initial goal high-speed IP forwarding using fixed length label (instead of IP address) fast lookup using fixed length identifier (rather than
shortest prefix matching)
borrowing ideas from Virtual Circuit (VC) approach
but IP datagram still keeps IP address
PPP or Ethernet
header IP header remainder of link-layer frame MPLS header
label Exp S TTL
20 3 1 5
Link Layer 5-78
MPLS capable routers
aka label-switched router
forward packets to outgoing interface based only on label value (donrsquot inspect IP address) MPLS forwarding table distinct from IP forwarding tables
flexibility MPLS forwarding decisions can differ from those of IP use destination and source addresses to route flows to
same destination differently (traffic engineering)
re-route flows quickly if link fails pre-computed backup paths (useful for VoIP)
Link Layer 5-79
R2
D
R3 R5
A
R6
MPLS versus IP paths
IP router IP routing path to destination determined
by destination address alone
R4
Link Layer 5-80
R2
D
R3 R4
R5
A
R6
MPLS versus IP paths
IP-only router
IP routing path to destination determined by destination address alone
MPLS and IP router
MPLS routing path to destination can be based on source and dest address fast reroute precompute backup routes in
case of link failure
entry router (R4) can use different MPLS
routes to A based eg on source address
Link Layer 5-81
MPLS signaling
modify OSPF IS-IS link-state flooding protocols to carry info used by MPLS routing eg link bandwidth amount of ldquoreservedrdquo link bandwidth
D
R4
R5
A
R6
entry MPLS router uses RSVP-TE signaling protocol to set up MPLS forwarding at downstream routers
modified link state flooding
RSVP-TE
Link Layer 5-82
R1 R2
D
R3 R4 R5
0
1
0 0
A
R6
in out out
label label dest interface
6 - A 0
in out out
label label dest interface
10 6 A 1
12 9 D 0
in out out
label label dest interface
10 A 0
12 D 0
1
in out out
label label dest interface
8 6 A 0
0
8 A 1
MPLS forwarding tables
Link Layer 5-83
Link layer LANs outline
51 introduction services
52 error detection correction
53 multiple access protocols
54 LANs addressing ARP
Ethernet
switches
VLANS
55 link virtualization MPLS
56 data center networking
57 a day in the life of a web request
Link Layer 5-84
Data center networks
10rsquos to 100rsquos of thousands of hosts often closely coupled in close proximity e-business (eg Amazon)
content-servers (eg YouTube Akamai Apple Microsoft)
search engines data mining (eg Google)
challenges
multiple applications each serving massive numbers of clients
managingbalancing load avoiding processing networking data bottlenecks
Inside a 40-ft Microsoft container
Chicago data center
Link Layer 5-85
Server racks
TOR switches
Tier-1 switches
Tier-2 switches
Load balancer
Load balancer
B
1 2 3 4 5 6 7 8
A C
Border router
Access router
Internet
Data center networks
load balancer application-layer routing receives external client requests
directs workload within data center
returns results to external client (hiding data
center internals from client)
Server racks
TOR switches
Tier-1 switches
Tier-2 switches
1 2 3 4 5 6 7 8
Data center networks
rich interconnection among switches racks
increased throughput between racks (multiple routing
paths possible)
increased reliability via redundancy
Link Layer 5-87
Link layer LANs outline
51 introduction services
52 error detection correction
53 multiple access protocols
54 LANs addressing ARP
Ethernet
switches
VLANS
55 link virtualization MPLS
56 data center networking
57 a day in the life of a web request
Link Layer 5-88
Synthesis a day in the life of a web request
journey down protocol stack complete application transport network link
putting-it-all-together synthesis goal identify review understand protocols (at all
layers) involved in seemingly simple scenario requesting www page
scenario student attaches laptop to campus network requestsreceives wwwgooglecom
Link Layer 5-89
A day in the life scenario
Comcast network
68800013
Googlersquos network
64233160019 64233169105
web server
DNS server
school network
68802024
web page
browser
router
(runs DHCP)
Link Layer 5-90
A day in the lifehellip connecting to the Internet
connecting laptop needs to get its own IP address addr of first-hop router addr of DNS server use DHCP
DHCP
UDP
IP
Eth
Phy
DHCP
DHCP
DHCP
DHCP
DHCP
DHCP
UDP
IP
Eth
Phy
DHCP
DHCP
DHCP
DHCP DHCP
DHCP request encapsulated in UDP encapsulated in IP encapsulated in 8023 Ethernet
Ethernet frame broadcast
(dest FFFFFFFFFFFF) on LAN received at router running DHCP server
Ethernet demuxed to IP demuxed UDP demuxed to DHCP
router
(runs DHCP)
Link Layer 5-91
DHCP server formulates DHCP ACK containing clientrsquos IP address IP address of first-hop router for client name amp IP address of DNS server
DHCP
UDP
IP
Eth
Phy
DHCP
DHCP
DHCP
DHCP
DHCP
UDP
IP
Eth
Phy
DHCP
DHCP
DHCP
DHCP
DHCP
encapsulation at DHCP server frame forwarded (switch learning) through LAN demultiplexing at client
Client now has IP address knows name amp addr of DNS
server IP address of its first-hop router
DHCP client receives DHCP ACK reply
A day in the lifehellip connecting to the Internet
router
(runs DHCP)
Link Layer 5-92
A day in the lifehellip ARP (before DNS before HTTP)
before sending HTTP request need IP address of wwwgooglecom DNS
DNS
UDP
IP
Eth
Phy
DNS
DNS
DNS
DNS query created encapsulated in UDP encapsulated in IP encapsulated in Eth To send frame to router need MAC address of router interface ARP
ARP query broadcast received by
router which replies with ARP reply giving MAC address of router interface
client now knows MAC address of first hop router so can now send frame containing DNS query
ARP query
Eth
Phy
ARP
ARP
ARP reply
router
(runs DHCP)
Link Layer 5-93
DNS
UDP
IP
Eth
Phy
DNS
DNS
DNS
DNS
DNS
IP datagram containing DNS query forwarded via LAN switch from client to 1st hop router
IP datagram forwarded from campus network into comcast network routed (tables created by RIP OSPF IS-IS andor BGP routing protocols) to DNS server
demuxrsquoed to DNS server
DNS server replies to client with IP address of wwwgooglecom
Comcast network
68800013
DNS server
DNS
UDP
IP
Eth
Phy
DNS
DNS
DNS
DNS
A day in the lifehellip using DNS
router
(runs DHCP)
Link Layer 5-94
A day in the lifehellipTCP connection carrying HTTP
HTTP
TCP
IP
Eth
Phy
HTTP
to send HTTP request client first opens TCP socket to web server
TCP SYN segment (step 1 in 3-way handshake) inter-domain routed to web server
TCP connection established 64233169105
web server
SYN
SYN
SYN
SYN
TCP
IP
Eth
Phy
SYN
SYN
SYN
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
web server responds with TCP SYNACK (step 2 in 3-way handshake)
router
(runs DHCP)
Link Layer 5-95
A day in the lifehellip HTTP requestreply
HTTP
TCP
IP
Eth
Phy
HTTP
HTTP request sent into TCP socket
IP datagram containing HTTP request routed to wwwgooglecom
IP datagram containing HTTP reply routed back to client
64233169105
web server
HTTP
TCP
IP
Eth
Phy web server responds with
HTTP reply (containing web page)
HTTP
HTTP
HTTP HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
web page finally () displayed
Link Layer 5-96
Chapter 5 Summary
principles behind data link layer services error detection correction
sharing a broadcast channel multiple access
link layer addressing
instantiation and implementation of various link layer technologies Ethernet
switched LANS VLANs
virtualized networks as a link layer MPLS
synthesis a day in the life of a web request
Link Layer 5-97
Chapter 5 letrsquos take a breath
journey down protocol stack complete (except PHY)
solid understanding of networking principles practice
hellip could stop here hellip but lots of interesting topics wireless
multimedia
security
network management
Link Layer 5-43
LAN addresses and ARP
each adapter on LAN has unique LAN address
adapter
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN
(wired or
wireless)
Link Layer 5-44
LAN addresses (more)
MAC address allocation administered by IEEE
manufacturer buys portion of MAC address space (to assure uniqueness)
analogy MAC address like Social Security Number
IP address like postal address
MAC flat address portability can move LAN card from one LAN to another
IP hierarchical address not portable address depends on IP subnet to which node is
attached
Link Layer 5-45
ARP address resolution protocol
ARP table each IP node (host router) on LAN has table
IPMAC address mappings for some LAN nodes
lt IP address MAC address TTLgt
TTL (Time To Live) time after which address mapping will be forgotten (typically 20 min)
Question how to determine
interfacersquos MAC address
knowing its IP address
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN
137196723
137196778
137196714
137196788
Link Layer 5-46
ARP protocol same LAN
A wants to send datagram to B Brsquos MAC address not in
Arsquos ARP table
A broadcasts ARP query packet containing Bs IP address dest MAC address = FF-FF-
FF-FF-FF-FF
all nodes on LAN receive ARP query
B receives ARP packet replies to A with its (Bs) MAC address frame sent to Arsquos MAC
address (unicast)
A caches (saves) IP-to-MAC address pair in its ARP table until information becomes old (times out) soft state information that
times out (goes away) unless refreshed
ARP is ldquoplug-and-playrdquo nodes create their ARP
tables without intervention from net administrator
Link Layer 5-47
walkthrough send datagram from A to B via R
focus on addressing ndash at IP (datagram) and MAC layer (frame)
assume A knows Brsquos IP address
assume A knows IP address of first hop router R (how)
assume A knows Rrsquos MAC address (how)
Addressing routing to another LAN
R
1A-23-F9-CD-06-9B 222222222220
111111111110 E6-E9-00-17-BB-4B CC-49-DE-D0-AB-7D
111111111112
111111111111
74-29-9C-E8-FF-55
A
222222222222
49-BD-D2-C7-56-2A
222222222221 88-B2-2F-54-1A-0F
B
R
1A-23-F9-CD-06-9B 222222222220
111111111110 E6-E9-00-17-BB-4B CC-49-DE-D0-AB-7D
111111111112
111111111111
74-29-9C-E8-FF-55
A
222222222222
49-BD-D2-C7-56-2A
222222222221 88-B2-2F-54-1A-0F
B
Link Layer 5-48
Addressing routing to another LAN
IP
Eth
Phy
IP src 111111111111
IP dest 222222222222
A creates IP datagram with IP source A destination B
A creates link-layer frame with Rs MAC address as dest frame contains A-to-B IP datagram
MAC src 74-29-9C-E8-FF-55
MAC dest E6-E9-00-17-BB-4B
R
1A-23-F9-CD-06-9B 222222222220
111111111110 E6-E9-00-17-BB-4B CC-49-DE-D0-AB-7D
111111111112
111111111111
74-29-9C-E8-FF-55
A
222222222222
49-BD-D2-C7-56-2A
222222222221 88-B2-2F-54-1A-0F
B
Link Layer 5-49
Addressing routing to another LAN
IP
Eth
Phy
frame sent from A to R
IP
Eth
Phy
frame received at R datagram removed passed up to IP
MAC src 74-29-9C-E8-FF-55
MAC dest E6-E9-00-17-BB-4B
IP src 111111111111
IP dest 222222222222
IP src 111111111111
IP dest 222222222222
R
1A-23-F9-CD-06-9B 222222222220
111111111110 E6-E9-00-17-BB-4B CC-49-DE-D0-AB-7D
111111111112
111111111111
74-29-9C-E8-FF-55
A
222222222222
49-BD-D2-C7-56-2A
222222222221 88-B2-2F-54-1A-0F
B
Link Layer 5-50
Addressing routing to another LAN
IP src 111111111111
IP dest 222222222222
R forwards datagram with IP source A destination B
R creates link-layer frame with Bs MAC address as dest frame contains A-to-B IP datagram
MAC src 1A-23-F9-CD-06-9B
MAC dest 49-BD-D2-C7-56-2A
IP
Eth
Phy
IP
Eth
Phy
R
1A-23-F9-CD-06-9B 222222222220
111111111110 E6-E9-00-17-BB-4B CC-49-DE-D0-AB-7D
111111111112
111111111111
74-29-9C-E8-FF-55
A
222222222222
49-BD-D2-C7-56-2A
222222222221 88-B2-2F-54-1A-0F
B
Link Layer 5-51
Addressing routing to another LAN R forwards datagram with IP source A destination B
R creates link-layer frame with Bs MAC address as dest frame contains A-to-B IP datagram
IP src 111111111111
IP dest 222222222222
MAC src 1A-23-F9-CD-06-9B
MAC dest 49-BD-D2-C7-56-2A
IP
Eth
Phy
IP
Eth
Phy
R
1A-23-F9-CD-06-9B 222222222220
111111111110 E6-E9-00-17-BB-4B CC-49-DE-D0-AB-7D
111111111112
111111111111
74-29-9C-E8-FF-55
A
222222222222
49-BD-D2-C7-56-2A
222222222221 88-B2-2F-54-1A-0F
B
Link Layer 5-52
Addressing routing to another LAN R forwards datagram with IP source A destination B
R creates link-layer frame with Bs MAC address as dest frame contains A-to-B IP datagram
IP src 111111111111
IP dest 222222222222
MAC src 1A-23-F9-CD-06-9B
MAC dest 49-BD-D2-C7-56-2A
IP
Eth
Phy
Link Layer 5-53
Link layer LANs outline
51 introduction services
52 error detection correction
53 multiple access protocols
54 LANs addressing ARP
Ethernet
switches
VLANS
55 link virtualization MPLS
56 data center networking
57 a day in the life of a web request
Link Layer 5-54
Ethernet
ldquodominantrdquo wired LAN technology
cheap $20 for NIC
first widely used LAN technology
simpler cheaper than token LANs and ATM
kept up with speed race 10 Mbps ndash 10 Gbps
Metcalfersquos Ethernet sketch
Link Layer 5-55
Ethernet physical topology
bus popular through mid 90s all nodes in same collision domain (can collide with each
other)
star prevails today active switch in center
each ldquospokerdquo runs a (separate) Ethernet protocol (nodes
do not collide with each other)
switch
bus coaxial cable star
Link Layer 5-56
Ethernet frame structure
sending adapter encapsulates IP datagram (or other network layer protocol packet) in Ethernet frame
preamble
7 bytes with pattern 10101010 followed by one byte with pattern 10101011
used to synchronize receiver sender clock rates
dest address
source address
data (payload) CRC preamble
type
Link Layer 5-57
Ethernet frame structure (more)
addresses 6 byte source destination MAC addresses if adapter receives frame with matching destination
address or with broadcast address (eg ARP packet) it passes data in frame to network layer protocol
otherwise adapter discards frame
type indicates higher layer protocol (mostly IP but others possible eg Novell IPX AppleTalk)
CRC cyclic redundancy check at receiver error detected frame is dropped
dest address
source address
data (payload) CRC preamble
type
Link Layer 5-58
Ethernet unreliable connectionless
connectionless no handshaking between sending and receiving NICs
unreliable receiving NIC doesnt send acks or nacks to sending NIC
data in dropped frames recovered only if initial sender uses higher layer rdt (eg TCP) otherwise dropped data lost
Ethernetrsquos MAC protocol unslotted CSMACD wth binary backoff
Link Layer 5-59
8023 Ethernet standards link amp physical layers
many different Ethernet standards
common MAC protocol and frame format
different speeds 2 Mbps 10 Mbps 100 Mbps 1Gbps 10G bps
different physical layer media fiber cable
application
transport
network
link
physical
MAC protocol
and frame format
100BASE-TX
100BASE-T4
100BASE-FX 100BASE-T2
100BASE-SX 100BASE-BX
fiber physical layer copper (twister
pair) physical layer
Link Layer 5-60
Link layer LANs outline
51 introduction services
52 error detection correction
53 multiple access protocols
54 LANs addressing ARP
Ethernet
switches
VLANS
55 link virtualization MPLS
56 data center networking
57 a day in the life of a web request
Link Layer 5-61
Ethernet switch link-layer device takes an active role
store forward Ethernet frames
examine incoming framersquos MAC address selectively forward frame to one-or-more outgoing links when frame is to be forwarded on segment uses CSMACD to access segment
transparent
hosts are unaware of presence of switches
plug-and-play self-learning
switches do not need to be configured
Link Layer 5-62
Switch multiple simultaneous transmissions
hosts have dedicated direct connection to switch
switches buffer packets
Ethernet protocol used on each incoming link but no collisions full duplex
each link is its own collision domain
switching A-to-Arsquo and B-to-Brsquo can transmit simultaneously without collisions switch with six interfaces
(123456)
A
Arsquo
B
Brsquo C
Crsquo
1 2
3 4 5
6
Link Layer 5-63
Switch forwarding table
Q how does switch know Arsquo reachable via interface 4 Brsquo reachable via interface 5
switch with six interfaces
(123456)
A
Arsquo
B
Brsquo C
Crsquo
1 2
3 4 5
6 A each switch has a switch table each entry
(MAC address of host interface to
reach host time stamp)
looks like a routing table
Q how are entries created maintained in switch table
something like a routing protocol
A
Arsquo
B
Brsquo C
Crsquo
1 2
3 4 5
6
Link Layer 5-64
Switch self-learning
switch learns which hosts can be reached through which interfaces
when frame received switch ldquolearnsrdquo location of sender incoming LAN segment
records senderlocation pair in switch table
A Arsquo
Source A
Dest Arsquo
MAC addr interface TTL
Switch table
(initially empty) A 1 60
Link Layer 5-65
Switch frame filteringforwarding
when frame received at switch 1 record incoming link MAC address of sending host
2 index switch table using MAC destination address
3 if entry found for destination then
if destination on segment from which frame arrived then drop frame
else forward frame on interface indicated by entry
else flood forward on all interfaces except arriving
interface
A
Arsquo
B
Brsquo C
Crsquo
1 2
3 4 5
6
Link Layer 5-66
Self-learning forwarding example
A Arsquo
Source A
Dest Arsquo
MAC addr interface TTL
switch table
(initially empty) A 1 60
A Arsquo A Arsquo A Arsquo A Arsquo A Arsquo
frame destination Arsquo locaton unknown flood
Arsquo A
destination A location
known
Arsquo 4 60
selectively send
on just one link
Link Layer 5-67
Interconnecting switches
switches can be connected together
Q sending from A to G - how does S1 know to forward frame destined to F via S4 and S3
A self learning (works exactly the same as in single-switch case)
A
B
S1
C D
E
F
S2
S4
S3
H
I
G
Link Layer 5-68
Self-learning multi-switch example
Suppose C sends frame to I I responds to C
Q show switch tables and packet forwarding in S1 S2 S3 S4
A
B
S1
C D
E
F
S2
S4
S3
H
I
G
Link Layer 5-69
Institutional network
to external
network
router
IP subnet
mail server
web server
Link Layer 5-70
Switches vs routers
both are store-and-forward
routers network-layer devices (examine network-layer headers)
switches link-layer devices (examine link-layer headers)
both have forwarding tables
routers compute tables using routing algorithms IP addresses
switches learn forwarding table using flooding learning MAC addresses
application
transport
network
link
physical
network
link
physical
link
physical
switch
datagram
application
transport
network
link
physical
frame
frame
frame
datagram
Link Layer 5-71
VLANs motivation
consider CS user moves office to
EE but wants connect to CS switch
single broadcast domain
all layer-2 broadcast traffic (ARP DHCP unknown location of destination MAC address) must cross entire LAN
securityprivacy efficiency issues
Computer
Science Electrical
Engineering
Computer
Engineering
Link Layer 5-72
VLANs port-based VLAN switch ports
grouped (by switch management software) so that single physical switch helliphellip
switch(es) supporting
VLAN capabilities can
be configured to
define multiple virtual
LANS over single
physical LAN
infrastructure
Virtual Local
Area Network 1
8
9
16 10 2
7
hellip
Electrical Engineering
(VLAN ports 1-8)
Computer Science
(VLAN ports 9-15)
15
hellip
Electrical Engineering
(VLAN ports 1-8)
hellip
1
8 2
7 9
16 10
15
hellip
Computer Science
(VLAN ports 9-16)
hellip operates as multiple virtual switches
Link Layer 5-73
Port-based VLAN
1
8
9
16 10 2
7
hellip
Electrical Engineering
(VLAN ports 1-8)
Computer Science
(VLAN ports 9-15)
15
hellip
traffic isolation frames tofrom ports 1-8 can only reach ports 1-8 can also define VLAN based on
MAC addresses of endpoints rather than switch port
dynamic membership ports can be dynamically assigned among VLANs
router
forwarding between VLANS done via routing (just as with separate switches) in practice vendors sell combined
switches plus routers
Link Layer 5-74
VLANS spanning multiple switches
trunk port carries frames between VLANS defined over multiple physical switches frames forwarded within VLAN between switches canrsquot be vanilla
8021 frames (must carry VLAN ID info)
8021q protocol addsremoved additional header fields for frames forwarded between trunk ports
1
8
9
10 2
7
hellip
Electrical Engineering
(VLAN ports 1-8)
Computer Science
(VLAN ports 9-15)
15
hellip
2
7 3
Ports 235 belong to EE VLAN
Ports 4678 belong to CS VLAN
5
4 6 8 16
1
Link Layer 5-75
type
2-byte Tag Protocol Identifier
(value 81-00)
Tag Control Information (12 bit VLAN ID field
3 bit priority field like IP TOS)
Recomputed CRC
8021Q VLAN frame format
8021 frame
8021Q frame
dest address
source address
data (payload) CRC preamble
dest address
source address
preamble data (payload) CRC
type
Link Layer 5-76
Link layer LANs outline
51 introduction services
52 error detection correction
53 multiple access protocols
54 LANs addressing ARP
Ethernet
switches
VLANS
55 link virtualization MPLS
56 data center networking
57 a day in the life of a web request
Link Layer 5-77
Multiprotocol label switching (MPLS)
initial goal high-speed IP forwarding using fixed length label (instead of IP address) fast lookup using fixed length identifier (rather than
shortest prefix matching)
borrowing ideas from Virtual Circuit (VC) approach
but IP datagram still keeps IP address
PPP or Ethernet
header IP header remainder of link-layer frame MPLS header
label Exp S TTL
20 3 1 5
Link Layer 5-78
MPLS capable routers
aka label-switched router
forward packets to outgoing interface based only on label value (donrsquot inspect IP address) MPLS forwarding table distinct from IP forwarding tables
flexibility MPLS forwarding decisions can differ from those of IP use destination and source addresses to route flows to
same destination differently (traffic engineering)
re-route flows quickly if link fails pre-computed backup paths (useful for VoIP)
Link Layer 5-79
R2
D
R3 R5
A
R6
MPLS versus IP paths
IP router IP routing path to destination determined
by destination address alone
R4
Link Layer 5-80
R2
D
R3 R4
R5
A
R6
MPLS versus IP paths
IP-only router
IP routing path to destination determined by destination address alone
MPLS and IP router
MPLS routing path to destination can be based on source and dest address fast reroute precompute backup routes in
case of link failure
entry router (R4) can use different MPLS
routes to A based eg on source address
Link Layer 5-81
MPLS signaling
modify OSPF IS-IS link-state flooding protocols to carry info used by MPLS routing eg link bandwidth amount of ldquoreservedrdquo link bandwidth
D
R4
R5
A
R6
entry MPLS router uses RSVP-TE signaling protocol to set up MPLS forwarding at downstream routers
modified link state flooding
RSVP-TE
Link Layer 5-82
R1 R2
D
R3 R4 R5
0
1
0 0
A
R6
in out out
label label dest interface
6 - A 0
in out out
label label dest interface
10 6 A 1
12 9 D 0
in out out
label label dest interface
10 A 0
12 D 0
1
in out out
label label dest interface
8 6 A 0
0
8 A 1
MPLS forwarding tables
Link Layer 5-83
Link layer LANs outline
51 introduction services
52 error detection correction
53 multiple access protocols
54 LANs addressing ARP
Ethernet
switches
VLANS
55 link virtualization MPLS
56 data center networking
57 a day in the life of a web request
Link Layer 5-84
Data center networks
10rsquos to 100rsquos of thousands of hosts often closely coupled in close proximity e-business (eg Amazon)
content-servers (eg YouTube Akamai Apple Microsoft)
search engines data mining (eg Google)
challenges
multiple applications each serving massive numbers of clients
managingbalancing load avoiding processing networking data bottlenecks
Inside a 40-ft Microsoft container
Chicago data center
Link Layer 5-85
Server racks
TOR switches
Tier-1 switches
Tier-2 switches
Load balancer
Load balancer
B
1 2 3 4 5 6 7 8
A C
Border router
Access router
Internet
Data center networks
load balancer application-layer routing receives external client requests
directs workload within data center
returns results to external client (hiding data
center internals from client)
Server racks
TOR switches
Tier-1 switches
Tier-2 switches
1 2 3 4 5 6 7 8
Data center networks
rich interconnection among switches racks
increased throughput between racks (multiple routing
paths possible)
increased reliability via redundancy
Link Layer 5-87
Link layer LANs outline
51 introduction services
52 error detection correction
53 multiple access protocols
54 LANs addressing ARP
Ethernet
switches
VLANS
55 link virtualization MPLS
56 data center networking
57 a day in the life of a web request
Link Layer 5-88
Synthesis a day in the life of a web request
journey down protocol stack complete application transport network link
putting-it-all-together synthesis goal identify review understand protocols (at all
layers) involved in seemingly simple scenario requesting www page
scenario student attaches laptop to campus network requestsreceives wwwgooglecom
Link Layer 5-89
A day in the life scenario
Comcast network
68800013
Googlersquos network
64233160019 64233169105
web server
DNS server
school network
68802024
web page
browser
router
(runs DHCP)
Link Layer 5-90
A day in the lifehellip connecting to the Internet
connecting laptop needs to get its own IP address addr of first-hop router addr of DNS server use DHCP
DHCP
UDP
IP
Eth
Phy
DHCP
DHCP
DHCP
DHCP
DHCP
DHCP
UDP
IP
Eth
Phy
DHCP
DHCP
DHCP
DHCP DHCP
DHCP request encapsulated in UDP encapsulated in IP encapsulated in 8023 Ethernet
Ethernet frame broadcast
(dest FFFFFFFFFFFF) on LAN received at router running DHCP server
Ethernet demuxed to IP demuxed UDP demuxed to DHCP
router
(runs DHCP)
Link Layer 5-91
DHCP server formulates DHCP ACK containing clientrsquos IP address IP address of first-hop router for client name amp IP address of DNS server
DHCP
UDP
IP
Eth
Phy
DHCP
DHCP
DHCP
DHCP
DHCP
UDP
IP
Eth
Phy
DHCP
DHCP
DHCP
DHCP
DHCP
encapsulation at DHCP server frame forwarded (switch learning) through LAN demultiplexing at client
Client now has IP address knows name amp addr of DNS
server IP address of its first-hop router
DHCP client receives DHCP ACK reply
A day in the lifehellip connecting to the Internet
router
(runs DHCP)
Link Layer 5-92
A day in the lifehellip ARP (before DNS before HTTP)
before sending HTTP request need IP address of wwwgooglecom DNS
DNS
UDP
IP
Eth
Phy
DNS
DNS
DNS
DNS query created encapsulated in UDP encapsulated in IP encapsulated in Eth To send frame to router need MAC address of router interface ARP
ARP query broadcast received by
router which replies with ARP reply giving MAC address of router interface
client now knows MAC address of first hop router so can now send frame containing DNS query
ARP query
Eth
Phy
ARP
ARP
ARP reply
router
(runs DHCP)
Link Layer 5-93
DNS
UDP
IP
Eth
Phy
DNS
DNS
DNS
DNS
DNS
IP datagram containing DNS query forwarded via LAN switch from client to 1st hop router
IP datagram forwarded from campus network into comcast network routed (tables created by RIP OSPF IS-IS andor BGP routing protocols) to DNS server
demuxrsquoed to DNS server
DNS server replies to client with IP address of wwwgooglecom
Comcast network
68800013
DNS server
DNS
UDP
IP
Eth
Phy
DNS
DNS
DNS
DNS
A day in the lifehellip using DNS
router
(runs DHCP)
Link Layer 5-94
A day in the lifehellipTCP connection carrying HTTP
HTTP
TCP
IP
Eth
Phy
HTTP
to send HTTP request client first opens TCP socket to web server
TCP SYN segment (step 1 in 3-way handshake) inter-domain routed to web server
TCP connection established 64233169105
web server
SYN
SYN
SYN
SYN
TCP
IP
Eth
Phy
SYN
SYN
SYN
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
web server responds with TCP SYNACK (step 2 in 3-way handshake)
router
(runs DHCP)
Link Layer 5-95
A day in the lifehellip HTTP requestreply
HTTP
TCP
IP
Eth
Phy
HTTP
HTTP request sent into TCP socket
IP datagram containing HTTP request routed to wwwgooglecom
IP datagram containing HTTP reply routed back to client
64233169105
web server
HTTP
TCP
IP
Eth
Phy web server responds with
HTTP reply (containing web page)
HTTP
HTTP
HTTP HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
web page finally () displayed
Link Layer 5-96
Chapter 5 Summary
principles behind data link layer services error detection correction
sharing a broadcast channel multiple access
link layer addressing
instantiation and implementation of various link layer technologies Ethernet
switched LANS VLANs
virtualized networks as a link layer MPLS
synthesis a day in the life of a web request
Link Layer 5-97
Chapter 5 letrsquos take a breath
journey down protocol stack complete (except PHY)
solid understanding of networking principles practice
hellip could stop here hellip but lots of interesting topics wireless
multimedia
security
network management
Link Layer 5-44
LAN addresses (more)
MAC address allocation administered by IEEE
manufacturer buys portion of MAC address space (to assure uniqueness)
analogy MAC address like Social Security Number
IP address like postal address
MAC flat address portability can move LAN card from one LAN to another
IP hierarchical address not portable address depends on IP subnet to which node is
attached
Link Layer 5-45
ARP address resolution protocol
ARP table each IP node (host router) on LAN has table
IPMAC address mappings for some LAN nodes
lt IP address MAC address TTLgt
TTL (Time To Live) time after which address mapping will be forgotten (typically 20 min)
Question how to determine
interfacersquos MAC address
knowing its IP address
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN
137196723
137196778
137196714
137196788
Link Layer 5-46
ARP protocol same LAN
A wants to send datagram to B Brsquos MAC address not in
Arsquos ARP table
A broadcasts ARP query packet containing Bs IP address dest MAC address = FF-FF-
FF-FF-FF-FF
all nodes on LAN receive ARP query
B receives ARP packet replies to A with its (Bs) MAC address frame sent to Arsquos MAC
address (unicast)
A caches (saves) IP-to-MAC address pair in its ARP table until information becomes old (times out) soft state information that
times out (goes away) unless refreshed
ARP is ldquoplug-and-playrdquo nodes create their ARP
tables without intervention from net administrator
Link Layer 5-47
walkthrough send datagram from A to B via R
focus on addressing ndash at IP (datagram) and MAC layer (frame)
assume A knows Brsquos IP address
assume A knows IP address of first hop router R (how)
assume A knows Rrsquos MAC address (how)
Addressing routing to another LAN
R
1A-23-F9-CD-06-9B 222222222220
111111111110 E6-E9-00-17-BB-4B CC-49-DE-D0-AB-7D
111111111112
111111111111
74-29-9C-E8-FF-55
A
222222222222
49-BD-D2-C7-56-2A
222222222221 88-B2-2F-54-1A-0F
B
R
1A-23-F9-CD-06-9B 222222222220
111111111110 E6-E9-00-17-BB-4B CC-49-DE-D0-AB-7D
111111111112
111111111111
74-29-9C-E8-FF-55
A
222222222222
49-BD-D2-C7-56-2A
222222222221 88-B2-2F-54-1A-0F
B
Link Layer 5-48
Addressing routing to another LAN
IP
Eth
Phy
IP src 111111111111
IP dest 222222222222
A creates IP datagram with IP source A destination B
A creates link-layer frame with Rs MAC address as dest frame contains A-to-B IP datagram
MAC src 74-29-9C-E8-FF-55
MAC dest E6-E9-00-17-BB-4B
R
1A-23-F9-CD-06-9B 222222222220
111111111110 E6-E9-00-17-BB-4B CC-49-DE-D0-AB-7D
111111111112
111111111111
74-29-9C-E8-FF-55
A
222222222222
49-BD-D2-C7-56-2A
222222222221 88-B2-2F-54-1A-0F
B
Link Layer 5-49
Addressing routing to another LAN
IP
Eth
Phy
frame sent from A to R
IP
Eth
Phy
frame received at R datagram removed passed up to IP
MAC src 74-29-9C-E8-FF-55
MAC dest E6-E9-00-17-BB-4B
IP src 111111111111
IP dest 222222222222
IP src 111111111111
IP dest 222222222222
R
1A-23-F9-CD-06-9B 222222222220
111111111110 E6-E9-00-17-BB-4B CC-49-DE-D0-AB-7D
111111111112
111111111111
74-29-9C-E8-FF-55
A
222222222222
49-BD-D2-C7-56-2A
222222222221 88-B2-2F-54-1A-0F
B
Link Layer 5-50
Addressing routing to another LAN
IP src 111111111111
IP dest 222222222222
R forwards datagram with IP source A destination B
R creates link-layer frame with Bs MAC address as dest frame contains A-to-B IP datagram
MAC src 1A-23-F9-CD-06-9B
MAC dest 49-BD-D2-C7-56-2A
IP
Eth
Phy
IP
Eth
Phy
R
1A-23-F9-CD-06-9B 222222222220
111111111110 E6-E9-00-17-BB-4B CC-49-DE-D0-AB-7D
111111111112
111111111111
74-29-9C-E8-FF-55
A
222222222222
49-BD-D2-C7-56-2A
222222222221 88-B2-2F-54-1A-0F
B
Link Layer 5-51
Addressing routing to another LAN R forwards datagram with IP source A destination B
R creates link-layer frame with Bs MAC address as dest frame contains A-to-B IP datagram
IP src 111111111111
IP dest 222222222222
MAC src 1A-23-F9-CD-06-9B
MAC dest 49-BD-D2-C7-56-2A
IP
Eth
Phy
IP
Eth
Phy
R
1A-23-F9-CD-06-9B 222222222220
111111111110 E6-E9-00-17-BB-4B CC-49-DE-D0-AB-7D
111111111112
111111111111
74-29-9C-E8-FF-55
A
222222222222
49-BD-D2-C7-56-2A
222222222221 88-B2-2F-54-1A-0F
B
Link Layer 5-52
Addressing routing to another LAN R forwards datagram with IP source A destination B
R creates link-layer frame with Bs MAC address as dest frame contains A-to-B IP datagram
IP src 111111111111
IP dest 222222222222
MAC src 1A-23-F9-CD-06-9B
MAC dest 49-BD-D2-C7-56-2A
IP
Eth
Phy
Link Layer 5-53
Link layer LANs outline
51 introduction services
52 error detection correction
53 multiple access protocols
54 LANs addressing ARP
Ethernet
switches
VLANS
55 link virtualization MPLS
56 data center networking
57 a day in the life of a web request
Link Layer 5-54
Ethernet
ldquodominantrdquo wired LAN technology
cheap $20 for NIC
first widely used LAN technology
simpler cheaper than token LANs and ATM
kept up with speed race 10 Mbps ndash 10 Gbps
Metcalfersquos Ethernet sketch
Link Layer 5-55
Ethernet physical topology
bus popular through mid 90s all nodes in same collision domain (can collide with each
other)
star prevails today active switch in center
each ldquospokerdquo runs a (separate) Ethernet protocol (nodes
do not collide with each other)
switch
bus coaxial cable star
Link Layer 5-56
Ethernet frame structure
sending adapter encapsulates IP datagram (or other network layer protocol packet) in Ethernet frame
preamble
7 bytes with pattern 10101010 followed by one byte with pattern 10101011
used to synchronize receiver sender clock rates
dest address
source address
data (payload) CRC preamble
type
Link Layer 5-57
Ethernet frame structure (more)
addresses 6 byte source destination MAC addresses if adapter receives frame with matching destination
address or with broadcast address (eg ARP packet) it passes data in frame to network layer protocol
otherwise adapter discards frame
type indicates higher layer protocol (mostly IP but others possible eg Novell IPX AppleTalk)
CRC cyclic redundancy check at receiver error detected frame is dropped
dest address
source address
data (payload) CRC preamble
type
Link Layer 5-58
Ethernet unreliable connectionless
connectionless no handshaking between sending and receiving NICs
unreliable receiving NIC doesnt send acks or nacks to sending NIC
data in dropped frames recovered only if initial sender uses higher layer rdt (eg TCP) otherwise dropped data lost
Ethernetrsquos MAC protocol unslotted CSMACD wth binary backoff
Link Layer 5-59
8023 Ethernet standards link amp physical layers
many different Ethernet standards
common MAC protocol and frame format
different speeds 2 Mbps 10 Mbps 100 Mbps 1Gbps 10G bps
different physical layer media fiber cable
application
transport
network
link
physical
MAC protocol
and frame format
100BASE-TX
100BASE-T4
100BASE-FX 100BASE-T2
100BASE-SX 100BASE-BX
fiber physical layer copper (twister
pair) physical layer
Link Layer 5-60
Link layer LANs outline
51 introduction services
52 error detection correction
53 multiple access protocols
54 LANs addressing ARP
Ethernet
switches
VLANS
55 link virtualization MPLS
56 data center networking
57 a day in the life of a web request
Link Layer 5-61
Ethernet switch link-layer device takes an active role
store forward Ethernet frames
examine incoming framersquos MAC address selectively forward frame to one-or-more outgoing links when frame is to be forwarded on segment uses CSMACD to access segment
transparent
hosts are unaware of presence of switches
plug-and-play self-learning
switches do not need to be configured
Link Layer 5-62
Switch multiple simultaneous transmissions
hosts have dedicated direct connection to switch
switches buffer packets
Ethernet protocol used on each incoming link but no collisions full duplex
each link is its own collision domain
switching A-to-Arsquo and B-to-Brsquo can transmit simultaneously without collisions switch with six interfaces
(123456)
A
Arsquo
B
Brsquo C
Crsquo
1 2
3 4 5
6
Link Layer 5-63
Switch forwarding table
Q how does switch know Arsquo reachable via interface 4 Brsquo reachable via interface 5
switch with six interfaces
(123456)
A
Arsquo
B
Brsquo C
Crsquo
1 2
3 4 5
6 A each switch has a switch table each entry
(MAC address of host interface to
reach host time stamp)
looks like a routing table
Q how are entries created maintained in switch table
something like a routing protocol
A
Arsquo
B
Brsquo C
Crsquo
1 2
3 4 5
6
Link Layer 5-64
Switch self-learning
switch learns which hosts can be reached through which interfaces
when frame received switch ldquolearnsrdquo location of sender incoming LAN segment
records senderlocation pair in switch table
A Arsquo
Source A
Dest Arsquo
MAC addr interface TTL
Switch table
(initially empty) A 1 60
Link Layer 5-65
Switch frame filteringforwarding
when frame received at switch 1 record incoming link MAC address of sending host
2 index switch table using MAC destination address
3 if entry found for destination then
if destination on segment from which frame arrived then drop frame
else forward frame on interface indicated by entry
else flood forward on all interfaces except arriving
interface
A
Arsquo
B
Brsquo C
Crsquo
1 2
3 4 5
6
Link Layer 5-66
Self-learning forwarding example
A Arsquo
Source A
Dest Arsquo
MAC addr interface TTL
switch table
(initially empty) A 1 60
A Arsquo A Arsquo A Arsquo A Arsquo A Arsquo
frame destination Arsquo locaton unknown flood
Arsquo A
destination A location
known
Arsquo 4 60
selectively send
on just one link
Link Layer 5-67
Interconnecting switches
switches can be connected together
Q sending from A to G - how does S1 know to forward frame destined to F via S4 and S3
A self learning (works exactly the same as in single-switch case)
A
B
S1
C D
E
F
S2
S4
S3
H
I
G
Link Layer 5-68
Self-learning multi-switch example
Suppose C sends frame to I I responds to C
Q show switch tables and packet forwarding in S1 S2 S3 S4
A
B
S1
C D
E
F
S2
S4
S3
H
I
G
Link Layer 5-69
Institutional network
to external
network
router
IP subnet
mail server
web server
Link Layer 5-70
Switches vs routers
both are store-and-forward
routers network-layer devices (examine network-layer headers)
switches link-layer devices (examine link-layer headers)
both have forwarding tables
routers compute tables using routing algorithms IP addresses
switches learn forwarding table using flooding learning MAC addresses
application
transport
network
link
physical
network
link
physical
link
physical
switch
datagram
application
transport
network
link
physical
frame
frame
frame
datagram
Link Layer 5-71
VLANs motivation
consider CS user moves office to
EE but wants connect to CS switch
single broadcast domain
all layer-2 broadcast traffic (ARP DHCP unknown location of destination MAC address) must cross entire LAN
securityprivacy efficiency issues
Computer
Science Electrical
Engineering
Computer
Engineering
Link Layer 5-72
VLANs port-based VLAN switch ports
grouped (by switch management software) so that single physical switch helliphellip
switch(es) supporting
VLAN capabilities can
be configured to
define multiple virtual
LANS over single
physical LAN
infrastructure
Virtual Local
Area Network 1
8
9
16 10 2
7
hellip
Electrical Engineering
(VLAN ports 1-8)
Computer Science
(VLAN ports 9-15)
15
hellip
Electrical Engineering
(VLAN ports 1-8)
hellip
1
8 2
7 9
16 10
15
hellip
Computer Science
(VLAN ports 9-16)
hellip operates as multiple virtual switches
Link Layer 5-73
Port-based VLAN
1
8
9
16 10 2
7
hellip
Electrical Engineering
(VLAN ports 1-8)
Computer Science
(VLAN ports 9-15)
15
hellip
traffic isolation frames tofrom ports 1-8 can only reach ports 1-8 can also define VLAN based on
MAC addresses of endpoints rather than switch port
dynamic membership ports can be dynamically assigned among VLANs
router
forwarding between VLANS done via routing (just as with separate switches) in practice vendors sell combined
switches plus routers
Link Layer 5-74
VLANS spanning multiple switches
trunk port carries frames between VLANS defined over multiple physical switches frames forwarded within VLAN between switches canrsquot be vanilla
8021 frames (must carry VLAN ID info)
8021q protocol addsremoved additional header fields for frames forwarded between trunk ports
1
8
9
10 2
7
hellip
Electrical Engineering
(VLAN ports 1-8)
Computer Science
(VLAN ports 9-15)
15
hellip
2
7 3
Ports 235 belong to EE VLAN
Ports 4678 belong to CS VLAN
5
4 6 8 16
1
Link Layer 5-75
type
2-byte Tag Protocol Identifier
(value 81-00)
Tag Control Information (12 bit VLAN ID field
3 bit priority field like IP TOS)
Recomputed CRC
8021Q VLAN frame format
8021 frame
8021Q frame
dest address
source address
data (payload) CRC preamble
dest address
source address
preamble data (payload) CRC
type
Link Layer 5-76
Link layer LANs outline
51 introduction services
52 error detection correction
53 multiple access protocols
54 LANs addressing ARP
Ethernet
switches
VLANS
55 link virtualization MPLS
56 data center networking
57 a day in the life of a web request
Link Layer 5-77
Multiprotocol label switching (MPLS)
initial goal high-speed IP forwarding using fixed length label (instead of IP address) fast lookup using fixed length identifier (rather than
shortest prefix matching)
borrowing ideas from Virtual Circuit (VC) approach
but IP datagram still keeps IP address
PPP or Ethernet
header IP header remainder of link-layer frame MPLS header
label Exp S TTL
20 3 1 5
Link Layer 5-78
MPLS capable routers
aka label-switched router
forward packets to outgoing interface based only on label value (donrsquot inspect IP address) MPLS forwarding table distinct from IP forwarding tables
flexibility MPLS forwarding decisions can differ from those of IP use destination and source addresses to route flows to
same destination differently (traffic engineering)
re-route flows quickly if link fails pre-computed backup paths (useful for VoIP)
Link Layer 5-79
R2
D
R3 R5
A
R6
MPLS versus IP paths
IP router IP routing path to destination determined
by destination address alone
R4
Link Layer 5-80
R2
D
R3 R4
R5
A
R6
MPLS versus IP paths
IP-only router
IP routing path to destination determined by destination address alone
MPLS and IP router
MPLS routing path to destination can be based on source and dest address fast reroute precompute backup routes in
case of link failure
entry router (R4) can use different MPLS
routes to A based eg on source address
Link Layer 5-81
MPLS signaling
modify OSPF IS-IS link-state flooding protocols to carry info used by MPLS routing eg link bandwidth amount of ldquoreservedrdquo link bandwidth
D
R4
R5
A
R6
entry MPLS router uses RSVP-TE signaling protocol to set up MPLS forwarding at downstream routers
modified link state flooding
RSVP-TE
Link Layer 5-82
R1 R2
D
R3 R4 R5
0
1
0 0
A
R6
in out out
label label dest interface
6 - A 0
in out out
label label dest interface
10 6 A 1
12 9 D 0
in out out
label label dest interface
10 A 0
12 D 0
1
in out out
label label dest interface
8 6 A 0
0
8 A 1
MPLS forwarding tables
Link Layer 5-83
Link layer LANs outline
51 introduction services
52 error detection correction
53 multiple access protocols
54 LANs addressing ARP
Ethernet
switches
VLANS
55 link virtualization MPLS
56 data center networking
57 a day in the life of a web request
Link Layer 5-84
Data center networks
10rsquos to 100rsquos of thousands of hosts often closely coupled in close proximity e-business (eg Amazon)
content-servers (eg YouTube Akamai Apple Microsoft)
search engines data mining (eg Google)
challenges
multiple applications each serving massive numbers of clients
managingbalancing load avoiding processing networking data bottlenecks
Inside a 40-ft Microsoft container
Chicago data center
Link Layer 5-85
Server racks
TOR switches
Tier-1 switches
Tier-2 switches
Load balancer
Load balancer
B
1 2 3 4 5 6 7 8
A C
Border router
Access router
Internet
Data center networks
load balancer application-layer routing receives external client requests
directs workload within data center
returns results to external client (hiding data
center internals from client)
Server racks
TOR switches
Tier-1 switches
Tier-2 switches
1 2 3 4 5 6 7 8
Data center networks
rich interconnection among switches racks
increased throughput between racks (multiple routing
paths possible)
increased reliability via redundancy
Link Layer 5-87
Link layer LANs outline
51 introduction services
52 error detection correction
53 multiple access protocols
54 LANs addressing ARP
Ethernet
switches
VLANS
55 link virtualization MPLS
56 data center networking
57 a day in the life of a web request
Link Layer 5-88
Synthesis a day in the life of a web request
journey down protocol stack complete application transport network link
putting-it-all-together synthesis goal identify review understand protocols (at all
layers) involved in seemingly simple scenario requesting www page
scenario student attaches laptop to campus network requestsreceives wwwgooglecom
Link Layer 5-89
A day in the life scenario
Comcast network
68800013
Googlersquos network
64233160019 64233169105
web server
DNS server
school network
68802024
web page
browser
router
(runs DHCP)
Link Layer 5-90
A day in the lifehellip connecting to the Internet
connecting laptop needs to get its own IP address addr of first-hop router addr of DNS server use DHCP
DHCP
UDP
IP
Eth
Phy
DHCP
DHCP
DHCP
DHCP
DHCP
DHCP
UDP
IP
Eth
Phy
DHCP
DHCP
DHCP
DHCP DHCP
DHCP request encapsulated in UDP encapsulated in IP encapsulated in 8023 Ethernet
Ethernet frame broadcast
(dest FFFFFFFFFFFF) on LAN received at router running DHCP server
Ethernet demuxed to IP demuxed UDP demuxed to DHCP
router
(runs DHCP)
Link Layer 5-91
DHCP server formulates DHCP ACK containing clientrsquos IP address IP address of first-hop router for client name amp IP address of DNS server
DHCP
UDP
IP
Eth
Phy
DHCP
DHCP
DHCP
DHCP
DHCP
UDP
IP
Eth
Phy
DHCP
DHCP
DHCP
DHCP
DHCP
encapsulation at DHCP server frame forwarded (switch learning) through LAN demultiplexing at client
Client now has IP address knows name amp addr of DNS
server IP address of its first-hop router
DHCP client receives DHCP ACK reply
A day in the lifehellip connecting to the Internet
router
(runs DHCP)
Link Layer 5-92
A day in the lifehellip ARP (before DNS before HTTP)
before sending HTTP request need IP address of wwwgooglecom DNS
DNS
UDP
IP
Eth
Phy
DNS
DNS
DNS
DNS query created encapsulated in UDP encapsulated in IP encapsulated in Eth To send frame to router need MAC address of router interface ARP
ARP query broadcast received by
router which replies with ARP reply giving MAC address of router interface
client now knows MAC address of first hop router so can now send frame containing DNS query
ARP query
Eth
Phy
ARP
ARP
ARP reply
router
(runs DHCP)
Link Layer 5-93
DNS
UDP
IP
Eth
Phy
DNS
DNS
DNS
DNS
DNS
IP datagram containing DNS query forwarded via LAN switch from client to 1st hop router
IP datagram forwarded from campus network into comcast network routed (tables created by RIP OSPF IS-IS andor BGP routing protocols) to DNS server
demuxrsquoed to DNS server
DNS server replies to client with IP address of wwwgooglecom
Comcast network
68800013
DNS server
DNS
UDP
IP
Eth
Phy
DNS
DNS
DNS
DNS
A day in the lifehellip using DNS
router
(runs DHCP)
Link Layer 5-94
A day in the lifehellipTCP connection carrying HTTP
HTTP
TCP
IP
Eth
Phy
HTTP
to send HTTP request client first opens TCP socket to web server
TCP SYN segment (step 1 in 3-way handshake) inter-domain routed to web server
TCP connection established 64233169105
web server
SYN
SYN
SYN
SYN
TCP
IP
Eth
Phy
SYN
SYN
SYN
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
web server responds with TCP SYNACK (step 2 in 3-way handshake)
router
(runs DHCP)
Link Layer 5-95
A day in the lifehellip HTTP requestreply
HTTP
TCP
IP
Eth
Phy
HTTP
HTTP request sent into TCP socket
IP datagram containing HTTP request routed to wwwgooglecom
IP datagram containing HTTP reply routed back to client
64233169105
web server
HTTP
TCP
IP
Eth
Phy web server responds with
HTTP reply (containing web page)
HTTP
HTTP
HTTP HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
web page finally () displayed
Link Layer 5-96
Chapter 5 Summary
principles behind data link layer services error detection correction
sharing a broadcast channel multiple access
link layer addressing
instantiation and implementation of various link layer technologies Ethernet
switched LANS VLANs
virtualized networks as a link layer MPLS
synthesis a day in the life of a web request
Link Layer 5-97
Chapter 5 letrsquos take a breath
journey down protocol stack complete (except PHY)
solid understanding of networking principles practice
hellip could stop here hellip but lots of interesting topics wireless
multimedia
security
network management
Link Layer 5-45
ARP address resolution protocol
ARP table each IP node (host router) on LAN has table
IPMAC address mappings for some LAN nodes
lt IP address MAC address TTLgt
TTL (Time To Live) time after which address mapping will be forgotten (typically 20 min)
Question how to determine
interfacersquos MAC address
knowing its IP address
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN
137196723
137196778
137196714
137196788
Link Layer 5-46
ARP protocol same LAN
A wants to send datagram to B Brsquos MAC address not in
Arsquos ARP table
A broadcasts ARP query packet containing Bs IP address dest MAC address = FF-FF-
FF-FF-FF-FF
all nodes on LAN receive ARP query
B receives ARP packet replies to A with its (Bs) MAC address frame sent to Arsquos MAC
address (unicast)
A caches (saves) IP-to-MAC address pair in its ARP table until information becomes old (times out) soft state information that
times out (goes away) unless refreshed
ARP is ldquoplug-and-playrdquo nodes create their ARP
tables without intervention from net administrator
Link Layer 5-47
walkthrough send datagram from A to B via R
focus on addressing ndash at IP (datagram) and MAC layer (frame)
assume A knows Brsquos IP address
assume A knows IP address of first hop router R (how)
assume A knows Rrsquos MAC address (how)
Addressing routing to another LAN
R
1A-23-F9-CD-06-9B 222222222220
111111111110 E6-E9-00-17-BB-4B CC-49-DE-D0-AB-7D
111111111112
111111111111
74-29-9C-E8-FF-55
A
222222222222
49-BD-D2-C7-56-2A
222222222221 88-B2-2F-54-1A-0F
B
R
1A-23-F9-CD-06-9B 222222222220
111111111110 E6-E9-00-17-BB-4B CC-49-DE-D0-AB-7D
111111111112
111111111111
74-29-9C-E8-FF-55
A
222222222222
49-BD-D2-C7-56-2A
222222222221 88-B2-2F-54-1A-0F
B
Link Layer 5-48
Addressing routing to another LAN
IP
Eth
Phy
IP src 111111111111
IP dest 222222222222
A creates IP datagram with IP source A destination B
A creates link-layer frame with Rs MAC address as dest frame contains A-to-B IP datagram
MAC src 74-29-9C-E8-FF-55
MAC dest E6-E9-00-17-BB-4B
R
1A-23-F9-CD-06-9B 222222222220
111111111110 E6-E9-00-17-BB-4B CC-49-DE-D0-AB-7D
111111111112
111111111111
74-29-9C-E8-FF-55
A
222222222222
49-BD-D2-C7-56-2A
222222222221 88-B2-2F-54-1A-0F
B
Link Layer 5-49
Addressing routing to another LAN
IP
Eth
Phy
frame sent from A to R
IP
Eth
Phy
frame received at R datagram removed passed up to IP
MAC src 74-29-9C-E8-FF-55
MAC dest E6-E9-00-17-BB-4B
IP src 111111111111
IP dest 222222222222
IP src 111111111111
IP dest 222222222222
R
1A-23-F9-CD-06-9B 222222222220
111111111110 E6-E9-00-17-BB-4B CC-49-DE-D0-AB-7D
111111111112
111111111111
74-29-9C-E8-FF-55
A
222222222222
49-BD-D2-C7-56-2A
222222222221 88-B2-2F-54-1A-0F
B
Link Layer 5-50
Addressing routing to another LAN
IP src 111111111111
IP dest 222222222222
R forwards datagram with IP source A destination B
R creates link-layer frame with Bs MAC address as dest frame contains A-to-B IP datagram
MAC src 1A-23-F9-CD-06-9B
MAC dest 49-BD-D2-C7-56-2A
IP
Eth
Phy
IP
Eth
Phy
R
1A-23-F9-CD-06-9B 222222222220
111111111110 E6-E9-00-17-BB-4B CC-49-DE-D0-AB-7D
111111111112
111111111111
74-29-9C-E8-FF-55
A
222222222222
49-BD-D2-C7-56-2A
222222222221 88-B2-2F-54-1A-0F
B
Link Layer 5-51
Addressing routing to another LAN R forwards datagram with IP source A destination B
R creates link-layer frame with Bs MAC address as dest frame contains A-to-B IP datagram
IP src 111111111111
IP dest 222222222222
MAC src 1A-23-F9-CD-06-9B
MAC dest 49-BD-D2-C7-56-2A
IP
Eth
Phy
IP
Eth
Phy
R
1A-23-F9-CD-06-9B 222222222220
111111111110 E6-E9-00-17-BB-4B CC-49-DE-D0-AB-7D
111111111112
111111111111
74-29-9C-E8-FF-55
A
222222222222
49-BD-D2-C7-56-2A
222222222221 88-B2-2F-54-1A-0F
B
Link Layer 5-52
Addressing routing to another LAN R forwards datagram with IP source A destination B
R creates link-layer frame with Bs MAC address as dest frame contains A-to-B IP datagram
IP src 111111111111
IP dest 222222222222
MAC src 1A-23-F9-CD-06-9B
MAC dest 49-BD-D2-C7-56-2A
IP
Eth
Phy
Link Layer 5-53
Link layer LANs outline
51 introduction services
52 error detection correction
53 multiple access protocols
54 LANs addressing ARP
Ethernet
switches
VLANS
55 link virtualization MPLS
56 data center networking
57 a day in the life of a web request
Link Layer 5-54
Ethernet
ldquodominantrdquo wired LAN technology
cheap $20 for NIC
first widely used LAN technology
simpler cheaper than token LANs and ATM
kept up with speed race 10 Mbps ndash 10 Gbps
Metcalfersquos Ethernet sketch
Link Layer 5-55
Ethernet physical topology
bus popular through mid 90s all nodes in same collision domain (can collide with each
other)
star prevails today active switch in center
each ldquospokerdquo runs a (separate) Ethernet protocol (nodes
do not collide with each other)
switch
bus coaxial cable star
Link Layer 5-56
Ethernet frame structure
sending adapter encapsulates IP datagram (or other network layer protocol packet) in Ethernet frame
preamble
7 bytes with pattern 10101010 followed by one byte with pattern 10101011
used to synchronize receiver sender clock rates
dest address
source address
data (payload) CRC preamble
type
Link Layer 5-57
Ethernet frame structure (more)
addresses 6 byte source destination MAC addresses if adapter receives frame with matching destination
address or with broadcast address (eg ARP packet) it passes data in frame to network layer protocol
otherwise adapter discards frame
type indicates higher layer protocol (mostly IP but others possible eg Novell IPX AppleTalk)
CRC cyclic redundancy check at receiver error detected frame is dropped
dest address
source address
data (payload) CRC preamble
type
Link Layer 5-58
Ethernet unreliable connectionless
connectionless no handshaking between sending and receiving NICs
unreliable receiving NIC doesnt send acks or nacks to sending NIC
data in dropped frames recovered only if initial sender uses higher layer rdt (eg TCP) otherwise dropped data lost
Ethernetrsquos MAC protocol unslotted CSMACD wth binary backoff
Link Layer 5-59
8023 Ethernet standards link amp physical layers
many different Ethernet standards
common MAC protocol and frame format
different speeds 2 Mbps 10 Mbps 100 Mbps 1Gbps 10G bps
different physical layer media fiber cable
application
transport
network
link
physical
MAC protocol
and frame format
100BASE-TX
100BASE-T4
100BASE-FX 100BASE-T2
100BASE-SX 100BASE-BX
fiber physical layer copper (twister
pair) physical layer
Link Layer 5-60
Link layer LANs outline
51 introduction services
52 error detection correction
53 multiple access protocols
54 LANs addressing ARP
Ethernet
switches
VLANS
55 link virtualization MPLS
56 data center networking
57 a day in the life of a web request
Link Layer 5-61
Ethernet switch link-layer device takes an active role
store forward Ethernet frames
examine incoming framersquos MAC address selectively forward frame to one-or-more outgoing links when frame is to be forwarded on segment uses CSMACD to access segment
transparent
hosts are unaware of presence of switches
plug-and-play self-learning
switches do not need to be configured
Link Layer 5-62
Switch multiple simultaneous transmissions
hosts have dedicated direct connection to switch
switches buffer packets
Ethernet protocol used on each incoming link but no collisions full duplex
each link is its own collision domain
switching A-to-Arsquo and B-to-Brsquo can transmit simultaneously without collisions switch with six interfaces
(123456)
A
Arsquo
B
Brsquo C
Crsquo
1 2
3 4 5
6
Link Layer 5-63
Switch forwarding table
Q how does switch know Arsquo reachable via interface 4 Brsquo reachable via interface 5
switch with six interfaces
(123456)
A
Arsquo
B
Brsquo C
Crsquo
1 2
3 4 5
6 A each switch has a switch table each entry
(MAC address of host interface to
reach host time stamp)
looks like a routing table
Q how are entries created maintained in switch table
something like a routing protocol
A
Arsquo
B
Brsquo C
Crsquo
1 2
3 4 5
6
Link Layer 5-64
Switch self-learning
switch learns which hosts can be reached through which interfaces
when frame received switch ldquolearnsrdquo location of sender incoming LAN segment
records senderlocation pair in switch table
A Arsquo
Source A
Dest Arsquo
MAC addr interface TTL
Switch table
(initially empty) A 1 60
Link Layer 5-65
Switch frame filteringforwarding
when frame received at switch 1 record incoming link MAC address of sending host
2 index switch table using MAC destination address
3 if entry found for destination then
if destination on segment from which frame arrived then drop frame
else forward frame on interface indicated by entry
else flood forward on all interfaces except arriving
interface
A
Arsquo
B
Brsquo C
Crsquo
1 2
3 4 5
6
Link Layer 5-66
Self-learning forwarding example
A Arsquo
Source A
Dest Arsquo
MAC addr interface TTL
switch table
(initially empty) A 1 60
A Arsquo A Arsquo A Arsquo A Arsquo A Arsquo
frame destination Arsquo locaton unknown flood
Arsquo A
destination A location
known
Arsquo 4 60
selectively send
on just one link
Link Layer 5-67
Interconnecting switches
switches can be connected together
Q sending from A to G - how does S1 know to forward frame destined to F via S4 and S3
A self learning (works exactly the same as in single-switch case)
A
B
S1
C D
E
F
S2
S4
S3
H
I
G
Link Layer 5-68
Self-learning multi-switch example
Suppose C sends frame to I I responds to C
Q show switch tables and packet forwarding in S1 S2 S3 S4
A
B
S1
C D
E
F
S2
S4
S3
H
I
G
Link Layer 5-69
Institutional network
to external
network
router
IP subnet
mail server
web server
Link Layer 5-70
Switches vs routers
both are store-and-forward
routers network-layer devices (examine network-layer headers)
switches link-layer devices (examine link-layer headers)
both have forwarding tables
routers compute tables using routing algorithms IP addresses
switches learn forwarding table using flooding learning MAC addresses
application
transport
network
link
physical
network
link
physical
link
physical
switch
datagram
application
transport
network
link
physical
frame
frame
frame
datagram
Link Layer 5-71
VLANs motivation
consider CS user moves office to
EE but wants connect to CS switch
single broadcast domain
all layer-2 broadcast traffic (ARP DHCP unknown location of destination MAC address) must cross entire LAN
securityprivacy efficiency issues
Computer
Science Electrical
Engineering
Computer
Engineering
Link Layer 5-72
VLANs port-based VLAN switch ports
grouped (by switch management software) so that single physical switch helliphellip
switch(es) supporting
VLAN capabilities can
be configured to
define multiple virtual
LANS over single
physical LAN
infrastructure
Virtual Local
Area Network 1
8
9
16 10 2
7
hellip
Electrical Engineering
(VLAN ports 1-8)
Computer Science
(VLAN ports 9-15)
15
hellip
Electrical Engineering
(VLAN ports 1-8)
hellip
1
8 2
7 9
16 10
15
hellip
Computer Science
(VLAN ports 9-16)
hellip operates as multiple virtual switches
Link Layer 5-73
Port-based VLAN
1
8
9
16 10 2
7
hellip
Electrical Engineering
(VLAN ports 1-8)
Computer Science
(VLAN ports 9-15)
15
hellip
traffic isolation frames tofrom ports 1-8 can only reach ports 1-8 can also define VLAN based on
MAC addresses of endpoints rather than switch port
dynamic membership ports can be dynamically assigned among VLANs
router
forwarding between VLANS done via routing (just as with separate switches) in practice vendors sell combined
switches plus routers
Link Layer 5-74
VLANS spanning multiple switches
trunk port carries frames between VLANS defined over multiple physical switches frames forwarded within VLAN between switches canrsquot be vanilla
8021 frames (must carry VLAN ID info)
8021q protocol addsremoved additional header fields for frames forwarded between trunk ports
1
8
9
10 2
7
hellip
Electrical Engineering
(VLAN ports 1-8)
Computer Science
(VLAN ports 9-15)
15
hellip
2
7 3
Ports 235 belong to EE VLAN
Ports 4678 belong to CS VLAN
5
4 6 8 16
1
Link Layer 5-75
type
2-byte Tag Protocol Identifier
(value 81-00)
Tag Control Information (12 bit VLAN ID field
3 bit priority field like IP TOS)
Recomputed CRC
8021Q VLAN frame format
8021 frame
8021Q frame
dest address
source address
data (payload) CRC preamble
dest address
source address
preamble data (payload) CRC
type
Link Layer 5-76
Link layer LANs outline
51 introduction services
52 error detection correction
53 multiple access protocols
54 LANs addressing ARP
Ethernet
switches
VLANS
55 link virtualization MPLS
56 data center networking
57 a day in the life of a web request
Link Layer 5-77
Multiprotocol label switching (MPLS)
initial goal high-speed IP forwarding using fixed length label (instead of IP address) fast lookup using fixed length identifier (rather than
shortest prefix matching)
borrowing ideas from Virtual Circuit (VC) approach
but IP datagram still keeps IP address
PPP or Ethernet
header IP header remainder of link-layer frame MPLS header
label Exp S TTL
20 3 1 5
Link Layer 5-78
MPLS capable routers
aka label-switched router
forward packets to outgoing interface based only on label value (donrsquot inspect IP address) MPLS forwarding table distinct from IP forwarding tables
flexibility MPLS forwarding decisions can differ from those of IP use destination and source addresses to route flows to
same destination differently (traffic engineering)
re-route flows quickly if link fails pre-computed backup paths (useful for VoIP)
Link Layer 5-79
R2
D
R3 R5
A
R6
MPLS versus IP paths
IP router IP routing path to destination determined
by destination address alone
R4
Link Layer 5-80
R2
D
R3 R4
R5
A
R6
MPLS versus IP paths
IP-only router
IP routing path to destination determined by destination address alone
MPLS and IP router
MPLS routing path to destination can be based on source and dest address fast reroute precompute backup routes in
case of link failure
entry router (R4) can use different MPLS
routes to A based eg on source address
Link Layer 5-81
MPLS signaling
modify OSPF IS-IS link-state flooding protocols to carry info used by MPLS routing eg link bandwidth amount of ldquoreservedrdquo link bandwidth
D
R4
R5
A
R6
entry MPLS router uses RSVP-TE signaling protocol to set up MPLS forwarding at downstream routers
modified link state flooding
RSVP-TE
Link Layer 5-82
R1 R2
D
R3 R4 R5
0
1
0 0
A
R6
in out out
label label dest interface
6 - A 0
in out out
label label dest interface
10 6 A 1
12 9 D 0
in out out
label label dest interface
10 A 0
12 D 0
1
in out out
label label dest interface
8 6 A 0
0
8 A 1
MPLS forwarding tables
Link Layer 5-83
Link layer LANs outline
51 introduction services
52 error detection correction
53 multiple access protocols
54 LANs addressing ARP
Ethernet
switches
VLANS
55 link virtualization MPLS
56 data center networking
57 a day in the life of a web request
Link Layer 5-84
Data center networks
10rsquos to 100rsquos of thousands of hosts often closely coupled in close proximity e-business (eg Amazon)
content-servers (eg YouTube Akamai Apple Microsoft)
search engines data mining (eg Google)
challenges
multiple applications each serving massive numbers of clients
managingbalancing load avoiding processing networking data bottlenecks
Inside a 40-ft Microsoft container
Chicago data center
Link Layer 5-85
Server racks
TOR switches
Tier-1 switches
Tier-2 switches
Load balancer
Load balancer
B
1 2 3 4 5 6 7 8
A C
Border router
Access router
Internet
Data center networks
load balancer application-layer routing receives external client requests
directs workload within data center
returns results to external client (hiding data
center internals from client)
Server racks
TOR switches
Tier-1 switches
Tier-2 switches
1 2 3 4 5 6 7 8
Data center networks
rich interconnection among switches racks
increased throughput between racks (multiple routing
paths possible)
increased reliability via redundancy
Link Layer 5-87
Link layer LANs outline
51 introduction services
52 error detection correction
53 multiple access protocols
54 LANs addressing ARP
Ethernet
switches
VLANS
55 link virtualization MPLS
56 data center networking
57 a day in the life of a web request
Link Layer 5-88
Synthesis a day in the life of a web request
journey down protocol stack complete application transport network link
putting-it-all-together synthesis goal identify review understand protocols (at all
layers) involved in seemingly simple scenario requesting www page
scenario student attaches laptop to campus network requestsreceives wwwgooglecom
Link Layer 5-89
A day in the life scenario
Comcast network
68800013
Googlersquos network
64233160019 64233169105
web server
DNS server
school network
68802024
web page
browser
router
(runs DHCP)
Link Layer 5-90
A day in the lifehellip connecting to the Internet
connecting laptop needs to get its own IP address addr of first-hop router addr of DNS server use DHCP
DHCP
UDP
IP
Eth
Phy
DHCP
DHCP
DHCP
DHCP
DHCP
DHCP
UDP
IP
Eth
Phy
DHCP
DHCP
DHCP
DHCP DHCP
DHCP request encapsulated in UDP encapsulated in IP encapsulated in 8023 Ethernet
Ethernet frame broadcast
(dest FFFFFFFFFFFF) on LAN received at router running DHCP server
Ethernet demuxed to IP demuxed UDP demuxed to DHCP
router
(runs DHCP)
Link Layer 5-91
DHCP server formulates DHCP ACK containing clientrsquos IP address IP address of first-hop router for client name amp IP address of DNS server
DHCP
UDP
IP
Eth
Phy
DHCP
DHCP
DHCP
DHCP
DHCP
UDP
IP
Eth
Phy
DHCP
DHCP
DHCP
DHCP
DHCP
encapsulation at DHCP server frame forwarded (switch learning) through LAN demultiplexing at client
Client now has IP address knows name amp addr of DNS
server IP address of its first-hop router
DHCP client receives DHCP ACK reply
A day in the lifehellip connecting to the Internet
router
(runs DHCP)
Link Layer 5-92
A day in the lifehellip ARP (before DNS before HTTP)
before sending HTTP request need IP address of wwwgooglecom DNS
DNS
UDP
IP
Eth
Phy
DNS
DNS
DNS
DNS query created encapsulated in UDP encapsulated in IP encapsulated in Eth To send frame to router need MAC address of router interface ARP
ARP query broadcast received by
router which replies with ARP reply giving MAC address of router interface
client now knows MAC address of first hop router so can now send frame containing DNS query
ARP query
Eth
Phy
ARP
ARP
ARP reply
router
(runs DHCP)
Link Layer 5-93
DNS
UDP
IP
Eth
Phy
DNS
DNS
DNS
DNS
DNS
IP datagram containing DNS query forwarded via LAN switch from client to 1st hop router
IP datagram forwarded from campus network into comcast network routed (tables created by RIP OSPF IS-IS andor BGP routing protocols) to DNS server
demuxrsquoed to DNS server
DNS server replies to client with IP address of wwwgooglecom
Comcast network
68800013
DNS server
DNS
UDP
IP
Eth
Phy
DNS
DNS
DNS
DNS
A day in the lifehellip using DNS
router
(runs DHCP)
Link Layer 5-94
A day in the lifehellipTCP connection carrying HTTP
HTTP
TCP
IP
Eth
Phy
HTTP
to send HTTP request client first opens TCP socket to web server
TCP SYN segment (step 1 in 3-way handshake) inter-domain routed to web server
TCP connection established 64233169105
web server
SYN
SYN
SYN
SYN
TCP
IP
Eth
Phy
SYN
SYN
SYN
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
web server responds with TCP SYNACK (step 2 in 3-way handshake)
router
(runs DHCP)
Link Layer 5-95
A day in the lifehellip HTTP requestreply
HTTP
TCP
IP
Eth
Phy
HTTP
HTTP request sent into TCP socket
IP datagram containing HTTP request routed to wwwgooglecom
IP datagram containing HTTP reply routed back to client
64233169105
web server
HTTP
TCP
IP
Eth
Phy web server responds with
HTTP reply (containing web page)
HTTP
HTTP
HTTP HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
web page finally () displayed
Link Layer 5-96
Chapter 5 Summary
principles behind data link layer services error detection correction
sharing a broadcast channel multiple access
link layer addressing
instantiation and implementation of various link layer technologies Ethernet
switched LANS VLANs
virtualized networks as a link layer MPLS
synthesis a day in the life of a web request
Link Layer 5-97
Chapter 5 letrsquos take a breath
journey down protocol stack complete (except PHY)
solid understanding of networking principles practice
hellip could stop here hellip but lots of interesting topics wireless
multimedia
security
network management
Link Layer 5-46
ARP protocol same LAN
A wants to send datagram to B Brsquos MAC address not in
Arsquos ARP table
A broadcasts ARP query packet containing Bs IP address dest MAC address = FF-FF-
FF-FF-FF-FF
all nodes on LAN receive ARP query
B receives ARP packet replies to A with its (Bs) MAC address frame sent to Arsquos MAC
address (unicast)
A caches (saves) IP-to-MAC address pair in its ARP table until information becomes old (times out) soft state information that
times out (goes away) unless refreshed
ARP is ldquoplug-and-playrdquo nodes create their ARP
tables without intervention from net administrator
Link Layer 5-47
walkthrough send datagram from A to B via R
focus on addressing ndash at IP (datagram) and MAC layer (frame)
assume A knows Brsquos IP address
assume A knows IP address of first hop router R (how)
assume A knows Rrsquos MAC address (how)
Addressing routing to another LAN
R
1A-23-F9-CD-06-9B 222222222220
111111111110 E6-E9-00-17-BB-4B CC-49-DE-D0-AB-7D
111111111112
111111111111
74-29-9C-E8-FF-55
A
222222222222
49-BD-D2-C7-56-2A
222222222221 88-B2-2F-54-1A-0F
B
R
1A-23-F9-CD-06-9B 222222222220
111111111110 E6-E9-00-17-BB-4B CC-49-DE-D0-AB-7D
111111111112
111111111111
74-29-9C-E8-FF-55
A
222222222222
49-BD-D2-C7-56-2A
222222222221 88-B2-2F-54-1A-0F
B
Link Layer 5-48
Addressing routing to another LAN
IP
Eth
Phy
IP src 111111111111
IP dest 222222222222
A creates IP datagram with IP source A destination B
A creates link-layer frame with Rs MAC address as dest frame contains A-to-B IP datagram
MAC src 74-29-9C-E8-FF-55
MAC dest E6-E9-00-17-BB-4B
R
1A-23-F9-CD-06-9B 222222222220
111111111110 E6-E9-00-17-BB-4B CC-49-DE-D0-AB-7D
111111111112
111111111111
74-29-9C-E8-FF-55
A
222222222222
49-BD-D2-C7-56-2A
222222222221 88-B2-2F-54-1A-0F
B
Link Layer 5-49
Addressing routing to another LAN
IP
Eth
Phy
frame sent from A to R
IP
Eth
Phy
frame received at R datagram removed passed up to IP
MAC src 74-29-9C-E8-FF-55
MAC dest E6-E9-00-17-BB-4B
IP src 111111111111
IP dest 222222222222
IP src 111111111111
IP dest 222222222222
R
1A-23-F9-CD-06-9B 222222222220
111111111110 E6-E9-00-17-BB-4B CC-49-DE-D0-AB-7D
111111111112
111111111111
74-29-9C-E8-FF-55
A
222222222222
49-BD-D2-C7-56-2A
222222222221 88-B2-2F-54-1A-0F
B
Link Layer 5-50
Addressing routing to another LAN
IP src 111111111111
IP dest 222222222222
R forwards datagram with IP source A destination B
R creates link-layer frame with Bs MAC address as dest frame contains A-to-B IP datagram
MAC src 1A-23-F9-CD-06-9B
MAC dest 49-BD-D2-C7-56-2A
IP
Eth
Phy
IP
Eth
Phy
R
1A-23-F9-CD-06-9B 222222222220
111111111110 E6-E9-00-17-BB-4B CC-49-DE-D0-AB-7D
111111111112
111111111111
74-29-9C-E8-FF-55
A
222222222222
49-BD-D2-C7-56-2A
222222222221 88-B2-2F-54-1A-0F
B
Link Layer 5-51
Addressing routing to another LAN R forwards datagram with IP source A destination B
R creates link-layer frame with Bs MAC address as dest frame contains A-to-B IP datagram
IP src 111111111111
IP dest 222222222222
MAC src 1A-23-F9-CD-06-9B
MAC dest 49-BD-D2-C7-56-2A
IP
Eth
Phy
IP
Eth
Phy
R
1A-23-F9-CD-06-9B 222222222220
111111111110 E6-E9-00-17-BB-4B CC-49-DE-D0-AB-7D
111111111112
111111111111
74-29-9C-E8-FF-55
A
222222222222
49-BD-D2-C7-56-2A
222222222221 88-B2-2F-54-1A-0F
B
Link Layer 5-52
Addressing routing to another LAN R forwards datagram with IP source A destination B
R creates link-layer frame with Bs MAC address as dest frame contains A-to-B IP datagram
IP src 111111111111
IP dest 222222222222
MAC src 1A-23-F9-CD-06-9B
MAC dest 49-BD-D2-C7-56-2A
IP
Eth
Phy
Link Layer 5-53
Link layer LANs outline
51 introduction services
52 error detection correction
53 multiple access protocols
54 LANs addressing ARP
Ethernet
switches
VLANS
55 link virtualization MPLS
56 data center networking
57 a day in the life of a web request
Link Layer 5-54
Ethernet
ldquodominantrdquo wired LAN technology
cheap $20 for NIC
first widely used LAN technology
simpler cheaper than token LANs and ATM
kept up with speed race 10 Mbps ndash 10 Gbps
Metcalfersquos Ethernet sketch
Link Layer 5-55
Ethernet physical topology
bus popular through mid 90s all nodes in same collision domain (can collide with each
other)
star prevails today active switch in center
each ldquospokerdquo runs a (separate) Ethernet protocol (nodes
do not collide with each other)
switch
bus coaxial cable star
Link Layer 5-56
Ethernet frame structure
sending adapter encapsulates IP datagram (or other network layer protocol packet) in Ethernet frame
preamble
7 bytes with pattern 10101010 followed by one byte with pattern 10101011
used to synchronize receiver sender clock rates
dest address
source address
data (payload) CRC preamble
type
Link Layer 5-57
Ethernet frame structure (more)
addresses 6 byte source destination MAC addresses if adapter receives frame with matching destination
address or with broadcast address (eg ARP packet) it passes data in frame to network layer protocol
otherwise adapter discards frame
type indicates higher layer protocol (mostly IP but others possible eg Novell IPX AppleTalk)
CRC cyclic redundancy check at receiver error detected frame is dropped
dest address
source address
data (payload) CRC preamble
type
Link Layer 5-58
Ethernet unreliable connectionless
connectionless no handshaking between sending and receiving NICs
unreliable receiving NIC doesnt send acks or nacks to sending NIC
data in dropped frames recovered only if initial sender uses higher layer rdt (eg TCP) otherwise dropped data lost
Ethernetrsquos MAC protocol unslotted CSMACD wth binary backoff
Link Layer 5-59
8023 Ethernet standards link amp physical layers
many different Ethernet standards
common MAC protocol and frame format
different speeds 2 Mbps 10 Mbps 100 Mbps 1Gbps 10G bps
different physical layer media fiber cable
application
transport
network
link
physical
MAC protocol
and frame format
100BASE-TX
100BASE-T4
100BASE-FX 100BASE-T2
100BASE-SX 100BASE-BX
fiber physical layer copper (twister
pair) physical layer
Link Layer 5-60
Link layer LANs outline
51 introduction services
52 error detection correction
53 multiple access protocols
54 LANs addressing ARP
Ethernet
switches
VLANS
55 link virtualization MPLS
56 data center networking
57 a day in the life of a web request
Link Layer 5-61
Ethernet switch link-layer device takes an active role
store forward Ethernet frames
examine incoming framersquos MAC address selectively forward frame to one-or-more outgoing links when frame is to be forwarded on segment uses CSMACD to access segment
transparent
hosts are unaware of presence of switches
plug-and-play self-learning
switches do not need to be configured
Link Layer 5-62
Switch multiple simultaneous transmissions
hosts have dedicated direct connection to switch
switches buffer packets
Ethernet protocol used on each incoming link but no collisions full duplex
each link is its own collision domain
switching A-to-Arsquo and B-to-Brsquo can transmit simultaneously without collisions switch with six interfaces
(123456)
A
Arsquo
B
Brsquo C
Crsquo
1 2
3 4 5
6
Link Layer 5-63
Switch forwarding table
Q how does switch know Arsquo reachable via interface 4 Brsquo reachable via interface 5
switch with six interfaces
(123456)
A
Arsquo
B
Brsquo C
Crsquo
1 2
3 4 5
6 A each switch has a switch table each entry
(MAC address of host interface to
reach host time stamp)
looks like a routing table
Q how are entries created maintained in switch table
something like a routing protocol
A
Arsquo
B
Brsquo C
Crsquo
1 2
3 4 5
6
Link Layer 5-64
Switch self-learning
switch learns which hosts can be reached through which interfaces
when frame received switch ldquolearnsrdquo location of sender incoming LAN segment
records senderlocation pair in switch table
A Arsquo
Source A
Dest Arsquo
MAC addr interface TTL
Switch table
(initially empty) A 1 60
Link Layer 5-65
Switch frame filteringforwarding
when frame received at switch 1 record incoming link MAC address of sending host
2 index switch table using MAC destination address
3 if entry found for destination then
if destination on segment from which frame arrived then drop frame
else forward frame on interface indicated by entry
else flood forward on all interfaces except arriving
interface
A
Arsquo
B
Brsquo C
Crsquo
1 2
3 4 5
6
Link Layer 5-66
Self-learning forwarding example
A Arsquo
Source A
Dest Arsquo
MAC addr interface TTL
switch table
(initially empty) A 1 60
A Arsquo A Arsquo A Arsquo A Arsquo A Arsquo
frame destination Arsquo locaton unknown flood
Arsquo A
destination A location
known
Arsquo 4 60
selectively send
on just one link
Link Layer 5-67
Interconnecting switches
switches can be connected together
Q sending from A to G - how does S1 know to forward frame destined to F via S4 and S3
A self learning (works exactly the same as in single-switch case)
A
B
S1
C D
E
F
S2
S4
S3
H
I
G
Link Layer 5-68
Self-learning multi-switch example
Suppose C sends frame to I I responds to C
Q show switch tables and packet forwarding in S1 S2 S3 S4
A
B
S1
C D
E
F
S2
S4
S3
H
I
G
Link Layer 5-69
Institutional network
to external
network
router
IP subnet
mail server
web server
Link Layer 5-70
Switches vs routers
both are store-and-forward
routers network-layer devices (examine network-layer headers)
switches link-layer devices (examine link-layer headers)
both have forwarding tables
routers compute tables using routing algorithms IP addresses
switches learn forwarding table using flooding learning MAC addresses
application
transport
network
link
physical
network
link
physical
link
physical
switch
datagram
application
transport
network
link
physical
frame
frame
frame
datagram
Link Layer 5-71
VLANs motivation
consider CS user moves office to
EE but wants connect to CS switch
single broadcast domain
all layer-2 broadcast traffic (ARP DHCP unknown location of destination MAC address) must cross entire LAN
securityprivacy efficiency issues
Computer
Science Electrical
Engineering
Computer
Engineering
Link Layer 5-72
VLANs port-based VLAN switch ports
grouped (by switch management software) so that single physical switch helliphellip
switch(es) supporting
VLAN capabilities can
be configured to
define multiple virtual
LANS over single
physical LAN
infrastructure
Virtual Local
Area Network 1
8
9
16 10 2
7
hellip
Electrical Engineering
(VLAN ports 1-8)
Computer Science
(VLAN ports 9-15)
15
hellip
Electrical Engineering
(VLAN ports 1-8)
hellip
1
8 2
7 9
16 10
15
hellip
Computer Science
(VLAN ports 9-16)
hellip operates as multiple virtual switches
Link Layer 5-73
Port-based VLAN
1
8
9
16 10 2
7
hellip
Electrical Engineering
(VLAN ports 1-8)
Computer Science
(VLAN ports 9-15)
15
hellip
traffic isolation frames tofrom ports 1-8 can only reach ports 1-8 can also define VLAN based on
MAC addresses of endpoints rather than switch port
dynamic membership ports can be dynamically assigned among VLANs
router
forwarding between VLANS done via routing (just as with separate switches) in practice vendors sell combined
switches plus routers
Link Layer 5-74
VLANS spanning multiple switches
trunk port carries frames between VLANS defined over multiple physical switches frames forwarded within VLAN between switches canrsquot be vanilla
8021 frames (must carry VLAN ID info)
8021q protocol addsremoved additional header fields for frames forwarded between trunk ports
1
8
9
10 2
7
hellip
Electrical Engineering
(VLAN ports 1-8)
Computer Science
(VLAN ports 9-15)
15
hellip
2
7 3
Ports 235 belong to EE VLAN
Ports 4678 belong to CS VLAN
5
4 6 8 16
1
Link Layer 5-75
type
2-byte Tag Protocol Identifier
(value 81-00)
Tag Control Information (12 bit VLAN ID field
3 bit priority field like IP TOS)
Recomputed CRC
8021Q VLAN frame format
8021 frame
8021Q frame
dest address
source address
data (payload) CRC preamble
dest address
source address
preamble data (payload) CRC
type
Link Layer 5-76
Link layer LANs outline
51 introduction services
52 error detection correction
53 multiple access protocols
54 LANs addressing ARP
Ethernet
switches
VLANS
55 link virtualization MPLS
56 data center networking
57 a day in the life of a web request
Link Layer 5-77
Multiprotocol label switching (MPLS)
initial goal high-speed IP forwarding using fixed length label (instead of IP address) fast lookup using fixed length identifier (rather than
shortest prefix matching)
borrowing ideas from Virtual Circuit (VC) approach
but IP datagram still keeps IP address
PPP or Ethernet
header IP header remainder of link-layer frame MPLS header
label Exp S TTL
20 3 1 5
Link Layer 5-78
MPLS capable routers
aka label-switched router
forward packets to outgoing interface based only on label value (donrsquot inspect IP address) MPLS forwarding table distinct from IP forwarding tables
flexibility MPLS forwarding decisions can differ from those of IP use destination and source addresses to route flows to
same destination differently (traffic engineering)
re-route flows quickly if link fails pre-computed backup paths (useful for VoIP)
Link Layer 5-79
R2
D
R3 R5
A
R6
MPLS versus IP paths
IP router IP routing path to destination determined
by destination address alone
R4
Link Layer 5-80
R2
D
R3 R4
R5
A
R6
MPLS versus IP paths
IP-only router
IP routing path to destination determined by destination address alone
MPLS and IP router
MPLS routing path to destination can be based on source and dest address fast reroute precompute backup routes in
case of link failure
entry router (R4) can use different MPLS
routes to A based eg on source address
Link Layer 5-81
MPLS signaling
modify OSPF IS-IS link-state flooding protocols to carry info used by MPLS routing eg link bandwidth amount of ldquoreservedrdquo link bandwidth
D
R4
R5
A
R6
entry MPLS router uses RSVP-TE signaling protocol to set up MPLS forwarding at downstream routers
modified link state flooding
RSVP-TE
Link Layer 5-82
R1 R2
D
R3 R4 R5
0
1
0 0
A
R6
in out out
label label dest interface
6 - A 0
in out out
label label dest interface
10 6 A 1
12 9 D 0
in out out
label label dest interface
10 A 0
12 D 0
1
in out out
label label dest interface
8 6 A 0
0
8 A 1
MPLS forwarding tables
Link Layer 5-83
Link layer LANs outline
51 introduction services
52 error detection correction
53 multiple access protocols
54 LANs addressing ARP
Ethernet
switches
VLANS
55 link virtualization MPLS
56 data center networking
57 a day in the life of a web request
Link Layer 5-84
Data center networks
10rsquos to 100rsquos of thousands of hosts often closely coupled in close proximity e-business (eg Amazon)
content-servers (eg YouTube Akamai Apple Microsoft)
search engines data mining (eg Google)
challenges
multiple applications each serving massive numbers of clients
managingbalancing load avoiding processing networking data bottlenecks
Inside a 40-ft Microsoft container
Chicago data center
Link Layer 5-85
Server racks
TOR switches
Tier-1 switches
Tier-2 switches
Load balancer
Load balancer
B
1 2 3 4 5 6 7 8
A C
Border router
Access router
Internet
Data center networks
load balancer application-layer routing receives external client requests
directs workload within data center
returns results to external client (hiding data
center internals from client)
Server racks
TOR switches
Tier-1 switches
Tier-2 switches
1 2 3 4 5 6 7 8
Data center networks
rich interconnection among switches racks
increased throughput between racks (multiple routing
paths possible)
increased reliability via redundancy
Link Layer 5-87
Link layer LANs outline
51 introduction services
52 error detection correction
53 multiple access protocols
54 LANs addressing ARP
Ethernet
switches
VLANS
55 link virtualization MPLS
56 data center networking
57 a day in the life of a web request
Link Layer 5-88
Synthesis a day in the life of a web request
journey down protocol stack complete application transport network link
putting-it-all-together synthesis goal identify review understand protocols (at all
layers) involved in seemingly simple scenario requesting www page
scenario student attaches laptop to campus network requestsreceives wwwgooglecom
Link Layer 5-89
A day in the life scenario
Comcast network
68800013
Googlersquos network
64233160019 64233169105
web server
DNS server
school network
68802024
web page
browser
router
(runs DHCP)
Link Layer 5-90
A day in the lifehellip connecting to the Internet
connecting laptop needs to get its own IP address addr of first-hop router addr of DNS server use DHCP
DHCP
UDP
IP
Eth
Phy
DHCP
DHCP
DHCP
DHCP
DHCP
DHCP
UDP
IP
Eth
Phy
DHCP
DHCP
DHCP
DHCP DHCP
DHCP request encapsulated in UDP encapsulated in IP encapsulated in 8023 Ethernet
Ethernet frame broadcast
(dest FFFFFFFFFFFF) on LAN received at router running DHCP server
Ethernet demuxed to IP demuxed UDP demuxed to DHCP
router
(runs DHCP)
Link Layer 5-91
DHCP server formulates DHCP ACK containing clientrsquos IP address IP address of first-hop router for client name amp IP address of DNS server
DHCP
UDP
IP
Eth
Phy
DHCP
DHCP
DHCP
DHCP
DHCP
UDP
IP
Eth
Phy
DHCP
DHCP
DHCP
DHCP
DHCP
encapsulation at DHCP server frame forwarded (switch learning) through LAN demultiplexing at client
Client now has IP address knows name amp addr of DNS
server IP address of its first-hop router
DHCP client receives DHCP ACK reply
A day in the lifehellip connecting to the Internet
router
(runs DHCP)
Link Layer 5-92
A day in the lifehellip ARP (before DNS before HTTP)
before sending HTTP request need IP address of wwwgooglecom DNS
DNS
UDP
IP
Eth
Phy
DNS
DNS
DNS
DNS query created encapsulated in UDP encapsulated in IP encapsulated in Eth To send frame to router need MAC address of router interface ARP
ARP query broadcast received by
router which replies with ARP reply giving MAC address of router interface
client now knows MAC address of first hop router so can now send frame containing DNS query
ARP query
Eth
Phy
ARP
ARP
ARP reply
router
(runs DHCP)
Link Layer 5-93
DNS
UDP
IP
Eth
Phy
DNS
DNS
DNS
DNS
DNS
IP datagram containing DNS query forwarded via LAN switch from client to 1st hop router
IP datagram forwarded from campus network into comcast network routed (tables created by RIP OSPF IS-IS andor BGP routing protocols) to DNS server
demuxrsquoed to DNS server
DNS server replies to client with IP address of wwwgooglecom
Comcast network
68800013
DNS server
DNS
UDP
IP
Eth
Phy
DNS
DNS
DNS
DNS
A day in the lifehellip using DNS
router
(runs DHCP)
Link Layer 5-94
A day in the lifehellipTCP connection carrying HTTP
HTTP
TCP
IP
Eth
Phy
HTTP
to send HTTP request client first opens TCP socket to web server
TCP SYN segment (step 1 in 3-way handshake) inter-domain routed to web server
TCP connection established 64233169105
web server
SYN
SYN
SYN
SYN
TCP
IP
Eth
Phy
SYN
SYN
SYN
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
web server responds with TCP SYNACK (step 2 in 3-way handshake)
router
(runs DHCP)
Link Layer 5-95
A day in the lifehellip HTTP requestreply
HTTP
TCP
IP
Eth
Phy
HTTP
HTTP request sent into TCP socket
IP datagram containing HTTP request routed to wwwgooglecom
IP datagram containing HTTP reply routed back to client
64233169105
web server
HTTP
TCP
IP
Eth
Phy web server responds with
HTTP reply (containing web page)
HTTP
HTTP
HTTP HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
web page finally () displayed
Link Layer 5-96
Chapter 5 Summary
principles behind data link layer services error detection correction
sharing a broadcast channel multiple access
link layer addressing
instantiation and implementation of various link layer technologies Ethernet
switched LANS VLANs
virtualized networks as a link layer MPLS
synthesis a day in the life of a web request
Link Layer 5-97
Chapter 5 letrsquos take a breath
journey down protocol stack complete (except PHY)
solid understanding of networking principles practice
hellip could stop here hellip but lots of interesting topics wireless
multimedia
security
network management
Link Layer 5-47
walkthrough send datagram from A to B via R
focus on addressing ndash at IP (datagram) and MAC layer (frame)
assume A knows Brsquos IP address
assume A knows IP address of first hop router R (how)
assume A knows Rrsquos MAC address (how)
Addressing routing to another LAN
R
1A-23-F9-CD-06-9B 222222222220
111111111110 E6-E9-00-17-BB-4B CC-49-DE-D0-AB-7D
111111111112
111111111111
74-29-9C-E8-FF-55
A
222222222222
49-BD-D2-C7-56-2A
222222222221 88-B2-2F-54-1A-0F
B
R
1A-23-F9-CD-06-9B 222222222220
111111111110 E6-E9-00-17-BB-4B CC-49-DE-D0-AB-7D
111111111112
111111111111
74-29-9C-E8-FF-55
A
222222222222
49-BD-D2-C7-56-2A
222222222221 88-B2-2F-54-1A-0F
B
Link Layer 5-48
Addressing routing to another LAN
IP
Eth
Phy
IP src 111111111111
IP dest 222222222222
A creates IP datagram with IP source A destination B
A creates link-layer frame with Rs MAC address as dest frame contains A-to-B IP datagram
MAC src 74-29-9C-E8-FF-55
MAC dest E6-E9-00-17-BB-4B
R
1A-23-F9-CD-06-9B 222222222220
111111111110 E6-E9-00-17-BB-4B CC-49-DE-D0-AB-7D
111111111112
111111111111
74-29-9C-E8-FF-55
A
222222222222
49-BD-D2-C7-56-2A
222222222221 88-B2-2F-54-1A-0F
B
Link Layer 5-49
Addressing routing to another LAN
IP
Eth
Phy
frame sent from A to R
IP
Eth
Phy
frame received at R datagram removed passed up to IP
MAC src 74-29-9C-E8-FF-55
MAC dest E6-E9-00-17-BB-4B
IP src 111111111111
IP dest 222222222222
IP src 111111111111
IP dest 222222222222
R
1A-23-F9-CD-06-9B 222222222220
111111111110 E6-E9-00-17-BB-4B CC-49-DE-D0-AB-7D
111111111112
111111111111
74-29-9C-E8-FF-55
A
222222222222
49-BD-D2-C7-56-2A
222222222221 88-B2-2F-54-1A-0F
B
Link Layer 5-50
Addressing routing to another LAN
IP src 111111111111
IP dest 222222222222
R forwards datagram with IP source A destination B
R creates link-layer frame with Bs MAC address as dest frame contains A-to-B IP datagram
MAC src 1A-23-F9-CD-06-9B
MAC dest 49-BD-D2-C7-56-2A
IP
Eth
Phy
IP
Eth
Phy
R
1A-23-F9-CD-06-9B 222222222220
111111111110 E6-E9-00-17-BB-4B CC-49-DE-D0-AB-7D
111111111112
111111111111
74-29-9C-E8-FF-55
A
222222222222
49-BD-D2-C7-56-2A
222222222221 88-B2-2F-54-1A-0F
B
Link Layer 5-51
Addressing routing to another LAN R forwards datagram with IP source A destination B
R creates link-layer frame with Bs MAC address as dest frame contains A-to-B IP datagram
IP src 111111111111
IP dest 222222222222
MAC src 1A-23-F9-CD-06-9B
MAC dest 49-BD-D2-C7-56-2A
IP
Eth
Phy
IP
Eth
Phy
R
1A-23-F9-CD-06-9B 222222222220
111111111110 E6-E9-00-17-BB-4B CC-49-DE-D0-AB-7D
111111111112
111111111111
74-29-9C-E8-FF-55
A
222222222222
49-BD-D2-C7-56-2A
222222222221 88-B2-2F-54-1A-0F
B
Link Layer 5-52
Addressing routing to another LAN R forwards datagram with IP source A destination B
R creates link-layer frame with Bs MAC address as dest frame contains A-to-B IP datagram
IP src 111111111111
IP dest 222222222222
MAC src 1A-23-F9-CD-06-9B
MAC dest 49-BD-D2-C7-56-2A
IP
Eth
Phy
Link Layer 5-53
Link layer LANs outline
51 introduction services
52 error detection correction
53 multiple access protocols
54 LANs addressing ARP
Ethernet
switches
VLANS
55 link virtualization MPLS
56 data center networking
57 a day in the life of a web request
Link Layer 5-54
Ethernet
ldquodominantrdquo wired LAN technology
cheap $20 for NIC
first widely used LAN technology
simpler cheaper than token LANs and ATM
kept up with speed race 10 Mbps ndash 10 Gbps
Metcalfersquos Ethernet sketch
Link Layer 5-55
Ethernet physical topology
bus popular through mid 90s all nodes in same collision domain (can collide with each
other)
star prevails today active switch in center
each ldquospokerdquo runs a (separate) Ethernet protocol (nodes
do not collide with each other)
switch
bus coaxial cable star
Link Layer 5-56
Ethernet frame structure
sending adapter encapsulates IP datagram (or other network layer protocol packet) in Ethernet frame
preamble
7 bytes with pattern 10101010 followed by one byte with pattern 10101011
used to synchronize receiver sender clock rates
dest address
source address
data (payload) CRC preamble
type
Link Layer 5-57
Ethernet frame structure (more)
addresses 6 byte source destination MAC addresses if adapter receives frame with matching destination
address or with broadcast address (eg ARP packet) it passes data in frame to network layer protocol
otherwise adapter discards frame
type indicates higher layer protocol (mostly IP but others possible eg Novell IPX AppleTalk)
CRC cyclic redundancy check at receiver error detected frame is dropped
dest address
source address
data (payload) CRC preamble
type
Link Layer 5-58
Ethernet unreliable connectionless
connectionless no handshaking between sending and receiving NICs
unreliable receiving NIC doesnt send acks or nacks to sending NIC
data in dropped frames recovered only if initial sender uses higher layer rdt (eg TCP) otherwise dropped data lost
Ethernetrsquos MAC protocol unslotted CSMACD wth binary backoff
Link Layer 5-59
8023 Ethernet standards link amp physical layers
many different Ethernet standards
common MAC protocol and frame format
different speeds 2 Mbps 10 Mbps 100 Mbps 1Gbps 10G bps
different physical layer media fiber cable
application
transport
network
link
physical
MAC protocol
and frame format
100BASE-TX
100BASE-T4
100BASE-FX 100BASE-T2
100BASE-SX 100BASE-BX
fiber physical layer copper (twister
pair) physical layer
Link Layer 5-60
Link layer LANs outline
51 introduction services
52 error detection correction
53 multiple access protocols
54 LANs addressing ARP
Ethernet
switches
VLANS
55 link virtualization MPLS
56 data center networking
57 a day in the life of a web request
Link Layer 5-61
Ethernet switch link-layer device takes an active role
store forward Ethernet frames
examine incoming framersquos MAC address selectively forward frame to one-or-more outgoing links when frame is to be forwarded on segment uses CSMACD to access segment
transparent
hosts are unaware of presence of switches
plug-and-play self-learning
switches do not need to be configured
Link Layer 5-62
Switch multiple simultaneous transmissions
hosts have dedicated direct connection to switch
switches buffer packets
Ethernet protocol used on each incoming link but no collisions full duplex
each link is its own collision domain
switching A-to-Arsquo and B-to-Brsquo can transmit simultaneously without collisions switch with six interfaces
(123456)
A
Arsquo
B
Brsquo C
Crsquo
1 2
3 4 5
6
Link Layer 5-63
Switch forwarding table
Q how does switch know Arsquo reachable via interface 4 Brsquo reachable via interface 5
switch with six interfaces
(123456)
A
Arsquo
B
Brsquo C
Crsquo
1 2
3 4 5
6 A each switch has a switch table each entry
(MAC address of host interface to
reach host time stamp)
looks like a routing table
Q how are entries created maintained in switch table
something like a routing protocol
A
Arsquo
B
Brsquo C
Crsquo
1 2
3 4 5
6
Link Layer 5-64
Switch self-learning
switch learns which hosts can be reached through which interfaces
when frame received switch ldquolearnsrdquo location of sender incoming LAN segment
records senderlocation pair in switch table
A Arsquo
Source A
Dest Arsquo
MAC addr interface TTL
Switch table
(initially empty) A 1 60
Link Layer 5-65
Switch frame filteringforwarding
when frame received at switch 1 record incoming link MAC address of sending host
2 index switch table using MAC destination address
3 if entry found for destination then
if destination on segment from which frame arrived then drop frame
else forward frame on interface indicated by entry
else flood forward on all interfaces except arriving
interface
A
Arsquo
B
Brsquo C
Crsquo
1 2
3 4 5
6
Link Layer 5-66
Self-learning forwarding example
A Arsquo
Source A
Dest Arsquo
MAC addr interface TTL
switch table
(initially empty) A 1 60
A Arsquo A Arsquo A Arsquo A Arsquo A Arsquo
frame destination Arsquo locaton unknown flood
Arsquo A
destination A location
known
Arsquo 4 60
selectively send
on just one link
Link Layer 5-67
Interconnecting switches
switches can be connected together
Q sending from A to G - how does S1 know to forward frame destined to F via S4 and S3
A self learning (works exactly the same as in single-switch case)
A
B
S1
C D
E
F
S2
S4
S3
H
I
G
Link Layer 5-68
Self-learning multi-switch example
Suppose C sends frame to I I responds to C
Q show switch tables and packet forwarding in S1 S2 S3 S4
A
B
S1
C D
E
F
S2
S4
S3
H
I
G
Link Layer 5-69
Institutional network
to external
network
router
IP subnet
mail server
web server
Link Layer 5-70
Switches vs routers
both are store-and-forward
routers network-layer devices (examine network-layer headers)
switches link-layer devices (examine link-layer headers)
both have forwarding tables
routers compute tables using routing algorithms IP addresses
switches learn forwarding table using flooding learning MAC addresses
application
transport
network
link
physical
network
link
physical
link
physical
switch
datagram
application
transport
network
link
physical
frame
frame
frame
datagram
Link Layer 5-71
VLANs motivation
consider CS user moves office to
EE but wants connect to CS switch
single broadcast domain
all layer-2 broadcast traffic (ARP DHCP unknown location of destination MAC address) must cross entire LAN
securityprivacy efficiency issues
Computer
Science Electrical
Engineering
Computer
Engineering
Link Layer 5-72
VLANs port-based VLAN switch ports
grouped (by switch management software) so that single physical switch helliphellip
switch(es) supporting
VLAN capabilities can
be configured to
define multiple virtual
LANS over single
physical LAN
infrastructure
Virtual Local
Area Network 1
8
9
16 10 2
7
hellip
Electrical Engineering
(VLAN ports 1-8)
Computer Science
(VLAN ports 9-15)
15
hellip
Electrical Engineering
(VLAN ports 1-8)
hellip
1
8 2
7 9
16 10
15
hellip
Computer Science
(VLAN ports 9-16)
hellip operates as multiple virtual switches
Link Layer 5-73
Port-based VLAN
1
8
9
16 10 2
7
hellip
Electrical Engineering
(VLAN ports 1-8)
Computer Science
(VLAN ports 9-15)
15
hellip
traffic isolation frames tofrom ports 1-8 can only reach ports 1-8 can also define VLAN based on
MAC addresses of endpoints rather than switch port
dynamic membership ports can be dynamically assigned among VLANs
router
forwarding between VLANS done via routing (just as with separate switches) in practice vendors sell combined
switches plus routers
Link Layer 5-74
VLANS spanning multiple switches
trunk port carries frames between VLANS defined over multiple physical switches frames forwarded within VLAN between switches canrsquot be vanilla
8021 frames (must carry VLAN ID info)
8021q protocol addsremoved additional header fields for frames forwarded between trunk ports
1
8
9
10 2
7
hellip
Electrical Engineering
(VLAN ports 1-8)
Computer Science
(VLAN ports 9-15)
15
hellip
2
7 3
Ports 235 belong to EE VLAN
Ports 4678 belong to CS VLAN
5
4 6 8 16
1
Link Layer 5-75
type
2-byte Tag Protocol Identifier
(value 81-00)
Tag Control Information (12 bit VLAN ID field
3 bit priority field like IP TOS)
Recomputed CRC
8021Q VLAN frame format
8021 frame
8021Q frame
dest address
source address
data (payload) CRC preamble
dest address
source address
preamble data (payload) CRC
type
Link Layer 5-76
Link layer LANs outline
51 introduction services
52 error detection correction
53 multiple access protocols
54 LANs addressing ARP
Ethernet
switches
VLANS
55 link virtualization MPLS
56 data center networking
57 a day in the life of a web request
Link Layer 5-77
Multiprotocol label switching (MPLS)
initial goal high-speed IP forwarding using fixed length label (instead of IP address) fast lookup using fixed length identifier (rather than
shortest prefix matching)
borrowing ideas from Virtual Circuit (VC) approach
but IP datagram still keeps IP address
PPP or Ethernet
header IP header remainder of link-layer frame MPLS header
label Exp S TTL
20 3 1 5
Link Layer 5-78
MPLS capable routers
aka label-switched router
forward packets to outgoing interface based only on label value (donrsquot inspect IP address) MPLS forwarding table distinct from IP forwarding tables
flexibility MPLS forwarding decisions can differ from those of IP use destination and source addresses to route flows to
same destination differently (traffic engineering)
re-route flows quickly if link fails pre-computed backup paths (useful for VoIP)
Link Layer 5-79
R2
D
R3 R5
A
R6
MPLS versus IP paths
IP router IP routing path to destination determined
by destination address alone
R4
Link Layer 5-80
R2
D
R3 R4
R5
A
R6
MPLS versus IP paths
IP-only router
IP routing path to destination determined by destination address alone
MPLS and IP router
MPLS routing path to destination can be based on source and dest address fast reroute precompute backup routes in
case of link failure
entry router (R4) can use different MPLS
routes to A based eg on source address
Link Layer 5-81
MPLS signaling
modify OSPF IS-IS link-state flooding protocols to carry info used by MPLS routing eg link bandwidth amount of ldquoreservedrdquo link bandwidth
D
R4
R5
A
R6
entry MPLS router uses RSVP-TE signaling protocol to set up MPLS forwarding at downstream routers
modified link state flooding
RSVP-TE
Link Layer 5-82
R1 R2
D
R3 R4 R5
0
1
0 0
A
R6
in out out
label label dest interface
6 - A 0
in out out
label label dest interface
10 6 A 1
12 9 D 0
in out out
label label dest interface
10 A 0
12 D 0
1
in out out
label label dest interface
8 6 A 0
0
8 A 1
MPLS forwarding tables
Link Layer 5-83
Link layer LANs outline
51 introduction services
52 error detection correction
53 multiple access protocols
54 LANs addressing ARP
Ethernet
switches
VLANS
55 link virtualization MPLS
56 data center networking
57 a day in the life of a web request
Link Layer 5-84
Data center networks
10rsquos to 100rsquos of thousands of hosts often closely coupled in close proximity e-business (eg Amazon)
content-servers (eg YouTube Akamai Apple Microsoft)
search engines data mining (eg Google)
challenges
multiple applications each serving massive numbers of clients
managingbalancing load avoiding processing networking data bottlenecks
Inside a 40-ft Microsoft container
Chicago data center
Link Layer 5-85
Server racks
TOR switches
Tier-1 switches
Tier-2 switches
Load balancer
Load balancer
B
1 2 3 4 5 6 7 8
A C
Border router
Access router
Internet
Data center networks
load balancer application-layer routing receives external client requests
directs workload within data center
returns results to external client (hiding data
center internals from client)
Server racks
TOR switches
Tier-1 switches
Tier-2 switches
1 2 3 4 5 6 7 8
Data center networks
rich interconnection among switches racks
increased throughput between racks (multiple routing
paths possible)
increased reliability via redundancy
Link Layer 5-87
Link layer LANs outline
51 introduction services
52 error detection correction
53 multiple access protocols
54 LANs addressing ARP
Ethernet
switches
VLANS
55 link virtualization MPLS
56 data center networking
57 a day in the life of a web request
Link Layer 5-88
Synthesis a day in the life of a web request
journey down protocol stack complete application transport network link
putting-it-all-together synthesis goal identify review understand protocols (at all
layers) involved in seemingly simple scenario requesting www page
scenario student attaches laptop to campus network requestsreceives wwwgooglecom
Link Layer 5-89
A day in the life scenario
Comcast network
68800013
Googlersquos network
64233160019 64233169105
web server
DNS server
school network
68802024
web page
browser
router
(runs DHCP)
Link Layer 5-90
A day in the lifehellip connecting to the Internet
connecting laptop needs to get its own IP address addr of first-hop router addr of DNS server use DHCP
DHCP
UDP
IP
Eth
Phy
DHCP
DHCP
DHCP
DHCP
DHCP
DHCP
UDP
IP
Eth
Phy
DHCP
DHCP
DHCP
DHCP DHCP
DHCP request encapsulated in UDP encapsulated in IP encapsulated in 8023 Ethernet
Ethernet frame broadcast
(dest FFFFFFFFFFFF) on LAN received at router running DHCP server
Ethernet demuxed to IP demuxed UDP demuxed to DHCP
router
(runs DHCP)
Link Layer 5-91
DHCP server formulates DHCP ACK containing clientrsquos IP address IP address of first-hop router for client name amp IP address of DNS server
DHCP
UDP
IP
Eth
Phy
DHCP
DHCP
DHCP
DHCP
DHCP
UDP
IP
Eth
Phy
DHCP
DHCP
DHCP
DHCP
DHCP
encapsulation at DHCP server frame forwarded (switch learning) through LAN demultiplexing at client
Client now has IP address knows name amp addr of DNS
server IP address of its first-hop router
DHCP client receives DHCP ACK reply
A day in the lifehellip connecting to the Internet
router
(runs DHCP)
Link Layer 5-92
A day in the lifehellip ARP (before DNS before HTTP)
before sending HTTP request need IP address of wwwgooglecom DNS
DNS
UDP
IP
Eth
Phy
DNS
DNS
DNS
DNS query created encapsulated in UDP encapsulated in IP encapsulated in Eth To send frame to router need MAC address of router interface ARP
ARP query broadcast received by
router which replies with ARP reply giving MAC address of router interface
client now knows MAC address of first hop router so can now send frame containing DNS query
ARP query
Eth
Phy
ARP
ARP
ARP reply
router
(runs DHCP)
Link Layer 5-93
DNS
UDP
IP
Eth
Phy
DNS
DNS
DNS
DNS
DNS
IP datagram containing DNS query forwarded via LAN switch from client to 1st hop router
IP datagram forwarded from campus network into comcast network routed (tables created by RIP OSPF IS-IS andor BGP routing protocols) to DNS server
demuxrsquoed to DNS server
DNS server replies to client with IP address of wwwgooglecom
Comcast network
68800013
DNS server
DNS
UDP
IP
Eth
Phy
DNS
DNS
DNS
DNS
A day in the lifehellip using DNS
router
(runs DHCP)
Link Layer 5-94
A day in the lifehellipTCP connection carrying HTTP
HTTP
TCP
IP
Eth
Phy
HTTP
to send HTTP request client first opens TCP socket to web server
TCP SYN segment (step 1 in 3-way handshake) inter-domain routed to web server
TCP connection established 64233169105
web server
SYN
SYN
SYN
SYN
TCP
IP
Eth
Phy
SYN
SYN
SYN
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
web server responds with TCP SYNACK (step 2 in 3-way handshake)
router
(runs DHCP)
Link Layer 5-95
A day in the lifehellip HTTP requestreply
HTTP
TCP
IP
Eth
Phy
HTTP
HTTP request sent into TCP socket
IP datagram containing HTTP request routed to wwwgooglecom
IP datagram containing HTTP reply routed back to client
64233169105
web server
HTTP
TCP
IP
Eth
Phy web server responds with
HTTP reply (containing web page)
HTTP
HTTP
HTTP HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
web page finally () displayed
Link Layer 5-96
Chapter 5 Summary
principles behind data link layer services error detection correction
sharing a broadcast channel multiple access
link layer addressing
instantiation and implementation of various link layer technologies Ethernet
switched LANS VLANs
virtualized networks as a link layer MPLS
synthesis a day in the life of a web request
Link Layer 5-97
Chapter 5 letrsquos take a breath
journey down protocol stack complete (except PHY)
solid understanding of networking principles practice
hellip could stop here hellip but lots of interesting topics wireless
multimedia
security
network management
R
1A-23-F9-CD-06-9B 222222222220
111111111110 E6-E9-00-17-BB-4B CC-49-DE-D0-AB-7D
111111111112
111111111111
74-29-9C-E8-FF-55
A
222222222222
49-BD-D2-C7-56-2A
222222222221 88-B2-2F-54-1A-0F
B
Link Layer 5-48
Addressing routing to another LAN
IP
Eth
Phy
IP src 111111111111
IP dest 222222222222
A creates IP datagram with IP source A destination B
A creates link-layer frame with Rs MAC address as dest frame contains A-to-B IP datagram
MAC src 74-29-9C-E8-FF-55
MAC dest E6-E9-00-17-BB-4B
R
1A-23-F9-CD-06-9B 222222222220
111111111110 E6-E9-00-17-BB-4B CC-49-DE-D0-AB-7D
111111111112
111111111111
74-29-9C-E8-FF-55
A
222222222222
49-BD-D2-C7-56-2A
222222222221 88-B2-2F-54-1A-0F
B
Link Layer 5-49
Addressing routing to another LAN
IP
Eth
Phy
frame sent from A to R
IP
Eth
Phy
frame received at R datagram removed passed up to IP
MAC src 74-29-9C-E8-FF-55
MAC dest E6-E9-00-17-BB-4B
IP src 111111111111
IP dest 222222222222
IP src 111111111111
IP dest 222222222222
R
1A-23-F9-CD-06-9B 222222222220
111111111110 E6-E9-00-17-BB-4B CC-49-DE-D0-AB-7D
111111111112
111111111111
74-29-9C-E8-FF-55
A
222222222222
49-BD-D2-C7-56-2A
222222222221 88-B2-2F-54-1A-0F
B
Link Layer 5-50
Addressing routing to another LAN
IP src 111111111111
IP dest 222222222222
R forwards datagram with IP source A destination B
R creates link-layer frame with Bs MAC address as dest frame contains A-to-B IP datagram
MAC src 1A-23-F9-CD-06-9B
MAC dest 49-BD-D2-C7-56-2A
IP
Eth
Phy
IP
Eth
Phy
R
1A-23-F9-CD-06-9B 222222222220
111111111110 E6-E9-00-17-BB-4B CC-49-DE-D0-AB-7D
111111111112
111111111111
74-29-9C-E8-FF-55
A
222222222222
49-BD-D2-C7-56-2A
222222222221 88-B2-2F-54-1A-0F
B
Link Layer 5-51
Addressing routing to another LAN R forwards datagram with IP source A destination B
R creates link-layer frame with Bs MAC address as dest frame contains A-to-B IP datagram
IP src 111111111111
IP dest 222222222222
MAC src 1A-23-F9-CD-06-9B
MAC dest 49-BD-D2-C7-56-2A
IP
Eth
Phy
IP
Eth
Phy
R
1A-23-F9-CD-06-9B 222222222220
111111111110 E6-E9-00-17-BB-4B CC-49-DE-D0-AB-7D
111111111112
111111111111
74-29-9C-E8-FF-55
A
222222222222
49-BD-D2-C7-56-2A
222222222221 88-B2-2F-54-1A-0F
B
Link Layer 5-52
Addressing routing to another LAN R forwards datagram with IP source A destination B
R creates link-layer frame with Bs MAC address as dest frame contains A-to-B IP datagram
IP src 111111111111
IP dest 222222222222
MAC src 1A-23-F9-CD-06-9B
MAC dest 49-BD-D2-C7-56-2A
IP
Eth
Phy
Link Layer 5-53
Link layer LANs outline
51 introduction services
52 error detection correction
53 multiple access protocols
54 LANs addressing ARP
Ethernet
switches
VLANS
55 link virtualization MPLS
56 data center networking
57 a day in the life of a web request
Link Layer 5-54
Ethernet
ldquodominantrdquo wired LAN technology
cheap $20 for NIC
first widely used LAN technology
simpler cheaper than token LANs and ATM
kept up with speed race 10 Mbps ndash 10 Gbps
Metcalfersquos Ethernet sketch
Link Layer 5-55
Ethernet physical topology
bus popular through mid 90s all nodes in same collision domain (can collide with each
other)
star prevails today active switch in center
each ldquospokerdquo runs a (separate) Ethernet protocol (nodes
do not collide with each other)
switch
bus coaxial cable star
Link Layer 5-56
Ethernet frame structure
sending adapter encapsulates IP datagram (or other network layer protocol packet) in Ethernet frame
preamble
7 bytes with pattern 10101010 followed by one byte with pattern 10101011
used to synchronize receiver sender clock rates
dest address
source address
data (payload) CRC preamble
type
Link Layer 5-57
Ethernet frame structure (more)
addresses 6 byte source destination MAC addresses if adapter receives frame with matching destination
address or with broadcast address (eg ARP packet) it passes data in frame to network layer protocol
otherwise adapter discards frame
type indicates higher layer protocol (mostly IP but others possible eg Novell IPX AppleTalk)
CRC cyclic redundancy check at receiver error detected frame is dropped
dest address
source address
data (payload) CRC preamble
type
Link Layer 5-58
Ethernet unreliable connectionless
connectionless no handshaking between sending and receiving NICs
unreliable receiving NIC doesnt send acks or nacks to sending NIC
data in dropped frames recovered only if initial sender uses higher layer rdt (eg TCP) otherwise dropped data lost
Ethernetrsquos MAC protocol unslotted CSMACD wth binary backoff
Link Layer 5-59
8023 Ethernet standards link amp physical layers
many different Ethernet standards
common MAC protocol and frame format
different speeds 2 Mbps 10 Mbps 100 Mbps 1Gbps 10G bps
different physical layer media fiber cable
application
transport
network
link
physical
MAC protocol
and frame format
100BASE-TX
100BASE-T4
100BASE-FX 100BASE-T2
100BASE-SX 100BASE-BX
fiber physical layer copper (twister
pair) physical layer
Link Layer 5-60
Link layer LANs outline
51 introduction services
52 error detection correction
53 multiple access protocols
54 LANs addressing ARP
Ethernet
switches
VLANS
55 link virtualization MPLS
56 data center networking
57 a day in the life of a web request
Link Layer 5-61
Ethernet switch link-layer device takes an active role
store forward Ethernet frames
examine incoming framersquos MAC address selectively forward frame to one-or-more outgoing links when frame is to be forwarded on segment uses CSMACD to access segment
transparent
hosts are unaware of presence of switches
plug-and-play self-learning
switches do not need to be configured
Link Layer 5-62
Switch multiple simultaneous transmissions
hosts have dedicated direct connection to switch
switches buffer packets
Ethernet protocol used on each incoming link but no collisions full duplex
each link is its own collision domain
switching A-to-Arsquo and B-to-Brsquo can transmit simultaneously without collisions switch with six interfaces
(123456)
A
Arsquo
B
Brsquo C
Crsquo
1 2
3 4 5
6
Link Layer 5-63
Switch forwarding table
Q how does switch know Arsquo reachable via interface 4 Brsquo reachable via interface 5
switch with six interfaces
(123456)
A
Arsquo
B
Brsquo C
Crsquo
1 2
3 4 5
6 A each switch has a switch table each entry
(MAC address of host interface to
reach host time stamp)
looks like a routing table
Q how are entries created maintained in switch table
something like a routing protocol
A
Arsquo
B
Brsquo C
Crsquo
1 2
3 4 5
6
Link Layer 5-64
Switch self-learning
switch learns which hosts can be reached through which interfaces
when frame received switch ldquolearnsrdquo location of sender incoming LAN segment
records senderlocation pair in switch table
A Arsquo
Source A
Dest Arsquo
MAC addr interface TTL
Switch table
(initially empty) A 1 60
Link Layer 5-65
Switch frame filteringforwarding
when frame received at switch 1 record incoming link MAC address of sending host
2 index switch table using MAC destination address
3 if entry found for destination then
if destination on segment from which frame arrived then drop frame
else forward frame on interface indicated by entry
else flood forward on all interfaces except arriving
interface
A
Arsquo
B
Brsquo C
Crsquo
1 2
3 4 5
6
Link Layer 5-66
Self-learning forwarding example
A Arsquo
Source A
Dest Arsquo
MAC addr interface TTL
switch table
(initially empty) A 1 60
A Arsquo A Arsquo A Arsquo A Arsquo A Arsquo
frame destination Arsquo locaton unknown flood
Arsquo A
destination A location
known
Arsquo 4 60
selectively send
on just one link
Link Layer 5-67
Interconnecting switches
switches can be connected together
Q sending from A to G - how does S1 know to forward frame destined to F via S4 and S3
A self learning (works exactly the same as in single-switch case)
A
B
S1
C D
E
F
S2
S4
S3
H
I
G
Link Layer 5-68
Self-learning multi-switch example
Suppose C sends frame to I I responds to C
Q show switch tables and packet forwarding in S1 S2 S3 S4
A
B
S1
C D
E
F
S2
S4
S3
H
I
G
Link Layer 5-69
Institutional network
to external
network
router
IP subnet
mail server
web server
Link Layer 5-70
Switches vs routers
both are store-and-forward
routers network-layer devices (examine network-layer headers)
switches link-layer devices (examine link-layer headers)
both have forwarding tables
routers compute tables using routing algorithms IP addresses
switches learn forwarding table using flooding learning MAC addresses
application
transport
network
link
physical
network
link
physical
link
physical
switch
datagram
application
transport
network
link
physical
frame
frame
frame
datagram
Link Layer 5-71
VLANs motivation
consider CS user moves office to
EE but wants connect to CS switch
single broadcast domain
all layer-2 broadcast traffic (ARP DHCP unknown location of destination MAC address) must cross entire LAN
securityprivacy efficiency issues
Computer
Science Electrical
Engineering
Computer
Engineering
Link Layer 5-72
VLANs port-based VLAN switch ports
grouped (by switch management software) so that single physical switch helliphellip
switch(es) supporting
VLAN capabilities can
be configured to
define multiple virtual
LANS over single
physical LAN
infrastructure
Virtual Local
Area Network 1
8
9
16 10 2
7
hellip
Electrical Engineering
(VLAN ports 1-8)
Computer Science
(VLAN ports 9-15)
15
hellip
Electrical Engineering
(VLAN ports 1-8)
hellip
1
8 2
7 9
16 10
15
hellip
Computer Science
(VLAN ports 9-16)
hellip operates as multiple virtual switches
Link Layer 5-73
Port-based VLAN
1
8
9
16 10 2
7
hellip
Electrical Engineering
(VLAN ports 1-8)
Computer Science
(VLAN ports 9-15)
15
hellip
traffic isolation frames tofrom ports 1-8 can only reach ports 1-8 can also define VLAN based on
MAC addresses of endpoints rather than switch port
dynamic membership ports can be dynamically assigned among VLANs
router
forwarding between VLANS done via routing (just as with separate switches) in practice vendors sell combined
switches plus routers
Link Layer 5-74
VLANS spanning multiple switches
trunk port carries frames between VLANS defined over multiple physical switches frames forwarded within VLAN between switches canrsquot be vanilla
8021 frames (must carry VLAN ID info)
8021q protocol addsremoved additional header fields for frames forwarded between trunk ports
1
8
9
10 2
7
hellip
Electrical Engineering
(VLAN ports 1-8)
Computer Science
(VLAN ports 9-15)
15
hellip
2
7 3
Ports 235 belong to EE VLAN
Ports 4678 belong to CS VLAN
5
4 6 8 16
1
Link Layer 5-75
type
2-byte Tag Protocol Identifier
(value 81-00)
Tag Control Information (12 bit VLAN ID field
3 bit priority field like IP TOS)
Recomputed CRC
8021Q VLAN frame format
8021 frame
8021Q frame
dest address
source address
data (payload) CRC preamble
dest address
source address
preamble data (payload) CRC
type
Link Layer 5-76
Link layer LANs outline
51 introduction services
52 error detection correction
53 multiple access protocols
54 LANs addressing ARP
Ethernet
switches
VLANS
55 link virtualization MPLS
56 data center networking
57 a day in the life of a web request
Link Layer 5-77
Multiprotocol label switching (MPLS)
initial goal high-speed IP forwarding using fixed length label (instead of IP address) fast lookup using fixed length identifier (rather than
shortest prefix matching)
borrowing ideas from Virtual Circuit (VC) approach
but IP datagram still keeps IP address
PPP or Ethernet
header IP header remainder of link-layer frame MPLS header
label Exp S TTL
20 3 1 5
Link Layer 5-78
MPLS capable routers
aka label-switched router
forward packets to outgoing interface based only on label value (donrsquot inspect IP address) MPLS forwarding table distinct from IP forwarding tables
flexibility MPLS forwarding decisions can differ from those of IP use destination and source addresses to route flows to
same destination differently (traffic engineering)
re-route flows quickly if link fails pre-computed backup paths (useful for VoIP)
Link Layer 5-79
R2
D
R3 R5
A
R6
MPLS versus IP paths
IP router IP routing path to destination determined
by destination address alone
R4
Link Layer 5-80
R2
D
R3 R4
R5
A
R6
MPLS versus IP paths
IP-only router
IP routing path to destination determined by destination address alone
MPLS and IP router
MPLS routing path to destination can be based on source and dest address fast reroute precompute backup routes in
case of link failure
entry router (R4) can use different MPLS
routes to A based eg on source address
Link Layer 5-81
MPLS signaling
modify OSPF IS-IS link-state flooding protocols to carry info used by MPLS routing eg link bandwidth amount of ldquoreservedrdquo link bandwidth
D
R4
R5
A
R6
entry MPLS router uses RSVP-TE signaling protocol to set up MPLS forwarding at downstream routers
modified link state flooding
RSVP-TE
Link Layer 5-82
R1 R2
D
R3 R4 R5
0
1
0 0
A
R6
in out out
label label dest interface
6 - A 0
in out out
label label dest interface
10 6 A 1
12 9 D 0
in out out
label label dest interface
10 A 0
12 D 0
1
in out out
label label dest interface
8 6 A 0
0
8 A 1
MPLS forwarding tables
Link Layer 5-83
Link layer LANs outline
51 introduction services
52 error detection correction
53 multiple access protocols
54 LANs addressing ARP
Ethernet
switches
VLANS
55 link virtualization MPLS
56 data center networking
57 a day in the life of a web request
Link Layer 5-84
Data center networks
10rsquos to 100rsquos of thousands of hosts often closely coupled in close proximity e-business (eg Amazon)
content-servers (eg YouTube Akamai Apple Microsoft)
search engines data mining (eg Google)
challenges
multiple applications each serving massive numbers of clients
managingbalancing load avoiding processing networking data bottlenecks
Inside a 40-ft Microsoft container
Chicago data center
Link Layer 5-85
Server racks
TOR switches
Tier-1 switches
Tier-2 switches
Load balancer
Load balancer
B
1 2 3 4 5 6 7 8
A C
Border router
Access router
Internet
Data center networks
load balancer application-layer routing receives external client requests
directs workload within data center
returns results to external client (hiding data
center internals from client)
Server racks
TOR switches
Tier-1 switches
Tier-2 switches
1 2 3 4 5 6 7 8
Data center networks
rich interconnection among switches racks
increased throughput between racks (multiple routing
paths possible)
increased reliability via redundancy
Link Layer 5-87
Link layer LANs outline
51 introduction services
52 error detection correction
53 multiple access protocols
54 LANs addressing ARP
Ethernet
switches
VLANS
55 link virtualization MPLS
56 data center networking
57 a day in the life of a web request
Link Layer 5-88
Synthesis a day in the life of a web request
journey down protocol stack complete application transport network link
putting-it-all-together synthesis goal identify review understand protocols (at all
layers) involved in seemingly simple scenario requesting www page
scenario student attaches laptop to campus network requestsreceives wwwgooglecom
Link Layer 5-89
A day in the life scenario
Comcast network
68800013
Googlersquos network
64233160019 64233169105
web server
DNS server
school network
68802024
web page
browser
router
(runs DHCP)
Link Layer 5-90
A day in the lifehellip connecting to the Internet
connecting laptop needs to get its own IP address addr of first-hop router addr of DNS server use DHCP
DHCP
UDP
IP
Eth
Phy
DHCP
DHCP
DHCP
DHCP
DHCP
DHCP
UDP
IP
Eth
Phy
DHCP
DHCP
DHCP
DHCP DHCP
DHCP request encapsulated in UDP encapsulated in IP encapsulated in 8023 Ethernet
Ethernet frame broadcast
(dest FFFFFFFFFFFF) on LAN received at router running DHCP server
Ethernet demuxed to IP demuxed UDP demuxed to DHCP
router
(runs DHCP)
Link Layer 5-91
DHCP server formulates DHCP ACK containing clientrsquos IP address IP address of first-hop router for client name amp IP address of DNS server
DHCP
UDP
IP
Eth
Phy
DHCP
DHCP
DHCP
DHCP
DHCP
UDP
IP
Eth
Phy
DHCP
DHCP
DHCP
DHCP
DHCP
encapsulation at DHCP server frame forwarded (switch learning) through LAN demultiplexing at client
Client now has IP address knows name amp addr of DNS
server IP address of its first-hop router
DHCP client receives DHCP ACK reply
A day in the lifehellip connecting to the Internet
router
(runs DHCP)
Link Layer 5-92
A day in the lifehellip ARP (before DNS before HTTP)
before sending HTTP request need IP address of wwwgooglecom DNS
DNS
UDP
IP
Eth
Phy
DNS
DNS
DNS
DNS query created encapsulated in UDP encapsulated in IP encapsulated in Eth To send frame to router need MAC address of router interface ARP
ARP query broadcast received by
router which replies with ARP reply giving MAC address of router interface
client now knows MAC address of first hop router so can now send frame containing DNS query
ARP query
Eth
Phy
ARP
ARP
ARP reply
router
(runs DHCP)
Link Layer 5-93
DNS
UDP
IP
Eth
Phy
DNS
DNS
DNS
DNS
DNS
IP datagram containing DNS query forwarded via LAN switch from client to 1st hop router
IP datagram forwarded from campus network into comcast network routed (tables created by RIP OSPF IS-IS andor BGP routing protocols) to DNS server
demuxrsquoed to DNS server
DNS server replies to client with IP address of wwwgooglecom
Comcast network
68800013
DNS server
DNS
UDP
IP
Eth
Phy
DNS
DNS
DNS
DNS
A day in the lifehellip using DNS
router
(runs DHCP)
Link Layer 5-94
A day in the lifehellipTCP connection carrying HTTP
HTTP
TCP
IP
Eth
Phy
HTTP
to send HTTP request client first opens TCP socket to web server
TCP SYN segment (step 1 in 3-way handshake) inter-domain routed to web server
TCP connection established 64233169105
web server
SYN
SYN
SYN
SYN
TCP
IP
Eth
Phy
SYN
SYN
SYN
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
web server responds with TCP SYNACK (step 2 in 3-way handshake)
router
(runs DHCP)
Link Layer 5-95
A day in the lifehellip HTTP requestreply
HTTP
TCP
IP
Eth
Phy
HTTP
HTTP request sent into TCP socket
IP datagram containing HTTP request routed to wwwgooglecom
IP datagram containing HTTP reply routed back to client
64233169105
web server
HTTP
TCP
IP
Eth
Phy web server responds with
HTTP reply (containing web page)
HTTP
HTTP
HTTP HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
web page finally () displayed
Link Layer 5-96
Chapter 5 Summary
principles behind data link layer services error detection correction
sharing a broadcast channel multiple access
link layer addressing
instantiation and implementation of various link layer technologies Ethernet
switched LANS VLANs
virtualized networks as a link layer MPLS
synthesis a day in the life of a web request
Link Layer 5-97
Chapter 5 letrsquos take a breath
journey down protocol stack complete (except PHY)
solid understanding of networking principles practice
hellip could stop here hellip but lots of interesting topics wireless
multimedia
security
network management
R
1A-23-F9-CD-06-9B 222222222220
111111111110 E6-E9-00-17-BB-4B CC-49-DE-D0-AB-7D
111111111112
111111111111
74-29-9C-E8-FF-55
A
222222222222
49-BD-D2-C7-56-2A
222222222221 88-B2-2F-54-1A-0F
B
Link Layer 5-49
Addressing routing to another LAN
IP
Eth
Phy
frame sent from A to R
IP
Eth
Phy
frame received at R datagram removed passed up to IP
MAC src 74-29-9C-E8-FF-55
MAC dest E6-E9-00-17-BB-4B
IP src 111111111111
IP dest 222222222222
IP src 111111111111
IP dest 222222222222
R
1A-23-F9-CD-06-9B 222222222220
111111111110 E6-E9-00-17-BB-4B CC-49-DE-D0-AB-7D
111111111112
111111111111
74-29-9C-E8-FF-55
A
222222222222
49-BD-D2-C7-56-2A
222222222221 88-B2-2F-54-1A-0F
B
Link Layer 5-50
Addressing routing to another LAN
IP src 111111111111
IP dest 222222222222
R forwards datagram with IP source A destination B
R creates link-layer frame with Bs MAC address as dest frame contains A-to-B IP datagram
MAC src 1A-23-F9-CD-06-9B
MAC dest 49-BD-D2-C7-56-2A
IP
Eth
Phy
IP
Eth
Phy
R
1A-23-F9-CD-06-9B 222222222220
111111111110 E6-E9-00-17-BB-4B CC-49-DE-D0-AB-7D
111111111112
111111111111
74-29-9C-E8-FF-55
A
222222222222
49-BD-D2-C7-56-2A
222222222221 88-B2-2F-54-1A-0F
B
Link Layer 5-51
Addressing routing to another LAN R forwards datagram with IP source A destination B
R creates link-layer frame with Bs MAC address as dest frame contains A-to-B IP datagram
IP src 111111111111
IP dest 222222222222
MAC src 1A-23-F9-CD-06-9B
MAC dest 49-BD-D2-C7-56-2A
IP
Eth
Phy
IP
Eth
Phy
R
1A-23-F9-CD-06-9B 222222222220
111111111110 E6-E9-00-17-BB-4B CC-49-DE-D0-AB-7D
111111111112
111111111111
74-29-9C-E8-FF-55
A
222222222222
49-BD-D2-C7-56-2A
222222222221 88-B2-2F-54-1A-0F
B
Link Layer 5-52
Addressing routing to another LAN R forwards datagram with IP source A destination B
R creates link-layer frame with Bs MAC address as dest frame contains A-to-B IP datagram
IP src 111111111111
IP dest 222222222222
MAC src 1A-23-F9-CD-06-9B
MAC dest 49-BD-D2-C7-56-2A
IP
Eth
Phy
Link Layer 5-53
Link layer LANs outline
51 introduction services
52 error detection correction
53 multiple access protocols
54 LANs addressing ARP
Ethernet
switches
VLANS
55 link virtualization MPLS
56 data center networking
57 a day in the life of a web request
Link Layer 5-54
Ethernet
ldquodominantrdquo wired LAN technology
cheap $20 for NIC
first widely used LAN technology
simpler cheaper than token LANs and ATM
kept up with speed race 10 Mbps ndash 10 Gbps
Metcalfersquos Ethernet sketch
Link Layer 5-55
Ethernet physical topology
bus popular through mid 90s all nodes in same collision domain (can collide with each
other)
star prevails today active switch in center
each ldquospokerdquo runs a (separate) Ethernet protocol (nodes
do not collide with each other)
switch
bus coaxial cable star
Link Layer 5-56
Ethernet frame structure
sending adapter encapsulates IP datagram (or other network layer protocol packet) in Ethernet frame
preamble
7 bytes with pattern 10101010 followed by one byte with pattern 10101011
used to synchronize receiver sender clock rates
dest address
source address
data (payload) CRC preamble
type
Link Layer 5-57
Ethernet frame structure (more)
addresses 6 byte source destination MAC addresses if adapter receives frame with matching destination
address or with broadcast address (eg ARP packet) it passes data in frame to network layer protocol
otherwise adapter discards frame
type indicates higher layer protocol (mostly IP but others possible eg Novell IPX AppleTalk)
CRC cyclic redundancy check at receiver error detected frame is dropped
dest address
source address
data (payload) CRC preamble
type
Link Layer 5-58
Ethernet unreliable connectionless
connectionless no handshaking between sending and receiving NICs
unreliable receiving NIC doesnt send acks or nacks to sending NIC
data in dropped frames recovered only if initial sender uses higher layer rdt (eg TCP) otherwise dropped data lost
Ethernetrsquos MAC protocol unslotted CSMACD wth binary backoff
Link Layer 5-59
8023 Ethernet standards link amp physical layers
many different Ethernet standards
common MAC protocol and frame format
different speeds 2 Mbps 10 Mbps 100 Mbps 1Gbps 10G bps
different physical layer media fiber cable
application
transport
network
link
physical
MAC protocol
and frame format
100BASE-TX
100BASE-T4
100BASE-FX 100BASE-T2
100BASE-SX 100BASE-BX
fiber physical layer copper (twister
pair) physical layer
Link Layer 5-60
Link layer LANs outline
51 introduction services
52 error detection correction
53 multiple access protocols
54 LANs addressing ARP
Ethernet
switches
VLANS
55 link virtualization MPLS
56 data center networking
57 a day in the life of a web request
Link Layer 5-61
Ethernet switch link-layer device takes an active role
store forward Ethernet frames
examine incoming framersquos MAC address selectively forward frame to one-or-more outgoing links when frame is to be forwarded on segment uses CSMACD to access segment
transparent
hosts are unaware of presence of switches
plug-and-play self-learning
switches do not need to be configured
Link Layer 5-62
Switch multiple simultaneous transmissions
hosts have dedicated direct connection to switch
switches buffer packets
Ethernet protocol used on each incoming link but no collisions full duplex
each link is its own collision domain
switching A-to-Arsquo and B-to-Brsquo can transmit simultaneously without collisions switch with six interfaces
(123456)
A
Arsquo
B
Brsquo C
Crsquo
1 2
3 4 5
6
Link Layer 5-63
Switch forwarding table
Q how does switch know Arsquo reachable via interface 4 Brsquo reachable via interface 5
switch with six interfaces
(123456)
A
Arsquo
B
Brsquo C
Crsquo
1 2
3 4 5
6 A each switch has a switch table each entry
(MAC address of host interface to
reach host time stamp)
looks like a routing table
Q how are entries created maintained in switch table
something like a routing protocol
A
Arsquo
B
Brsquo C
Crsquo
1 2
3 4 5
6
Link Layer 5-64
Switch self-learning
switch learns which hosts can be reached through which interfaces
when frame received switch ldquolearnsrdquo location of sender incoming LAN segment
records senderlocation pair in switch table
A Arsquo
Source A
Dest Arsquo
MAC addr interface TTL
Switch table
(initially empty) A 1 60
Link Layer 5-65
Switch frame filteringforwarding
when frame received at switch 1 record incoming link MAC address of sending host
2 index switch table using MAC destination address
3 if entry found for destination then
if destination on segment from which frame arrived then drop frame
else forward frame on interface indicated by entry
else flood forward on all interfaces except arriving
interface
A
Arsquo
B
Brsquo C
Crsquo
1 2
3 4 5
6
Link Layer 5-66
Self-learning forwarding example
A Arsquo
Source A
Dest Arsquo
MAC addr interface TTL
switch table
(initially empty) A 1 60
A Arsquo A Arsquo A Arsquo A Arsquo A Arsquo
frame destination Arsquo locaton unknown flood
Arsquo A
destination A location
known
Arsquo 4 60
selectively send
on just one link
Link Layer 5-67
Interconnecting switches
switches can be connected together
Q sending from A to G - how does S1 know to forward frame destined to F via S4 and S3
A self learning (works exactly the same as in single-switch case)
A
B
S1
C D
E
F
S2
S4
S3
H
I
G
Link Layer 5-68
Self-learning multi-switch example
Suppose C sends frame to I I responds to C
Q show switch tables and packet forwarding in S1 S2 S3 S4
A
B
S1
C D
E
F
S2
S4
S3
H
I
G
Link Layer 5-69
Institutional network
to external
network
router
IP subnet
mail server
web server
Link Layer 5-70
Switches vs routers
both are store-and-forward
routers network-layer devices (examine network-layer headers)
switches link-layer devices (examine link-layer headers)
both have forwarding tables
routers compute tables using routing algorithms IP addresses
switches learn forwarding table using flooding learning MAC addresses
application
transport
network
link
physical
network
link
physical
link
physical
switch
datagram
application
transport
network
link
physical
frame
frame
frame
datagram
Link Layer 5-71
VLANs motivation
consider CS user moves office to
EE but wants connect to CS switch
single broadcast domain
all layer-2 broadcast traffic (ARP DHCP unknown location of destination MAC address) must cross entire LAN
securityprivacy efficiency issues
Computer
Science Electrical
Engineering
Computer
Engineering
Link Layer 5-72
VLANs port-based VLAN switch ports
grouped (by switch management software) so that single physical switch helliphellip
switch(es) supporting
VLAN capabilities can
be configured to
define multiple virtual
LANS over single
physical LAN
infrastructure
Virtual Local
Area Network 1
8
9
16 10 2
7
hellip
Electrical Engineering
(VLAN ports 1-8)
Computer Science
(VLAN ports 9-15)
15
hellip
Electrical Engineering
(VLAN ports 1-8)
hellip
1
8 2
7 9
16 10
15
hellip
Computer Science
(VLAN ports 9-16)
hellip operates as multiple virtual switches
Link Layer 5-73
Port-based VLAN
1
8
9
16 10 2
7
hellip
Electrical Engineering
(VLAN ports 1-8)
Computer Science
(VLAN ports 9-15)
15
hellip
traffic isolation frames tofrom ports 1-8 can only reach ports 1-8 can also define VLAN based on
MAC addresses of endpoints rather than switch port
dynamic membership ports can be dynamically assigned among VLANs
router
forwarding between VLANS done via routing (just as with separate switches) in practice vendors sell combined
switches plus routers
Link Layer 5-74
VLANS spanning multiple switches
trunk port carries frames between VLANS defined over multiple physical switches frames forwarded within VLAN between switches canrsquot be vanilla
8021 frames (must carry VLAN ID info)
8021q protocol addsremoved additional header fields for frames forwarded between trunk ports
1
8
9
10 2
7
hellip
Electrical Engineering
(VLAN ports 1-8)
Computer Science
(VLAN ports 9-15)
15
hellip
2
7 3
Ports 235 belong to EE VLAN
Ports 4678 belong to CS VLAN
5
4 6 8 16
1
Link Layer 5-75
type
2-byte Tag Protocol Identifier
(value 81-00)
Tag Control Information (12 bit VLAN ID field
3 bit priority field like IP TOS)
Recomputed CRC
8021Q VLAN frame format
8021 frame
8021Q frame
dest address
source address
data (payload) CRC preamble
dest address
source address
preamble data (payload) CRC
type
Link Layer 5-76
Link layer LANs outline
51 introduction services
52 error detection correction
53 multiple access protocols
54 LANs addressing ARP
Ethernet
switches
VLANS
55 link virtualization MPLS
56 data center networking
57 a day in the life of a web request
Link Layer 5-77
Multiprotocol label switching (MPLS)
initial goal high-speed IP forwarding using fixed length label (instead of IP address) fast lookup using fixed length identifier (rather than
shortest prefix matching)
borrowing ideas from Virtual Circuit (VC) approach
but IP datagram still keeps IP address
PPP or Ethernet
header IP header remainder of link-layer frame MPLS header
label Exp S TTL
20 3 1 5
Link Layer 5-78
MPLS capable routers
aka label-switched router
forward packets to outgoing interface based only on label value (donrsquot inspect IP address) MPLS forwarding table distinct from IP forwarding tables
flexibility MPLS forwarding decisions can differ from those of IP use destination and source addresses to route flows to
same destination differently (traffic engineering)
re-route flows quickly if link fails pre-computed backup paths (useful for VoIP)
Link Layer 5-79
R2
D
R3 R5
A
R6
MPLS versus IP paths
IP router IP routing path to destination determined
by destination address alone
R4
Link Layer 5-80
R2
D
R3 R4
R5
A
R6
MPLS versus IP paths
IP-only router
IP routing path to destination determined by destination address alone
MPLS and IP router
MPLS routing path to destination can be based on source and dest address fast reroute precompute backup routes in
case of link failure
entry router (R4) can use different MPLS
routes to A based eg on source address
Link Layer 5-81
MPLS signaling
modify OSPF IS-IS link-state flooding protocols to carry info used by MPLS routing eg link bandwidth amount of ldquoreservedrdquo link bandwidth
D
R4
R5
A
R6
entry MPLS router uses RSVP-TE signaling protocol to set up MPLS forwarding at downstream routers
modified link state flooding
RSVP-TE
Link Layer 5-82
R1 R2
D
R3 R4 R5
0
1
0 0
A
R6
in out out
label label dest interface
6 - A 0
in out out
label label dest interface
10 6 A 1
12 9 D 0
in out out
label label dest interface
10 A 0
12 D 0
1
in out out
label label dest interface
8 6 A 0
0
8 A 1
MPLS forwarding tables
Link Layer 5-83
Link layer LANs outline
51 introduction services
52 error detection correction
53 multiple access protocols
54 LANs addressing ARP
Ethernet
switches
VLANS
55 link virtualization MPLS
56 data center networking
57 a day in the life of a web request
Link Layer 5-84
Data center networks
10rsquos to 100rsquos of thousands of hosts often closely coupled in close proximity e-business (eg Amazon)
content-servers (eg YouTube Akamai Apple Microsoft)
search engines data mining (eg Google)
challenges
multiple applications each serving massive numbers of clients
managingbalancing load avoiding processing networking data bottlenecks
Inside a 40-ft Microsoft container
Chicago data center
Link Layer 5-85
Server racks
TOR switches
Tier-1 switches
Tier-2 switches
Load balancer
Load balancer
B
1 2 3 4 5 6 7 8
A C
Border router
Access router
Internet
Data center networks
load balancer application-layer routing receives external client requests
directs workload within data center
returns results to external client (hiding data
center internals from client)
Server racks
TOR switches
Tier-1 switches
Tier-2 switches
1 2 3 4 5 6 7 8
Data center networks
rich interconnection among switches racks
increased throughput between racks (multiple routing
paths possible)
increased reliability via redundancy
Link Layer 5-87
Link layer LANs outline
51 introduction services
52 error detection correction
53 multiple access protocols
54 LANs addressing ARP
Ethernet
switches
VLANS
55 link virtualization MPLS
56 data center networking
57 a day in the life of a web request
Link Layer 5-88
Synthesis a day in the life of a web request
journey down protocol stack complete application transport network link
putting-it-all-together synthesis goal identify review understand protocols (at all
layers) involved in seemingly simple scenario requesting www page
scenario student attaches laptop to campus network requestsreceives wwwgooglecom
Link Layer 5-89
A day in the life scenario
Comcast network
68800013
Googlersquos network
64233160019 64233169105
web server
DNS server
school network
68802024
web page
browser
router
(runs DHCP)
Link Layer 5-90
A day in the lifehellip connecting to the Internet
connecting laptop needs to get its own IP address addr of first-hop router addr of DNS server use DHCP
DHCP
UDP
IP
Eth
Phy
DHCP
DHCP
DHCP
DHCP
DHCP
DHCP
UDP
IP
Eth
Phy
DHCP
DHCP
DHCP
DHCP DHCP
DHCP request encapsulated in UDP encapsulated in IP encapsulated in 8023 Ethernet
Ethernet frame broadcast
(dest FFFFFFFFFFFF) on LAN received at router running DHCP server
Ethernet demuxed to IP demuxed UDP demuxed to DHCP
router
(runs DHCP)
Link Layer 5-91
DHCP server formulates DHCP ACK containing clientrsquos IP address IP address of first-hop router for client name amp IP address of DNS server
DHCP
UDP
IP
Eth
Phy
DHCP
DHCP
DHCP
DHCP
DHCP
UDP
IP
Eth
Phy
DHCP
DHCP
DHCP
DHCP
DHCP
encapsulation at DHCP server frame forwarded (switch learning) through LAN demultiplexing at client
Client now has IP address knows name amp addr of DNS
server IP address of its first-hop router
DHCP client receives DHCP ACK reply
A day in the lifehellip connecting to the Internet
router
(runs DHCP)
Link Layer 5-92
A day in the lifehellip ARP (before DNS before HTTP)
before sending HTTP request need IP address of wwwgooglecom DNS
DNS
UDP
IP
Eth
Phy
DNS
DNS
DNS
DNS query created encapsulated in UDP encapsulated in IP encapsulated in Eth To send frame to router need MAC address of router interface ARP
ARP query broadcast received by
router which replies with ARP reply giving MAC address of router interface
client now knows MAC address of first hop router so can now send frame containing DNS query
ARP query
Eth
Phy
ARP
ARP
ARP reply
router
(runs DHCP)
Link Layer 5-93
DNS
UDP
IP
Eth
Phy
DNS
DNS
DNS
DNS
DNS
IP datagram containing DNS query forwarded via LAN switch from client to 1st hop router
IP datagram forwarded from campus network into comcast network routed (tables created by RIP OSPF IS-IS andor BGP routing protocols) to DNS server
demuxrsquoed to DNS server
DNS server replies to client with IP address of wwwgooglecom
Comcast network
68800013
DNS server
DNS
UDP
IP
Eth
Phy
DNS
DNS
DNS
DNS
A day in the lifehellip using DNS
router
(runs DHCP)
Link Layer 5-94
A day in the lifehellipTCP connection carrying HTTP
HTTP
TCP
IP
Eth
Phy
HTTP
to send HTTP request client first opens TCP socket to web server
TCP SYN segment (step 1 in 3-way handshake) inter-domain routed to web server
TCP connection established 64233169105
web server
SYN
SYN
SYN
SYN
TCP
IP
Eth
Phy
SYN
SYN
SYN
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
web server responds with TCP SYNACK (step 2 in 3-way handshake)
router
(runs DHCP)
Link Layer 5-95
A day in the lifehellip HTTP requestreply
HTTP
TCP
IP
Eth
Phy
HTTP
HTTP request sent into TCP socket
IP datagram containing HTTP request routed to wwwgooglecom
IP datagram containing HTTP reply routed back to client
64233169105
web server
HTTP
TCP
IP
Eth
Phy web server responds with
HTTP reply (containing web page)
HTTP
HTTP
HTTP HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
web page finally () displayed
Link Layer 5-96
Chapter 5 Summary
principles behind data link layer services error detection correction
sharing a broadcast channel multiple access
link layer addressing
instantiation and implementation of various link layer technologies Ethernet
switched LANS VLANs
virtualized networks as a link layer MPLS
synthesis a day in the life of a web request
Link Layer 5-97
Chapter 5 letrsquos take a breath
journey down protocol stack complete (except PHY)
solid understanding of networking principles practice
hellip could stop here hellip but lots of interesting topics wireless
multimedia
security
network management
R
1A-23-F9-CD-06-9B 222222222220
111111111110 E6-E9-00-17-BB-4B CC-49-DE-D0-AB-7D
111111111112
111111111111
74-29-9C-E8-FF-55
A
222222222222
49-BD-D2-C7-56-2A
222222222221 88-B2-2F-54-1A-0F
B
Link Layer 5-50
Addressing routing to another LAN
IP src 111111111111
IP dest 222222222222
R forwards datagram with IP source A destination B
R creates link-layer frame with Bs MAC address as dest frame contains A-to-B IP datagram
MAC src 1A-23-F9-CD-06-9B
MAC dest 49-BD-D2-C7-56-2A
IP
Eth
Phy
IP
Eth
Phy
R
1A-23-F9-CD-06-9B 222222222220
111111111110 E6-E9-00-17-BB-4B CC-49-DE-D0-AB-7D
111111111112
111111111111
74-29-9C-E8-FF-55
A
222222222222
49-BD-D2-C7-56-2A
222222222221 88-B2-2F-54-1A-0F
B
Link Layer 5-51
Addressing routing to another LAN R forwards datagram with IP source A destination B
R creates link-layer frame with Bs MAC address as dest frame contains A-to-B IP datagram
IP src 111111111111
IP dest 222222222222
MAC src 1A-23-F9-CD-06-9B
MAC dest 49-BD-D2-C7-56-2A
IP
Eth
Phy
IP
Eth
Phy
R
1A-23-F9-CD-06-9B 222222222220
111111111110 E6-E9-00-17-BB-4B CC-49-DE-D0-AB-7D
111111111112
111111111111
74-29-9C-E8-FF-55
A
222222222222
49-BD-D2-C7-56-2A
222222222221 88-B2-2F-54-1A-0F
B
Link Layer 5-52
Addressing routing to another LAN R forwards datagram with IP source A destination B
R creates link-layer frame with Bs MAC address as dest frame contains A-to-B IP datagram
IP src 111111111111
IP dest 222222222222
MAC src 1A-23-F9-CD-06-9B
MAC dest 49-BD-D2-C7-56-2A
IP
Eth
Phy
Link Layer 5-53
Link layer LANs outline
51 introduction services
52 error detection correction
53 multiple access protocols
54 LANs addressing ARP
Ethernet
switches
VLANS
55 link virtualization MPLS
56 data center networking
57 a day in the life of a web request
Link Layer 5-54
Ethernet
ldquodominantrdquo wired LAN technology
cheap $20 for NIC
first widely used LAN technology
simpler cheaper than token LANs and ATM
kept up with speed race 10 Mbps ndash 10 Gbps
Metcalfersquos Ethernet sketch
Link Layer 5-55
Ethernet physical topology
bus popular through mid 90s all nodes in same collision domain (can collide with each
other)
star prevails today active switch in center
each ldquospokerdquo runs a (separate) Ethernet protocol (nodes
do not collide with each other)
switch
bus coaxial cable star
Link Layer 5-56
Ethernet frame structure
sending adapter encapsulates IP datagram (or other network layer protocol packet) in Ethernet frame
preamble
7 bytes with pattern 10101010 followed by one byte with pattern 10101011
used to synchronize receiver sender clock rates
dest address
source address
data (payload) CRC preamble
type
Link Layer 5-57
Ethernet frame structure (more)
addresses 6 byte source destination MAC addresses if adapter receives frame with matching destination
address or with broadcast address (eg ARP packet) it passes data in frame to network layer protocol
otherwise adapter discards frame
type indicates higher layer protocol (mostly IP but others possible eg Novell IPX AppleTalk)
CRC cyclic redundancy check at receiver error detected frame is dropped
dest address
source address
data (payload) CRC preamble
type
Link Layer 5-58
Ethernet unreliable connectionless
connectionless no handshaking between sending and receiving NICs
unreliable receiving NIC doesnt send acks or nacks to sending NIC
data in dropped frames recovered only if initial sender uses higher layer rdt (eg TCP) otherwise dropped data lost
Ethernetrsquos MAC protocol unslotted CSMACD wth binary backoff
Link Layer 5-59
8023 Ethernet standards link amp physical layers
many different Ethernet standards
common MAC protocol and frame format
different speeds 2 Mbps 10 Mbps 100 Mbps 1Gbps 10G bps
different physical layer media fiber cable
application
transport
network
link
physical
MAC protocol
and frame format
100BASE-TX
100BASE-T4
100BASE-FX 100BASE-T2
100BASE-SX 100BASE-BX
fiber physical layer copper (twister
pair) physical layer
Link Layer 5-60
Link layer LANs outline
51 introduction services
52 error detection correction
53 multiple access protocols
54 LANs addressing ARP
Ethernet
switches
VLANS
55 link virtualization MPLS
56 data center networking
57 a day in the life of a web request
Link Layer 5-61
Ethernet switch link-layer device takes an active role
store forward Ethernet frames
examine incoming framersquos MAC address selectively forward frame to one-or-more outgoing links when frame is to be forwarded on segment uses CSMACD to access segment
transparent
hosts are unaware of presence of switches
plug-and-play self-learning
switches do not need to be configured
Link Layer 5-62
Switch multiple simultaneous transmissions
hosts have dedicated direct connection to switch
switches buffer packets
Ethernet protocol used on each incoming link but no collisions full duplex
each link is its own collision domain
switching A-to-Arsquo and B-to-Brsquo can transmit simultaneously without collisions switch with six interfaces
(123456)
A
Arsquo
B
Brsquo C
Crsquo
1 2
3 4 5
6
Link Layer 5-63
Switch forwarding table
Q how does switch know Arsquo reachable via interface 4 Brsquo reachable via interface 5
switch with six interfaces
(123456)
A
Arsquo
B
Brsquo C
Crsquo
1 2
3 4 5
6 A each switch has a switch table each entry
(MAC address of host interface to
reach host time stamp)
looks like a routing table
Q how are entries created maintained in switch table
something like a routing protocol
A
Arsquo
B
Brsquo C
Crsquo
1 2
3 4 5
6
Link Layer 5-64
Switch self-learning
switch learns which hosts can be reached through which interfaces
when frame received switch ldquolearnsrdquo location of sender incoming LAN segment
records senderlocation pair in switch table
A Arsquo
Source A
Dest Arsquo
MAC addr interface TTL
Switch table
(initially empty) A 1 60
Link Layer 5-65
Switch frame filteringforwarding
when frame received at switch 1 record incoming link MAC address of sending host
2 index switch table using MAC destination address
3 if entry found for destination then
if destination on segment from which frame arrived then drop frame
else forward frame on interface indicated by entry
else flood forward on all interfaces except arriving
interface
A
Arsquo
B
Brsquo C
Crsquo
1 2
3 4 5
6
Link Layer 5-66
Self-learning forwarding example
A Arsquo
Source A
Dest Arsquo
MAC addr interface TTL
switch table
(initially empty) A 1 60
A Arsquo A Arsquo A Arsquo A Arsquo A Arsquo
frame destination Arsquo locaton unknown flood
Arsquo A
destination A location
known
Arsquo 4 60
selectively send
on just one link
Link Layer 5-67
Interconnecting switches
switches can be connected together
Q sending from A to G - how does S1 know to forward frame destined to F via S4 and S3
A self learning (works exactly the same as in single-switch case)
A
B
S1
C D
E
F
S2
S4
S3
H
I
G
Link Layer 5-68
Self-learning multi-switch example
Suppose C sends frame to I I responds to C
Q show switch tables and packet forwarding in S1 S2 S3 S4
A
B
S1
C D
E
F
S2
S4
S3
H
I
G
Link Layer 5-69
Institutional network
to external
network
router
IP subnet
mail server
web server
Link Layer 5-70
Switches vs routers
both are store-and-forward
routers network-layer devices (examine network-layer headers)
switches link-layer devices (examine link-layer headers)
both have forwarding tables
routers compute tables using routing algorithms IP addresses
switches learn forwarding table using flooding learning MAC addresses
application
transport
network
link
physical
network
link
physical
link
physical
switch
datagram
application
transport
network
link
physical
frame
frame
frame
datagram
Link Layer 5-71
VLANs motivation
consider CS user moves office to
EE but wants connect to CS switch
single broadcast domain
all layer-2 broadcast traffic (ARP DHCP unknown location of destination MAC address) must cross entire LAN
securityprivacy efficiency issues
Computer
Science Electrical
Engineering
Computer
Engineering
Link Layer 5-72
VLANs port-based VLAN switch ports
grouped (by switch management software) so that single physical switch helliphellip
switch(es) supporting
VLAN capabilities can
be configured to
define multiple virtual
LANS over single
physical LAN
infrastructure
Virtual Local
Area Network 1
8
9
16 10 2
7
hellip
Electrical Engineering
(VLAN ports 1-8)
Computer Science
(VLAN ports 9-15)
15
hellip
Electrical Engineering
(VLAN ports 1-8)
hellip
1
8 2
7 9
16 10
15
hellip
Computer Science
(VLAN ports 9-16)
hellip operates as multiple virtual switches
Link Layer 5-73
Port-based VLAN
1
8
9
16 10 2
7
hellip
Electrical Engineering
(VLAN ports 1-8)
Computer Science
(VLAN ports 9-15)
15
hellip
traffic isolation frames tofrom ports 1-8 can only reach ports 1-8 can also define VLAN based on
MAC addresses of endpoints rather than switch port
dynamic membership ports can be dynamically assigned among VLANs
router
forwarding between VLANS done via routing (just as with separate switches) in practice vendors sell combined
switches plus routers
Link Layer 5-74
VLANS spanning multiple switches
trunk port carries frames between VLANS defined over multiple physical switches frames forwarded within VLAN between switches canrsquot be vanilla
8021 frames (must carry VLAN ID info)
8021q protocol addsremoved additional header fields for frames forwarded between trunk ports
1
8
9
10 2
7
hellip
Electrical Engineering
(VLAN ports 1-8)
Computer Science
(VLAN ports 9-15)
15
hellip
2
7 3
Ports 235 belong to EE VLAN
Ports 4678 belong to CS VLAN
5
4 6 8 16
1
Link Layer 5-75
type
2-byte Tag Protocol Identifier
(value 81-00)
Tag Control Information (12 bit VLAN ID field
3 bit priority field like IP TOS)
Recomputed CRC
8021Q VLAN frame format
8021 frame
8021Q frame
dest address
source address
data (payload) CRC preamble
dest address
source address
preamble data (payload) CRC
type
Link Layer 5-76
Link layer LANs outline
51 introduction services
52 error detection correction
53 multiple access protocols
54 LANs addressing ARP
Ethernet
switches
VLANS
55 link virtualization MPLS
56 data center networking
57 a day in the life of a web request
Link Layer 5-77
Multiprotocol label switching (MPLS)
initial goal high-speed IP forwarding using fixed length label (instead of IP address) fast lookup using fixed length identifier (rather than
shortest prefix matching)
borrowing ideas from Virtual Circuit (VC) approach
but IP datagram still keeps IP address
PPP or Ethernet
header IP header remainder of link-layer frame MPLS header
label Exp S TTL
20 3 1 5
Link Layer 5-78
MPLS capable routers
aka label-switched router
forward packets to outgoing interface based only on label value (donrsquot inspect IP address) MPLS forwarding table distinct from IP forwarding tables
flexibility MPLS forwarding decisions can differ from those of IP use destination and source addresses to route flows to
same destination differently (traffic engineering)
re-route flows quickly if link fails pre-computed backup paths (useful for VoIP)
Link Layer 5-79
R2
D
R3 R5
A
R6
MPLS versus IP paths
IP router IP routing path to destination determined
by destination address alone
R4
Link Layer 5-80
R2
D
R3 R4
R5
A
R6
MPLS versus IP paths
IP-only router
IP routing path to destination determined by destination address alone
MPLS and IP router
MPLS routing path to destination can be based on source and dest address fast reroute precompute backup routes in
case of link failure
entry router (R4) can use different MPLS
routes to A based eg on source address
Link Layer 5-81
MPLS signaling
modify OSPF IS-IS link-state flooding protocols to carry info used by MPLS routing eg link bandwidth amount of ldquoreservedrdquo link bandwidth
D
R4
R5
A
R6
entry MPLS router uses RSVP-TE signaling protocol to set up MPLS forwarding at downstream routers
modified link state flooding
RSVP-TE
Link Layer 5-82
R1 R2
D
R3 R4 R5
0
1
0 0
A
R6
in out out
label label dest interface
6 - A 0
in out out
label label dest interface
10 6 A 1
12 9 D 0
in out out
label label dest interface
10 A 0
12 D 0
1
in out out
label label dest interface
8 6 A 0
0
8 A 1
MPLS forwarding tables
Link Layer 5-83
Link layer LANs outline
51 introduction services
52 error detection correction
53 multiple access protocols
54 LANs addressing ARP
Ethernet
switches
VLANS
55 link virtualization MPLS
56 data center networking
57 a day in the life of a web request
Link Layer 5-84
Data center networks
10rsquos to 100rsquos of thousands of hosts often closely coupled in close proximity e-business (eg Amazon)
content-servers (eg YouTube Akamai Apple Microsoft)
search engines data mining (eg Google)
challenges
multiple applications each serving massive numbers of clients
managingbalancing load avoiding processing networking data bottlenecks
Inside a 40-ft Microsoft container
Chicago data center
Link Layer 5-85
Server racks
TOR switches
Tier-1 switches
Tier-2 switches
Load balancer
Load balancer
B
1 2 3 4 5 6 7 8
A C
Border router
Access router
Internet
Data center networks
load balancer application-layer routing receives external client requests
directs workload within data center
returns results to external client (hiding data
center internals from client)
Server racks
TOR switches
Tier-1 switches
Tier-2 switches
1 2 3 4 5 6 7 8
Data center networks
rich interconnection among switches racks
increased throughput between racks (multiple routing
paths possible)
increased reliability via redundancy
Link Layer 5-87
Link layer LANs outline
51 introduction services
52 error detection correction
53 multiple access protocols
54 LANs addressing ARP
Ethernet
switches
VLANS
55 link virtualization MPLS
56 data center networking
57 a day in the life of a web request
Link Layer 5-88
Synthesis a day in the life of a web request
journey down protocol stack complete application transport network link
putting-it-all-together synthesis goal identify review understand protocols (at all
layers) involved in seemingly simple scenario requesting www page
scenario student attaches laptop to campus network requestsreceives wwwgooglecom
Link Layer 5-89
A day in the life scenario
Comcast network
68800013
Googlersquos network
64233160019 64233169105
web server
DNS server
school network
68802024
web page
browser
router
(runs DHCP)
Link Layer 5-90
A day in the lifehellip connecting to the Internet
connecting laptop needs to get its own IP address addr of first-hop router addr of DNS server use DHCP
DHCP
UDP
IP
Eth
Phy
DHCP
DHCP
DHCP
DHCP
DHCP
DHCP
UDP
IP
Eth
Phy
DHCP
DHCP
DHCP
DHCP DHCP
DHCP request encapsulated in UDP encapsulated in IP encapsulated in 8023 Ethernet
Ethernet frame broadcast
(dest FFFFFFFFFFFF) on LAN received at router running DHCP server
Ethernet demuxed to IP demuxed UDP demuxed to DHCP
router
(runs DHCP)
Link Layer 5-91
DHCP server formulates DHCP ACK containing clientrsquos IP address IP address of first-hop router for client name amp IP address of DNS server
DHCP
UDP
IP
Eth
Phy
DHCP
DHCP
DHCP
DHCP
DHCP
UDP
IP
Eth
Phy
DHCP
DHCP
DHCP
DHCP
DHCP
encapsulation at DHCP server frame forwarded (switch learning) through LAN demultiplexing at client
Client now has IP address knows name amp addr of DNS
server IP address of its first-hop router
DHCP client receives DHCP ACK reply
A day in the lifehellip connecting to the Internet
router
(runs DHCP)
Link Layer 5-92
A day in the lifehellip ARP (before DNS before HTTP)
before sending HTTP request need IP address of wwwgooglecom DNS
DNS
UDP
IP
Eth
Phy
DNS
DNS
DNS
DNS query created encapsulated in UDP encapsulated in IP encapsulated in Eth To send frame to router need MAC address of router interface ARP
ARP query broadcast received by
router which replies with ARP reply giving MAC address of router interface
client now knows MAC address of first hop router so can now send frame containing DNS query
ARP query
Eth
Phy
ARP
ARP
ARP reply
router
(runs DHCP)
Link Layer 5-93
DNS
UDP
IP
Eth
Phy
DNS
DNS
DNS
DNS
DNS
IP datagram containing DNS query forwarded via LAN switch from client to 1st hop router
IP datagram forwarded from campus network into comcast network routed (tables created by RIP OSPF IS-IS andor BGP routing protocols) to DNS server
demuxrsquoed to DNS server
DNS server replies to client with IP address of wwwgooglecom
Comcast network
68800013
DNS server
DNS
UDP
IP
Eth
Phy
DNS
DNS
DNS
DNS
A day in the lifehellip using DNS
router
(runs DHCP)
Link Layer 5-94
A day in the lifehellipTCP connection carrying HTTP
HTTP
TCP
IP
Eth
Phy
HTTP
to send HTTP request client first opens TCP socket to web server
TCP SYN segment (step 1 in 3-way handshake) inter-domain routed to web server
TCP connection established 64233169105
web server
SYN
SYN
SYN
SYN
TCP
IP
Eth
Phy
SYN
SYN
SYN
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
web server responds with TCP SYNACK (step 2 in 3-way handshake)
router
(runs DHCP)
Link Layer 5-95
A day in the lifehellip HTTP requestreply
HTTP
TCP
IP
Eth
Phy
HTTP
HTTP request sent into TCP socket
IP datagram containing HTTP request routed to wwwgooglecom
IP datagram containing HTTP reply routed back to client
64233169105
web server
HTTP
TCP
IP
Eth
Phy web server responds with
HTTP reply (containing web page)
HTTP
HTTP
HTTP HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
web page finally () displayed
Link Layer 5-96
Chapter 5 Summary
principles behind data link layer services error detection correction
sharing a broadcast channel multiple access
link layer addressing
instantiation and implementation of various link layer technologies Ethernet
switched LANS VLANs
virtualized networks as a link layer MPLS
synthesis a day in the life of a web request
Link Layer 5-97
Chapter 5 letrsquos take a breath
journey down protocol stack complete (except PHY)
solid understanding of networking principles practice
hellip could stop here hellip but lots of interesting topics wireless
multimedia
security
network management
R
1A-23-F9-CD-06-9B 222222222220
111111111110 E6-E9-00-17-BB-4B CC-49-DE-D0-AB-7D
111111111112
111111111111
74-29-9C-E8-FF-55
A
222222222222
49-BD-D2-C7-56-2A
222222222221 88-B2-2F-54-1A-0F
B
Link Layer 5-51
Addressing routing to another LAN R forwards datagram with IP source A destination B
R creates link-layer frame with Bs MAC address as dest frame contains A-to-B IP datagram
IP src 111111111111
IP dest 222222222222
MAC src 1A-23-F9-CD-06-9B
MAC dest 49-BD-D2-C7-56-2A
IP
Eth
Phy
IP
Eth
Phy
R
1A-23-F9-CD-06-9B 222222222220
111111111110 E6-E9-00-17-BB-4B CC-49-DE-D0-AB-7D
111111111112
111111111111
74-29-9C-E8-FF-55
A
222222222222
49-BD-D2-C7-56-2A
222222222221 88-B2-2F-54-1A-0F
B
Link Layer 5-52
Addressing routing to another LAN R forwards datagram with IP source A destination B
R creates link-layer frame with Bs MAC address as dest frame contains A-to-B IP datagram
IP src 111111111111
IP dest 222222222222
MAC src 1A-23-F9-CD-06-9B
MAC dest 49-BD-D2-C7-56-2A
IP
Eth
Phy
Link Layer 5-53
Link layer LANs outline
51 introduction services
52 error detection correction
53 multiple access protocols
54 LANs addressing ARP
Ethernet
switches
VLANS
55 link virtualization MPLS
56 data center networking
57 a day in the life of a web request
Link Layer 5-54
Ethernet
ldquodominantrdquo wired LAN technology
cheap $20 for NIC
first widely used LAN technology
simpler cheaper than token LANs and ATM
kept up with speed race 10 Mbps ndash 10 Gbps
Metcalfersquos Ethernet sketch
Link Layer 5-55
Ethernet physical topology
bus popular through mid 90s all nodes in same collision domain (can collide with each
other)
star prevails today active switch in center
each ldquospokerdquo runs a (separate) Ethernet protocol (nodes
do not collide with each other)
switch
bus coaxial cable star
Link Layer 5-56
Ethernet frame structure
sending adapter encapsulates IP datagram (or other network layer protocol packet) in Ethernet frame
preamble
7 bytes with pattern 10101010 followed by one byte with pattern 10101011
used to synchronize receiver sender clock rates
dest address
source address
data (payload) CRC preamble
type
Link Layer 5-57
Ethernet frame structure (more)
addresses 6 byte source destination MAC addresses if adapter receives frame with matching destination
address or with broadcast address (eg ARP packet) it passes data in frame to network layer protocol
otherwise adapter discards frame
type indicates higher layer protocol (mostly IP but others possible eg Novell IPX AppleTalk)
CRC cyclic redundancy check at receiver error detected frame is dropped
dest address
source address
data (payload) CRC preamble
type
Link Layer 5-58
Ethernet unreliable connectionless
connectionless no handshaking between sending and receiving NICs
unreliable receiving NIC doesnt send acks or nacks to sending NIC
data in dropped frames recovered only if initial sender uses higher layer rdt (eg TCP) otherwise dropped data lost
Ethernetrsquos MAC protocol unslotted CSMACD wth binary backoff
Link Layer 5-59
8023 Ethernet standards link amp physical layers
many different Ethernet standards
common MAC protocol and frame format
different speeds 2 Mbps 10 Mbps 100 Mbps 1Gbps 10G bps
different physical layer media fiber cable
application
transport
network
link
physical
MAC protocol
and frame format
100BASE-TX
100BASE-T4
100BASE-FX 100BASE-T2
100BASE-SX 100BASE-BX
fiber physical layer copper (twister
pair) physical layer
Link Layer 5-60
Link layer LANs outline
51 introduction services
52 error detection correction
53 multiple access protocols
54 LANs addressing ARP
Ethernet
switches
VLANS
55 link virtualization MPLS
56 data center networking
57 a day in the life of a web request
Link Layer 5-61
Ethernet switch link-layer device takes an active role
store forward Ethernet frames
examine incoming framersquos MAC address selectively forward frame to one-or-more outgoing links when frame is to be forwarded on segment uses CSMACD to access segment
transparent
hosts are unaware of presence of switches
plug-and-play self-learning
switches do not need to be configured
Link Layer 5-62
Switch multiple simultaneous transmissions
hosts have dedicated direct connection to switch
switches buffer packets
Ethernet protocol used on each incoming link but no collisions full duplex
each link is its own collision domain
switching A-to-Arsquo and B-to-Brsquo can transmit simultaneously without collisions switch with six interfaces
(123456)
A
Arsquo
B
Brsquo C
Crsquo
1 2
3 4 5
6
Link Layer 5-63
Switch forwarding table
Q how does switch know Arsquo reachable via interface 4 Brsquo reachable via interface 5
switch with six interfaces
(123456)
A
Arsquo
B
Brsquo C
Crsquo
1 2
3 4 5
6 A each switch has a switch table each entry
(MAC address of host interface to
reach host time stamp)
looks like a routing table
Q how are entries created maintained in switch table
something like a routing protocol
A
Arsquo
B
Brsquo C
Crsquo
1 2
3 4 5
6
Link Layer 5-64
Switch self-learning
switch learns which hosts can be reached through which interfaces
when frame received switch ldquolearnsrdquo location of sender incoming LAN segment
records senderlocation pair in switch table
A Arsquo
Source A
Dest Arsquo
MAC addr interface TTL
Switch table
(initially empty) A 1 60
Link Layer 5-65
Switch frame filteringforwarding
when frame received at switch 1 record incoming link MAC address of sending host
2 index switch table using MAC destination address
3 if entry found for destination then
if destination on segment from which frame arrived then drop frame
else forward frame on interface indicated by entry
else flood forward on all interfaces except arriving
interface
A
Arsquo
B
Brsquo C
Crsquo
1 2
3 4 5
6
Link Layer 5-66
Self-learning forwarding example
A Arsquo
Source A
Dest Arsquo
MAC addr interface TTL
switch table
(initially empty) A 1 60
A Arsquo A Arsquo A Arsquo A Arsquo A Arsquo
frame destination Arsquo locaton unknown flood
Arsquo A
destination A location
known
Arsquo 4 60
selectively send
on just one link
Link Layer 5-67
Interconnecting switches
switches can be connected together
Q sending from A to G - how does S1 know to forward frame destined to F via S4 and S3
A self learning (works exactly the same as in single-switch case)
A
B
S1
C D
E
F
S2
S4
S3
H
I
G
Link Layer 5-68
Self-learning multi-switch example
Suppose C sends frame to I I responds to C
Q show switch tables and packet forwarding in S1 S2 S3 S4
A
B
S1
C D
E
F
S2
S4
S3
H
I
G
Link Layer 5-69
Institutional network
to external
network
router
IP subnet
mail server
web server
Link Layer 5-70
Switches vs routers
both are store-and-forward
routers network-layer devices (examine network-layer headers)
switches link-layer devices (examine link-layer headers)
both have forwarding tables
routers compute tables using routing algorithms IP addresses
switches learn forwarding table using flooding learning MAC addresses
application
transport
network
link
physical
network
link
physical
link
physical
switch
datagram
application
transport
network
link
physical
frame
frame
frame
datagram
Link Layer 5-71
VLANs motivation
consider CS user moves office to
EE but wants connect to CS switch
single broadcast domain
all layer-2 broadcast traffic (ARP DHCP unknown location of destination MAC address) must cross entire LAN
securityprivacy efficiency issues
Computer
Science Electrical
Engineering
Computer
Engineering
Link Layer 5-72
VLANs port-based VLAN switch ports
grouped (by switch management software) so that single physical switch helliphellip
switch(es) supporting
VLAN capabilities can
be configured to
define multiple virtual
LANS over single
physical LAN
infrastructure
Virtual Local
Area Network 1
8
9
16 10 2
7
hellip
Electrical Engineering
(VLAN ports 1-8)
Computer Science
(VLAN ports 9-15)
15
hellip
Electrical Engineering
(VLAN ports 1-8)
hellip
1
8 2
7 9
16 10
15
hellip
Computer Science
(VLAN ports 9-16)
hellip operates as multiple virtual switches
Link Layer 5-73
Port-based VLAN
1
8
9
16 10 2
7
hellip
Electrical Engineering
(VLAN ports 1-8)
Computer Science
(VLAN ports 9-15)
15
hellip
traffic isolation frames tofrom ports 1-8 can only reach ports 1-8 can also define VLAN based on
MAC addresses of endpoints rather than switch port
dynamic membership ports can be dynamically assigned among VLANs
router
forwarding between VLANS done via routing (just as with separate switches) in practice vendors sell combined
switches plus routers
Link Layer 5-74
VLANS spanning multiple switches
trunk port carries frames between VLANS defined over multiple physical switches frames forwarded within VLAN between switches canrsquot be vanilla
8021 frames (must carry VLAN ID info)
8021q protocol addsremoved additional header fields for frames forwarded between trunk ports
1
8
9
10 2
7
hellip
Electrical Engineering
(VLAN ports 1-8)
Computer Science
(VLAN ports 9-15)
15
hellip
2
7 3
Ports 235 belong to EE VLAN
Ports 4678 belong to CS VLAN
5
4 6 8 16
1
Link Layer 5-75
type
2-byte Tag Protocol Identifier
(value 81-00)
Tag Control Information (12 bit VLAN ID field
3 bit priority field like IP TOS)
Recomputed CRC
8021Q VLAN frame format
8021 frame
8021Q frame
dest address
source address
data (payload) CRC preamble
dest address
source address
preamble data (payload) CRC
type
Link Layer 5-76
Link layer LANs outline
51 introduction services
52 error detection correction
53 multiple access protocols
54 LANs addressing ARP
Ethernet
switches
VLANS
55 link virtualization MPLS
56 data center networking
57 a day in the life of a web request
Link Layer 5-77
Multiprotocol label switching (MPLS)
initial goal high-speed IP forwarding using fixed length label (instead of IP address) fast lookup using fixed length identifier (rather than
shortest prefix matching)
borrowing ideas from Virtual Circuit (VC) approach
but IP datagram still keeps IP address
PPP or Ethernet
header IP header remainder of link-layer frame MPLS header
label Exp S TTL
20 3 1 5
Link Layer 5-78
MPLS capable routers
aka label-switched router
forward packets to outgoing interface based only on label value (donrsquot inspect IP address) MPLS forwarding table distinct from IP forwarding tables
flexibility MPLS forwarding decisions can differ from those of IP use destination and source addresses to route flows to
same destination differently (traffic engineering)
re-route flows quickly if link fails pre-computed backup paths (useful for VoIP)
Link Layer 5-79
R2
D
R3 R5
A
R6
MPLS versus IP paths
IP router IP routing path to destination determined
by destination address alone
R4
Link Layer 5-80
R2
D
R3 R4
R5
A
R6
MPLS versus IP paths
IP-only router
IP routing path to destination determined by destination address alone
MPLS and IP router
MPLS routing path to destination can be based on source and dest address fast reroute precompute backup routes in
case of link failure
entry router (R4) can use different MPLS
routes to A based eg on source address
Link Layer 5-81
MPLS signaling
modify OSPF IS-IS link-state flooding protocols to carry info used by MPLS routing eg link bandwidth amount of ldquoreservedrdquo link bandwidth
D
R4
R5
A
R6
entry MPLS router uses RSVP-TE signaling protocol to set up MPLS forwarding at downstream routers
modified link state flooding
RSVP-TE
Link Layer 5-82
R1 R2
D
R3 R4 R5
0
1
0 0
A
R6
in out out
label label dest interface
6 - A 0
in out out
label label dest interface
10 6 A 1
12 9 D 0
in out out
label label dest interface
10 A 0
12 D 0
1
in out out
label label dest interface
8 6 A 0
0
8 A 1
MPLS forwarding tables
Link Layer 5-83
Link layer LANs outline
51 introduction services
52 error detection correction
53 multiple access protocols
54 LANs addressing ARP
Ethernet
switches
VLANS
55 link virtualization MPLS
56 data center networking
57 a day in the life of a web request
Link Layer 5-84
Data center networks
10rsquos to 100rsquos of thousands of hosts often closely coupled in close proximity e-business (eg Amazon)
content-servers (eg YouTube Akamai Apple Microsoft)
search engines data mining (eg Google)
challenges
multiple applications each serving massive numbers of clients
managingbalancing load avoiding processing networking data bottlenecks
Inside a 40-ft Microsoft container
Chicago data center
Link Layer 5-85
Server racks
TOR switches
Tier-1 switches
Tier-2 switches
Load balancer
Load balancer
B
1 2 3 4 5 6 7 8
A C
Border router
Access router
Internet
Data center networks
load balancer application-layer routing receives external client requests
directs workload within data center
returns results to external client (hiding data
center internals from client)
Server racks
TOR switches
Tier-1 switches
Tier-2 switches
1 2 3 4 5 6 7 8
Data center networks
rich interconnection among switches racks
increased throughput between racks (multiple routing
paths possible)
increased reliability via redundancy
Link Layer 5-87
Link layer LANs outline
51 introduction services
52 error detection correction
53 multiple access protocols
54 LANs addressing ARP
Ethernet
switches
VLANS
55 link virtualization MPLS
56 data center networking
57 a day in the life of a web request
Link Layer 5-88
Synthesis a day in the life of a web request
journey down protocol stack complete application transport network link
putting-it-all-together synthesis goal identify review understand protocols (at all
layers) involved in seemingly simple scenario requesting www page
scenario student attaches laptop to campus network requestsreceives wwwgooglecom
Link Layer 5-89
A day in the life scenario
Comcast network
68800013
Googlersquos network
64233160019 64233169105
web server
DNS server
school network
68802024
web page
browser
router
(runs DHCP)
Link Layer 5-90
A day in the lifehellip connecting to the Internet
connecting laptop needs to get its own IP address addr of first-hop router addr of DNS server use DHCP
DHCP
UDP
IP
Eth
Phy
DHCP
DHCP
DHCP
DHCP
DHCP
DHCP
UDP
IP
Eth
Phy
DHCP
DHCP
DHCP
DHCP DHCP
DHCP request encapsulated in UDP encapsulated in IP encapsulated in 8023 Ethernet
Ethernet frame broadcast
(dest FFFFFFFFFFFF) on LAN received at router running DHCP server
Ethernet demuxed to IP demuxed UDP demuxed to DHCP
router
(runs DHCP)
Link Layer 5-91
DHCP server formulates DHCP ACK containing clientrsquos IP address IP address of first-hop router for client name amp IP address of DNS server
DHCP
UDP
IP
Eth
Phy
DHCP
DHCP
DHCP
DHCP
DHCP
UDP
IP
Eth
Phy
DHCP
DHCP
DHCP
DHCP
DHCP
encapsulation at DHCP server frame forwarded (switch learning) through LAN demultiplexing at client
Client now has IP address knows name amp addr of DNS
server IP address of its first-hop router
DHCP client receives DHCP ACK reply
A day in the lifehellip connecting to the Internet
router
(runs DHCP)
Link Layer 5-92
A day in the lifehellip ARP (before DNS before HTTP)
before sending HTTP request need IP address of wwwgooglecom DNS
DNS
UDP
IP
Eth
Phy
DNS
DNS
DNS
DNS query created encapsulated in UDP encapsulated in IP encapsulated in Eth To send frame to router need MAC address of router interface ARP
ARP query broadcast received by
router which replies with ARP reply giving MAC address of router interface
client now knows MAC address of first hop router so can now send frame containing DNS query
ARP query
Eth
Phy
ARP
ARP
ARP reply
router
(runs DHCP)
Link Layer 5-93
DNS
UDP
IP
Eth
Phy
DNS
DNS
DNS
DNS
DNS
IP datagram containing DNS query forwarded via LAN switch from client to 1st hop router
IP datagram forwarded from campus network into comcast network routed (tables created by RIP OSPF IS-IS andor BGP routing protocols) to DNS server
demuxrsquoed to DNS server
DNS server replies to client with IP address of wwwgooglecom
Comcast network
68800013
DNS server
DNS
UDP
IP
Eth
Phy
DNS
DNS
DNS
DNS
A day in the lifehellip using DNS
router
(runs DHCP)
Link Layer 5-94
A day in the lifehellipTCP connection carrying HTTP
HTTP
TCP
IP
Eth
Phy
HTTP
to send HTTP request client first opens TCP socket to web server
TCP SYN segment (step 1 in 3-way handshake) inter-domain routed to web server
TCP connection established 64233169105
web server
SYN
SYN
SYN
SYN
TCP
IP
Eth
Phy
SYN
SYN
SYN
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
web server responds with TCP SYNACK (step 2 in 3-way handshake)
router
(runs DHCP)
Link Layer 5-95
A day in the lifehellip HTTP requestreply
HTTP
TCP
IP
Eth
Phy
HTTP
HTTP request sent into TCP socket
IP datagram containing HTTP request routed to wwwgooglecom
IP datagram containing HTTP reply routed back to client
64233169105
web server
HTTP
TCP
IP
Eth
Phy web server responds with
HTTP reply (containing web page)
HTTP
HTTP
HTTP HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
web page finally () displayed
Link Layer 5-96
Chapter 5 Summary
principles behind data link layer services error detection correction
sharing a broadcast channel multiple access
link layer addressing
instantiation and implementation of various link layer technologies Ethernet
switched LANS VLANs
virtualized networks as a link layer MPLS
synthesis a day in the life of a web request
Link Layer 5-97
Chapter 5 letrsquos take a breath
journey down protocol stack complete (except PHY)
solid understanding of networking principles practice
hellip could stop here hellip but lots of interesting topics wireless
multimedia
security
network management
R
1A-23-F9-CD-06-9B 222222222220
111111111110 E6-E9-00-17-BB-4B CC-49-DE-D0-AB-7D
111111111112
111111111111
74-29-9C-E8-FF-55
A
222222222222
49-BD-D2-C7-56-2A
222222222221 88-B2-2F-54-1A-0F
B
Link Layer 5-52
Addressing routing to another LAN R forwards datagram with IP source A destination B
R creates link-layer frame with Bs MAC address as dest frame contains A-to-B IP datagram
IP src 111111111111
IP dest 222222222222
MAC src 1A-23-F9-CD-06-9B
MAC dest 49-BD-D2-C7-56-2A
IP
Eth
Phy
Link Layer 5-53
Link layer LANs outline
51 introduction services
52 error detection correction
53 multiple access protocols
54 LANs addressing ARP
Ethernet
switches
VLANS
55 link virtualization MPLS
56 data center networking
57 a day in the life of a web request
Link Layer 5-54
Ethernet
ldquodominantrdquo wired LAN technology
cheap $20 for NIC
first widely used LAN technology
simpler cheaper than token LANs and ATM
kept up with speed race 10 Mbps ndash 10 Gbps
Metcalfersquos Ethernet sketch
Link Layer 5-55
Ethernet physical topology
bus popular through mid 90s all nodes in same collision domain (can collide with each
other)
star prevails today active switch in center
each ldquospokerdquo runs a (separate) Ethernet protocol (nodes
do not collide with each other)
switch
bus coaxial cable star
Link Layer 5-56
Ethernet frame structure
sending adapter encapsulates IP datagram (or other network layer protocol packet) in Ethernet frame
preamble
7 bytes with pattern 10101010 followed by one byte with pattern 10101011
used to synchronize receiver sender clock rates
dest address
source address
data (payload) CRC preamble
type
Link Layer 5-57
Ethernet frame structure (more)
addresses 6 byte source destination MAC addresses if adapter receives frame with matching destination
address or with broadcast address (eg ARP packet) it passes data in frame to network layer protocol
otherwise adapter discards frame
type indicates higher layer protocol (mostly IP but others possible eg Novell IPX AppleTalk)
CRC cyclic redundancy check at receiver error detected frame is dropped
dest address
source address
data (payload) CRC preamble
type
Link Layer 5-58
Ethernet unreliable connectionless
connectionless no handshaking between sending and receiving NICs
unreliable receiving NIC doesnt send acks or nacks to sending NIC
data in dropped frames recovered only if initial sender uses higher layer rdt (eg TCP) otherwise dropped data lost
Ethernetrsquos MAC protocol unslotted CSMACD wth binary backoff
Link Layer 5-59
8023 Ethernet standards link amp physical layers
many different Ethernet standards
common MAC protocol and frame format
different speeds 2 Mbps 10 Mbps 100 Mbps 1Gbps 10G bps
different physical layer media fiber cable
application
transport
network
link
physical
MAC protocol
and frame format
100BASE-TX
100BASE-T4
100BASE-FX 100BASE-T2
100BASE-SX 100BASE-BX
fiber physical layer copper (twister
pair) physical layer
Link Layer 5-60
Link layer LANs outline
51 introduction services
52 error detection correction
53 multiple access protocols
54 LANs addressing ARP
Ethernet
switches
VLANS
55 link virtualization MPLS
56 data center networking
57 a day in the life of a web request
Link Layer 5-61
Ethernet switch link-layer device takes an active role
store forward Ethernet frames
examine incoming framersquos MAC address selectively forward frame to one-or-more outgoing links when frame is to be forwarded on segment uses CSMACD to access segment
transparent
hosts are unaware of presence of switches
plug-and-play self-learning
switches do not need to be configured
Link Layer 5-62
Switch multiple simultaneous transmissions
hosts have dedicated direct connection to switch
switches buffer packets
Ethernet protocol used on each incoming link but no collisions full duplex
each link is its own collision domain
switching A-to-Arsquo and B-to-Brsquo can transmit simultaneously without collisions switch with six interfaces
(123456)
A
Arsquo
B
Brsquo C
Crsquo
1 2
3 4 5
6
Link Layer 5-63
Switch forwarding table
Q how does switch know Arsquo reachable via interface 4 Brsquo reachable via interface 5
switch with six interfaces
(123456)
A
Arsquo
B
Brsquo C
Crsquo
1 2
3 4 5
6 A each switch has a switch table each entry
(MAC address of host interface to
reach host time stamp)
looks like a routing table
Q how are entries created maintained in switch table
something like a routing protocol
A
Arsquo
B
Brsquo C
Crsquo
1 2
3 4 5
6
Link Layer 5-64
Switch self-learning
switch learns which hosts can be reached through which interfaces
when frame received switch ldquolearnsrdquo location of sender incoming LAN segment
records senderlocation pair in switch table
A Arsquo
Source A
Dest Arsquo
MAC addr interface TTL
Switch table
(initially empty) A 1 60
Link Layer 5-65
Switch frame filteringforwarding
when frame received at switch 1 record incoming link MAC address of sending host
2 index switch table using MAC destination address
3 if entry found for destination then
if destination on segment from which frame arrived then drop frame
else forward frame on interface indicated by entry
else flood forward on all interfaces except arriving
interface
A
Arsquo
B
Brsquo C
Crsquo
1 2
3 4 5
6
Link Layer 5-66
Self-learning forwarding example
A Arsquo
Source A
Dest Arsquo
MAC addr interface TTL
switch table
(initially empty) A 1 60
A Arsquo A Arsquo A Arsquo A Arsquo A Arsquo
frame destination Arsquo locaton unknown flood
Arsquo A
destination A location
known
Arsquo 4 60
selectively send
on just one link
Link Layer 5-67
Interconnecting switches
switches can be connected together
Q sending from A to G - how does S1 know to forward frame destined to F via S4 and S3
A self learning (works exactly the same as in single-switch case)
A
B
S1
C D
E
F
S2
S4
S3
H
I
G
Link Layer 5-68
Self-learning multi-switch example
Suppose C sends frame to I I responds to C
Q show switch tables and packet forwarding in S1 S2 S3 S4
A
B
S1
C D
E
F
S2
S4
S3
H
I
G
Link Layer 5-69
Institutional network
to external
network
router
IP subnet
mail server
web server
Link Layer 5-70
Switches vs routers
both are store-and-forward
routers network-layer devices (examine network-layer headers)
switches link-layer devices (examine link-layer headers)
both have forwarding tables
routers compute tables using routing algorithms IP addresses
switches learn forwarding table using flooding learning MAC addresses
application
transport
network
link
physical
network
link
physical
link
physical
switch
datagram
application
transport
network
link
physical
frame
frame
frame
datagram
Link Layer 5-71
VLANs motivation
consider CS user moves office to
EE but wants connect to CS switch
single broadcast domain
all layer-2 broadcast traffic (ARP DHCP unknown location of destination MAC address) must cross entire LAN
securityprivacy efficiency issues
Computer
Science Electrical
Engineering
Computer
Engineering
Link Layer 5-72
VLANs port-based VLAN switch ports
grouped (by switch management software) so that single physical switch helliphellip
switch(es) supporting
VLAN capabilities can
be configured to
define multiple virtual
LANS over single
physical LAN
infrastructure
Virtual Local
Area Network 1
8
9
16 10 2
7
hellip
Electrical Engineering
(VLAN ports 1-8)
Computer Science
(VLAN ports 9-15)
15
hellip
Electrical Engineering
(VLAN ports 1-8)
hellip
1
8 2
7 9
16 10
15
hellip
Computer Science
(VLAN ports 9-16)
hellip operates as multiple virtual switches
Link Layer 5-73
Port-based VLAN
1
8
9
16 10 2
7
hellip
Electrical Engineering
(VLAN ports 1-8)
Computer Science
(VLAN ports 9-15)
15
hellip
traffic isolation frames tofrom ports 1-8 can only reach ports 1-8 can also define VLAN based on
MAC addresses of endpoints rather than switch port
dynamic membership ports can be dynamically assigned among VLANs
router
forwarding between VLANS done via routing (just as with separate switches) in practice vendors sell combined
switches plus routers
Link Layer 5-74
VLANS spanning multiple switches
trunk port carries frames between VLANS defined over multiple physical switches frames forwarded within VLAN between switches canrsquot be vanilla
8021 frames (must carry VLAN ID info)
8021q protocol addsremoved additional header fields for frames forwarded between trunk ports
1
8
9
10 2
7
hellip
Electrical Engineering
(VLAN ports 1-8)
Computer Science
(VLAN ports 9-15)
15
hellip
2
7 3
Ports 235 belong to EE VLAN
Ports 4678 belong to CS VLAN
5
4 6 8 16
1
Link Layer 5-75
type
2-byte Tag Protocol Identifier
(value 81-00)
Tag Control Information (12 bit VLAN ID field
3 bit priority field like IP TOS)
Recomputed CRC
8021Q VLAN frame format
8021 frame
8021Q frame
dest address
source address
data (payload) CRC preamble
dest address
source address
preamble data (payload) CRC
type
Link Layer 5-76
Link layer LANs outline
51 introduction services
52 error detection correction
53 multiple access protocols
54 LANs addressing ARP
Ethernet
switches
VLANS
55 link virtualization MPLS
56 data center networking
57 a day in the life of a web request
Link Layer 5-77
Multiprotocol label switching (MPLS)
initial goal high-speed IP forwarding using fixed length label (instead of IP address) fast lookup using fixed length identifier (rather than
shortest prefix matching)
borrowing ideas from Virtual Circuit (VC) approach
but IP datagram still keeps IP address
PPP or Ethernet
header IP header remainder of link-layer frame MPLS header
label Exp S TTL
20 3 1 5
Link Layer 5-78
MPLS capable routers
aka label-switched router
forward packets to outgoing interface based only on label value (donrsquot inspect IP address) MPLS forwarding table distinct from IP forwarding tables
flexibility MPLS forwarding decisions can differ from those of IP use destination and source addresses to route flows to
same destination differently (traffic engineering)
re-route flows quickly if link fails pre-computed backup paths (useful for VoIP)
Link Layer 5-79
R2
D
R3 R5
A
R6
MPLS versus IP paths
IP router IP routing path to destination determined
by destination address alone
R4
Link Layer 5-80
R2
D
R3 R4
R5
A
R6
MPLS versus IP paths
IP-only router
IP routing path to destination determined by destination address alone
MPLS and IP router
MPLS routing path to destination can be based on source and dest address fast reroute precompute backup routes in
case of link failure
entry router (R4) can use different MPLS
routes to A based eg on source address
Link Layer 5-81
MPLS signaling
modify OSPF IS-IS link-state flooding protocols to carry info used by MPLS routing eg link bandwidth amount of ldquoreservedrdquo link bandwidth
D
R4
R5
A
R6
entry MPLS router uses RSVP-TE signaling protocol to set up MPLS forwarding at downstream routers
modified link state flooding
RSVP-TE
Link Layer 5-82
R1 R2
D
R3 R4 R5
0
1
0 0
A
R6
in out out
label label dest interface
6 - A 0
in out out
label label dest interface
10 6 A 1
12 9 D 0
in out out
label label dest interface
10 A 0
12 D 0
1
in out out
label label dest interface
8 6 A 0
0
8 A 1
MPLS forwarding tables
Link Layer 5-83
Link layer LANs outline
51 introduction services
52 error detection correction
53 multiple access protocols
54 LANs addressing ARP
Ethernet
switches
VLANS
55 link virtualization MPLS
56 data center networking
57 a day in the life of a web request
Link Layer 5-84
Data center networks
10rsquos to 100rsquos of thousands of hosts often closely coupled in close proximity e-business (eg Amazon)
content-servers (eg YouTube Akamai Apple Microsoft)
search engines data mining (eg Google)
challenges
multiple applications each serving massive numbers of clients
managingbalancing load avoiding processing networking data bottlenecks
Inside a 40-ft Microsoft container
Chicago data center
Link Layer 5-85
Server racks
TOR switches
Tier-1 switches
Tier-2 switches
Load balancer
Load balancer
B
1 2 3 4 5 6 7 8
A C
Border router
Access router
Internet
Data center networks
load balancer application-layer routing receives external client requests
directs workload within data center
returns results to external client (hiding data
center internals from client)
Server racks
TOR switches
Tier-1 switches
Tier-2 switches
1 2 3 4 5 6 7 8
Data center networks
rich interconnection among switches racks
increased throughput between racks (multiple routing
paths possible)
increased reliability via redundancy
Link Layer 5-87
Link layer LANs outline
51 introduction services
52 error detection correction
53 multiple access protocols
54 LANs addressing ARP
Ethernet
switches
VLANS
55 link virtualization MPLS
56 data center networking
57 a day in the life of a web request
Link Layer 5-88
Synthesis a day in the life of a web request
journey down protocol stack complete application transport network link
putting-it-all-together synthesis goal identify review understand protocols (at all
layers) involved in seemingly simple scenario requesting www page
scenario student attaches laptop to campus network requestsreceives wwwgooglecom
Link Layer 5-89
A day in the life scenario
Comcast network
68800013
Googlersquos network
64233160019 64233169105
web server
DNS server
school network
68802024
web page
browser
router
(runs DHCP)
Link Layer 5-90
A day in the lifehellip connecting to the Internet
connecting laptop needs to get its own IP address addr of first-hop router addr of DNS server use DHCP
DHCP
UDP
IP
Eth
Phy
DHCP
DHCP
DHCP
DHCP
DHCP
DHCP
UDP
IP
Eth
Phy
DHCP
DHCP
DHCP
DHCP DHCP
DHCP request encapsulated in UDP encapsulated in IP encapsulated in 8023 Ethernet
Ethernet frame broadcast
(dest FFFFFFFFFFFF) on LAN received at router running DHCP server
Ethernet demuxed to IP demuxed UDP demuxed to DHCP
router
(runs DHCP)
Link Layer 5-91
DHCP server formulates DHCP ACK containing clientrsquos IP address IP address of first-hop router for client name amp IP address of DNS server
DHCP
UDP
IP
Eth
Phy
DHCP
DHCP
DHCP
DHCP
DHCP
UDP
IP
Eth
Phy
DHCP
DHCP
DHCP
DHCP
DHCP
encapsulation at DHCP server frame forwarded (switch learning) through LAN demultiplexing at client
Client now has IP address knows name amp addr of DNS
server IP address of its first-hop router
DHCP client receives DHCP ACK reply
A day in the lifehellip connecting to the Internet
router
(runs DHCP)
Link Layer 5-92
A day in the lifehellip ARP (before DNS before HTTP)
before sending HTTP request need IP address of wwwgooglecom DNS
DNS
UDP
IP
Eth
Phy
DNS
DNS
DNS
DNS query created encapsulated in UDP encapsulated in IP encapsulated in Eth To send frame to router need MAC address of router interface ARP
ARP query broadcast received by
router which replies with ARP reply giving MAC address of router interface
client now knows MAC address of first hop router so can now send frame containing DNS query
ARP query
Eth
Phy
ARP
ARP
ARP reply
router
(runs DHCP)
Link Layer 5-93
DNS
UDP
IP
Eth
Phy
DNS
DNS
DNS
DNS
DNS
IP datagram containing DNS query forwarded via LAN switch from client to 1st hop router
IP datagram forwarded from campus network into comcast network routed (tables created by RIP OSPF IS-IS andor BGP routing protocols) to DNS server
demuxrsquoed to DNS server
DNS server replies to client with IP address of wwwgooglecom
Comcast network
68800013
DNS server
DNS
UDP
IP
Eth
Phy
DNS
DNS
DNS
DNS
A day in the lifehellip using DNS
router
(runs DHCP)
Link Layer 5-94
A day in the lifehellipTCP connection carrying HTTP
HTTP
TCP
IP
Eth
Phy
HTTP
to send HTTP request client first opens TCP socket to web server
TCP SYN segment (step 1 in 3-way handshake) inter-domain routed to web server
TCP connection established 64233169105
web server
SYN
SYN
SYN
SYN
TCP
IP
Eth
Phy
SYN
SYN
SYN
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
web server responds with TCP SYNACK (step 2 in 3-way handshake)
router
(runs DHCP)
Link Layer 5-95
A day in the lifehellip HTTP requestreply
HTTP
TCP
IP
Eth
Phy
HTTP
HTTP request sent into TCP socket
IP datagram containing HTTP request routed to wwwgooglecom
IP datagram containing HTTP reply routed back to client
64233169105
web server
HTTP
TCP
IP
Eth
Phy web server responds with
HTTP reply (containing web page)
HTTP
HTTP
HTTP HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
web page finally () displayed
Link Layer 5-96
Chapter 5 Summary
principles behind data link layer services error detection correction
sharing a broadcast channel multiple access
link layer addressing
instantiation and implementation of various link layer technologies Ethernet
switched LANS VLANs
virtualized networks as a link layer MPLS
synthesis a day in the life of a web request
Link Layer 5-97
Chapter 5 letrsquos take a breath
journey down protocol stack complete (except PHY)
solid understanding of networking principles practice
hellip could stop here hellip but lots of interesting topics wireless
multimedia
security
network management
Link Layer 5-53
Link layer LANs outline
51 introduction services
52 error detection correction
53 multiple access protocols
54 LANs addressing ARP
Ethernet
switches
VLANS
55 link virtualization MPLS
56 data center networking
57 a day in the life of a web request
Link Layer 5-54
Ethernet
ldquodominantrdquo wired LAN technology
cheap $20 for NIC
first widely used LAN technology
simpler cheaper than token LANs and ATM
kept up with speed race 10 Mbps ndash 10 Gbps
Metcalfersquos Ethernet sketch
Link Layer 5-55
Ethernet physical topology
bus popular through mid 90s all nodes in same collision domain (can collide with each
other)
star prevails today active switch in center
each ldquospokerdquo runs a (separate) Ethernet protocol (nodes
do not collide with each other)
switch
bus coaxial cable star
Link Layer 5-56
Ethernet frame structure
sending adapter encapsulates IP datagram (or other network layer protocol packet) in Ethernet frame
preamble
7 bytes with pattern 10101010 followed by one byte with pattern 10101011
used to synchronize receiver sender clock rates
dest address
source address
data (payload) CRC preamble
type
Link Layer 5-57
Ethernet frame structure (more)
addresses 6 byte source destination MAC addresses if adapter receives frame with matching destination
address or with broadcast address (eg ARP packet) it passes data in frame to network layer protocol
otherwise adapter discards frame
type indicates higher layer protocol (mostly IP but others possible eg Novell IPX AppleTalk)
CRC cyclic redundancy check at receiver error detected frame is dropped
dest address
source address
data (payload) CRC preamble
type
Link Layer 5-58
Ethernet unreliable connectionless
connectionless no handshaking between sending and receiving NICs
unreliable receiving NIC doesnt send acks or nacks to sending NIC
data in dropped frames recovered only if initial sender uses higher layer rdt (eg TCP) otherwise dropped data lost
Ethernetrsquos MAC protocol unslotted CSMACD wth binary backoff
Link Layer 5-59
8023 Ethernet standards link amp physical layers
many different Ethernet standards
common MAC protocol and frame format
different speeds 2 Mbps 10 Mbps 100 Mbps 1Gbps 10G bps
different physical layer media fiber cable
application
transport
network
link
physical
MAC protocol
and frame format
100BASE-TX
100BASE-T4
100BASE-FX 100BASE-T2
100BASE-SX 100BASE-BX
fiber physical layer copper (twister
pair) physical layer
Link Layer 5-60
Link layer LANs outline
51 introduction services
52 error detection correction
53 multiple access protocols
54 LANs addressing ARP
Ethernet
switches
VLANS
55 link virtualization MPLS
56 data center networking
57 a day in the life of a web request
Link Layer 5-61
Ethernet switch link-layer device takes an active role
store forward Ethernet frames
examine incoming framersquos MAC address selectively forward frame to one-or-more outgoing links when frame is to be forwarded on segment uses CSMACD to access segment
transparent
hosts are unaware of presence of switches
plug-and-play self-learning
switches do not need to be configured
Link Layer 5-62
Switch multiple simultaneous transmissions
hosts have dedicated direct connection to switch
switches buffer packets
Ethernet protocol used on each incoming link but no collisions full duplex
each link is its own collision domain
switching A-to-Arsquo and B-to-Brsquo can transmit simultaneously without collisions switch with six interfaces
(123456)
A
Arsquo
B
Brsquo C
Crsquo
1 2
3 4 5
6
Link Layer 5-63
Switch forwarding table
Q how does switch know Arsquo reachable via interface 4 Brsquo reachable via interface 5
switch with six interfaces
(123456)
A
Arsquo
B
Brsquo C
Crsquo
1 2
3 4 5
6 A each switch has a switch table each entry
(MAC address of host interface to
reach host time stamp)
looks like a routing table
Q how are entries created maintained in switch table
something like a routing protocol
A
Arsquo
B
Brsquo C
Crsquo
1 2
3 4 5
6
Link Layer 5-64
Switch self-learning
switch learns which hosts can be reached through which interfaces
when frame received switch ldquolearnsrdquo location of sender incoming LAN segment
records senderlocation pair in switch table
A Arsquo
Source A
Dest Arsquo
MAC addr interface TTL
Switch table
(initially empty) A 1 60
Link Layer 5-65
Switch frame filteringforwarding
when frame received at switch 1 record incoming link MAC address of sending host
2 index switch table using MAC destination address
3 if entry found for destination then
if destination on segment from which frame arrived then drop frame
else forward frame on interface indicated by entry
else flood forward on all interfaces except arriving
interface
A
Arsquo
B
Brsquo C
Crsquo
1 2
3 4 5
6
Link Layer 5-66
Self-learning forwarding example
A Arsquo
Source A
Dest Arsquo
MAC addr interface TTL
switch table
(initially empty) A 1 60
A Arsquo A Arsquo A Arsquo A Arsquo A Arsquo
frame destination Arsquo locaton unknown flood
Arsquo A
destination A location
known
Arsquo 4 60
selectively send
on just one link
Link Layer 5-67
Interconnecting switches
switches can be connected together
Q sending from A to G - how does S1 know to forward frame destined to F via S4 and S3
A self learning (works exactly the same as in single-switch case)
A
B
S1
C D
E
F
S2
S4
S3
H
I
G
Link Layer 5-68
Self-learning multi-switch example
Suppose C sends frame to I I responds to C
Q show switch tables and packet forwarding in S1 S2 S3 S4
A
B
S1
C D
E
F
S2
S4
S3
H
I
G
Link Layer 5-69
Institutional network
to external
network
router
IP subnet
mail server
web server
Link Layer 5-70
Switches vs routers
both are store-and-forward
routers network-layer devices (examine network-layer headers)
switches link-layer devices (examine link-layer headers)
both have forwarding tables
routers compute tables using routing algorithms IP addresses
switches learn forwarding table using flooding learning MAC addresses
application
transport
network
link
physical
network
link
physical
link
physical
switch
datagram
application
transport
network
link
physical
frame
frame
frame
datagram
Link Layer 5-71
VLANs motivation
consider CS user moves office to
EE but wants connect to CS switch
single broadcast domain
all layer-2 broadcast traffic (ARP DHCP unknown location of destination MAC address) must cross entire LAN
securityprivacy efficiency issues
Computer
Science Electrical
Engineering
Computer
Engineering
Link Layer 5-72
VLANs port-based VLAN switch ports
grouped (by switch management software) so that single physical switch helliphellip
switch(es) supporting
VLAN capabilities can
be configured to
define multiple virtual
LANS over single
physical LAN
infrastructure
Virtual Local
Area Network 1
8
9
16 10 2
7
hellip
Electrical Engineering
(VLAN ports 1-8)
Computer Science
(VLAN ports 9-15)
15
hellip
Electrical Engineering
(VLAN ports 1-8)
hellip
1
8 2
7 9
16 10
15
hellip
Computer Science
(VLAN ports 9-16)
hellip operates as multiple virtual switches
Link Layer 5-73
Port-based VLAN
1
8
9
16 10 2
7
hellip
Electrical Engineering
(VLAN ports 1-8)
Computer Science
(VLAN ports 9-15)
15
hellip
traffic isolation frames tofrom ports 1-8 can only reach ports 1-8 can also define VLAN based on
MAC addresses of endpoints rather than switch port
dynamic membership ports can be dynamically assigned among VLANs
router
forwarding between VLANS done via routing (just as with separate switches) in practice vendors sell combined
switches plus routers
Link Layer 5-74
VLANS spanning multiple switches
trunk port carries frames between VLANS defined over multiple physical switches frames forwarded within VLAN between switches canrsquot be vanilla
8021 frames (must carry VLAN ID info)
8021q protocol addsremoved additional header fields for frames forwarded between trunk ports
1
8
9
10 2
7
hellip
Electrical Engineering
(VLAN ports 1-8)
Computer Science
(VLAN ports 9-15)
15
hellip
2
7 3
Ports 235 belong to EE VLAN
Ports 4678 belong to CS VLAN
5
4 6 8 16
1
Link Layer 5-75
type
2-byte Tag Protocol Identifier
(value 81-00)
Tag Control Information (12 bit VLAN ID field
3 bit priority field like IP TOS)
Recomputed CRC
8021Q VLAN frame format
8021 frame
8021Q frame
dest address
source address
data (payload) CRC preamble
dest address
source address
preamble data (payload) CRC
type
Link Layer 5-76
Link layer LANs outline
51 introduction services
52 error detection correction
53 multiple access protocols
54 LANs addressing ARP
Ethernet
switches
VLANS
55 link virtualization MPLS
56 data center networking
57 a day in the life of a web request
Link Layer 5-77
Multiprotocol label switching (MPLS)
initial goal high-speed IP forwarding using fixed length label (instead of IP address) fast lookup using fixed length identifier (rather than
shortest prefix matching)
borrowing ideas from Virtual Circuit (VC) approach
but IP datagram still keeps IP address
PPP or Ethernet
header IP header remainder of link-layer frame MPLS header
label Exp S TTL
20 3 1 5
Link Layer 5-78
MPLS capable routers
aka label-switched router
forward packets to outgoing interface based only on label value (donrsquot inspect IP address) MPLS forwarding table distinct from IP forwarding tables
flexibility MPLS forwarding decisions can differ from those of IP use destination and source addresses to route flows to
same destination differently (traffic engineering)
re-route flows quickly if link fails pre-computed backup paths (useful for VoIP)
Link Layer 5-79
R2
D
R3 R5
A
R6
MPLS versus IP paths
IP router IP routing path to destination determined
by destination address alone
R4
Link Layer 5-80
R2
D
R3 R4
R5
A
R6
MPLS versus IP paths
IP-only router
IP routing path to destination determined by destination address alone
MPLS and IP router
MPLS routing path to destination can be based on source and dest address fast reroute precompute backup routes in
case of link failure
entry router (R4) can use different MPLS
routes to A based eg on source address
Link Layer 5-81
MPLS signaling
modify OSPF IS-IS link-state flooding protocols to carry info used by MPLS routing eg link bandwidth amount of ldquoreservedrdquo link bandwidth
D
R4
R5
A
R6
entry MPLS router uses RSVP-TE signaling protocol to set up MPLS forwarding at downstream routers
modified link state flooding
RSVP-TE
Link Layer 5-82
R1 R2
D
R3 R4 R5
0
1
0 0
A
R6
in out out
label label dest interface
6 - A 0
in out out
label label dest interface
10 6 A 1
12 9 D 0
in out out
label label dest interface
10 A 0
12 D 0
1
in out out
label label dest interface
8 6 A 0
0
8 A 1
MPLS forwarding tables
Link Layer 5-83
Link layer LANs outline
51 introduction services
52 error detection correction
53 multiple access protocols
54 LANs addressing ARP
Ethernet
switches
VLANS
55 link virtualization MPLS
56 data center networking
57 a day in the life of a web request
Link Layer 5-84
Data center networks
10rsquos to 100rsquos of thousands of hosts often closely coupled in close proximity e-business (eg Amazon)
content-servers (eg YouTube Akamai Apple Microsoft)
search engines data mining (eg Google)
challenges
multiple applications each serving massive numbers of clients
managingbalancing load avoiding processing networking data bottlenecks
Inside a 40-ft Microsoft container
Chicago data center
Link Layer 5-85
Server racks
TOR switches
Tier-1 switches
Tier-2 switches
Load balancer
Load balancer
B
1 2 3 4 5 6 7 8
A C
Border router
Access router
Internet
Data center networks
load balancer application-layer routing receives external client requests
directs workload within data center
returns results to external client (hiding data
center internals from client)
Server racks
TOR switches
Tier-1 switches
Tier-2 switches
1 2 3 4 5 6 7 8
Data center networks
rich interconnection among switches racks
increased throughput between racks (multiple routing
paths possible)
increased reliability via redundancy
Link Layer 5-87
Link layer LANs outline
51 introduction services
52 error detection correction
53 multiple access protocols
54 LANs addressing ARP
Ethernet
switches
VLANS
55 link virtualization MPLS
56 data center networking
57 a day in the life of a web request
Link Layer 5-88
Synthesis a day in the life of a web request
journey down protocol stack complete application transport network link
putting-it-all-together synthesis goal identify review understand protocols (at all
layers) involved in seemingly simple scenario requesting www page
scenario student attaches laptop to campus network requestsreceives wwwgooglecom
Link Layer 5-89
A day in the life scenario
Comcast network
68800013
Googlersquos network
64233160019 64233169105
web server
DNS server
school network
68802024
web page
browser
router
(runs DHCP)
Link Layer 5-90
A day in the lifehellip connecting to the Internet
connecting laptop needs to get its own IP address addr of first-hop router addr of DNS server use DHCP
DHCP
UDP
IP
Eth
Phy
DHCP
DHCP
DHCP
DHCP
DHCP
DHCP
UDP
IP
Eth
Phy
DHCP
DHCP
DHCP
DHCP DHCP
DHCP request encapsulated in UDP encapsulated in IP encapsulated in 8023 Ethernet
Ethernet frame broadcast
(dest FFFFFFFFFFFF) on LAN received at router running DHCP server
Ethernet demuxed to IP demuxed UDP demuxed to DHCP
router
(runs DHCP)
Link Layer 5-91
DHCP server formulates DHCP ACK containing clientrsquos IP address IP address of first-hop router for client name amp IP address of DNS server
DHCP
UDP
IP
Eth
Phy
DHCP
DHCP
DHCP
DHCP
DHCP
UDP
IP
Eth
Phy
DHCP
DHCP
DHCP
DHCP
DHCP
encapsulation at DHCP server frame forwarded (switch learning) through LAN demultiplexing at client
Client now has IP address knows name amp addr of DNS
server IP address of its first-hop router
DHCP client receives DHCP ACK reply
A day in the lifehellip connecting to the Internet
router
(runs DHCP)
Link Layer 5-92
A day in the lifehellip ARP (before DNS before HTTP)
before sending HTTP request need IP address of wwwgooglecom DNS
DNS
UDP
IP
Eth
Phy
DNS
DNS
DNS
DNS query created encapsulated in UDP encapsulated in IP encapsulated in Eth To send frame to router need MAC address of router interface ARP
ARP query broadcast received by
router which replies with ARP reply giving MAC address of router interface
client now knows MAC address of first hop router so can now send frame containing DNS query
ARP query
Eth
Phy
ARP
ARP
ARP reply
router
(runs DHCP)
Link Layer 5-93
DNS
UDP
IP
Eth
Phy
DNS
DNS
DNS
DNS
DNS
IP datagram containing DNS query forwarded via LAN switch from client to 1st hop router
IP datagram forwarded from campus network into comcast network routed (tables created by RIP OSPF IS-IS andor BGP routing protocols) to DNS server
demuxrsquoed to DNS server
DNS server replies to client with IP address of wwwgooglecom
Comcast network
68800013
DNS server
DNS
UDP
IP
Eth
Phy
DNS
DNS
DNS
DNS
A day in the lifehellip using DNS
router
(runs DHCP)
Link Layer 5-94
A day in the lifehellipTCP connection carrying HTTP
HTTP
TCP
IP
Eth
Phy
HTTP
to send HTTP request client first opens TCP socket to web server
TCP SYN segment (step 1 in 3-way handshake) inter-domain routed to web server
TCP connection established 64233169105
web server
SYN
SYN
SYN
SYN
TCP
IP
Eth
Phy
SYN
SYN
SYN
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
web server responds with TCP SYNACK (step 2 in 3-way handshake)
router
(runs DHCP)
Link Layer 5-95
A day in the lifehellip HTTP requestreply
HTTP
TCP
IP
Eth
Phy
HTTP
HTTP request sent into TCP socket
IP datagram containing HTTP request routed to wwwgooglecom
IP datagram containing HTTP reply routed back to client
64233169105
web server
HTTP
TCP
IP
Eth
Phy web server responds with
HTTP reply (containing web page)
HTTP
HTTP
HTTP HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
web page finally () displayed
Link Layer 5-96
Chapter 5 Summary
principles behind data link layer services error detection correction
sharing a broadcast channel multiple access
link layer addressing
instantiation and implementation of various link layer technologies Ethernet
switched LANS VLANs
virtualized networks as a link layer MPLS
synthesis a day in the life of a web request
Link Layer 5-97
Chapter 5 letrsquos take a breath
journey down protocol stack complete (except PHY)
solid understanding of networking principles practice
hellip could stop here hellip but lots of interesting topics wireless
multimedia
security
network management
Link Layer 5-54
Ethernet
ldquodominantrdquo wired LAN technology
cheap $20 for NIC
first widely used LAN technology
simpler cheaper than token LANs and ATM
kept up with speed race 10 Mbps ndash 10 Gbps
Metcalfersquos Ethernet sketch
Link Layer 5-55
Ethernet physical topology
bus popular through mid 90s all nodes in same collision domain (can collide with each
other)
star prevails today active switch in center
each ldquospokerdquo runs a (separate) Ethernet protocol (nodes
do not collide with each other)
switch
bus coaxial cable star
Link Layer 5-56
Ethernet frame structure
sending adapter encapsulates IP datagram (or other network layer protocol packet) in Ethernet frame
preamble
7 bytes with pattern 10101010 followed by one byte with pattern 10101011
used to synchronize receiver sender clock rates
dest address
source address
data (payload) CRC preamble
type
Link Layer 5-57
Ethernet frame structure (more)
addresses 6 byte source destination MAC addresses if adapter receives frame with matching destination
address or with broadcast address (eg ARP packet) it passes data in frame to network layer protocol
otherwise adapter discards frame
type indicates higher layer protocol (mostly IP but others possible eg Novell IPX AppleTalk)
CRC cyclic redundancy check at receiver error detected frame is dropped
dest address
source address
data (payload) CRC preamble
type
Link Layer 5-58
Ethernet unreliable connectionless
connectionless no handshaking between sending and receiving NICs
unreliable receiving NIC doesnt send acks or nacks to sending NIC
data in dropped frames recovered only if initial sender uses higher layer rdt (eg TCP) otherwise dropped data lost
Ethernetrsquos MAC protocol unslotted CSMACD wth binary backoff
Link Layer 5-59
8023 Ethernet standards link amp physical layers
many different Ethernet standards
common MAC protocol and frame format
different speeds 2 Mbps 10 Mbps 100 Mbps 1Gbps 10G bps
different physical layer media fiber cable
application
transport
network
link
physical
MAC protocol
and frame format
100BASE-TX
100BASE-T4
100BASE-FX 100BASE-T2
100BASE-SX 100BASE-BX
fiber physical layer copper (twister
pair) physical layer
Link Layer 5-60
Link layer LANs outline
51 introduction services
52 error detection correction
53 multiple access protocols
54 LANs addressing ARP
Ethernet
switches
VLANS
55 link virtualization MPLS
56 data center networking
57 a day in the life of a web request
Link Layer 5-61
Ethernet switch link-layer device takes an active role
store forward Ethernet frames
examine incoming framersquos MAC address selectively forward frame to one-or-more outgoing links when frame is to be forwarded on segment uses CSMACD to access segment
transparent
hosts are unaware of presence of switches
plug-and-play self-learning
switches do not need to be configured
Link Layer 5-62
Switch multiple simultaneous transmissions
hosts have dedicated direct connection to switch
switches buffer packets
Ethernet protocol used on each incoming link but no collisions full duplex
each link is its own collision domain
switching A-to-Arsquo and B-to-Brsquo can transmit simultaneously without collisions switch with six interfaces
(123456)
A
Arsquo
B
Brsquo C
Crsquo
1 2
3 4 5
6
Link Layer 5-63
Switch forwarding table
Q how does switch know Arsquo reachable via interface 4 Brsquo reachable via interface 5
switch with six interfaces
(123456)
A
Arsquo
B
Brsquo C
Crsquo
1 2
3 4 5
6 A each switch has a switch table each entry
(MAC address of host interface to
reach host time stamp)
looks like a routing table
Q how are entries created maintained in switch table
something like a routing protocol
A
Arsquo
B
Brsquo C
Crsquo
1 2
3 4 5
6
Link Layer 5-64
Switch self-learning
switch learns which hosts can be reached through which interfaces
when frame received switch ldquolearnsrdquo location of sender incoming LAN segment
records senderlocation pair in switch table
A Arsquo
Source A
Dest Arsquo
MAC addr interface TTL
Switch table
(initially empty) A 1 60
Link Layer 5-65
Switch frame filteringforwarding
when frame received at switch 1 record incoming link MAC address of sending host
2 index switch table using MAC destination address
3 if entry found for destination then
if destination on segment from which frame arrived then drop frame
else forward frame on interface indicated by entry
else flood forward on all interfaces except arriving
interface
A
Arsquo
B
Brsquo C
Crsquo
1 2
3 4 5
6
Link Layer 5-66
Self-learning forwarding example
A Arsquo
Source A
Dest Arsquo
MAC addr interface TTL
switch table
(initially empty) A 1 60
A Arsquo A Arsquo A Arsquo A Arsquo A Arsquo
frame destination Arsquo locaton unknown flood
Arsquo A
destination A location
known
Arsquo 4 60
selectively send
on just one link
Link Layer 5-67
Interconnecting switches
switches can be connected together
Q sending from A to G - how does S1 know to forward frame destined to F via S4 and S3
A self learning (works exactly the same as in single-switch case)
A
B
S1
C D
E
F
S2
S4
S3
H
I
G
Link Layer 5-68
Self-learning multi-switch example
Suppose C sends frame to I I responds to C
Q show switch tables and packet forwarding in S1 S2 S3 S4
A
B
S1
C D
E
F
S2
S4
S3
H
I
G
Link Layer 5-69
Institutional network
to external
network
router
IP subnet
mail server
web server
Link Layer 5-70
Switches vs routers
both are store-and-forward
routers network-layer devices (examine network-layer headers)
switches link-layer devices (examine link-layer headers)
both have forwarding tables
routers compute tables using routing algorithms IP addresses
switches learn forwarding table using flooding learning MAC addresses
application
transport
network
link
physical
network
link
physical
link
physical
switch
datagram
application
transport
network
link
physical
frame
frame
frame
datagram
Link Layer 5-71
VLANs motivation
consider CS user moves office to
EE but wants connect to CS switch
single broadcast domain
all layer-2 broadcast traffic (ARP DHCP unknown location of destination MAC address) must cross entire LAN
securityprivacy efficiency issues
Computer
Science Electrical
Engineering
Computer
Engineering
Link Layer 5-72
VLANs port-based VLAN switch ports
grouped (by switch management software) so that single physical switch helliphellip
switch(es) supporting
VLAN capabilities can
be configured to
define multiple virtual
LANS over single
physical LAN
infrastructure
Virtual Local
Area Network 1
8
9
16 10 2
7
hellip
Electrical Engineering
(VLAN ports 1-8)
Computer Science
(VLAN ports 9-15)
15
hellip
Electrical Engineering
(VLAN ports 1-8)
hellip
1
8 2
7 9
16 10
15
hellip
Computer Science
(VLAN ports 9-16)
hellip operates as multiple virtual switches
Link Layer 5-73
Port-based VLAN
1
8
9
16 10 2
7
hellip
Electrical Engineering
(VLAN ports 1-8)
Computer Science
(VLAN ports 9-15)
15
hellip
traffic isolation frames tofrom ports 1-8 can only reach ports 1-8 can also define VLAN based on
MAC addresses of endpoints rather than switch port
dynamic membership ports can be dynamically assigned among VLANs
router
forwarding between VLANS done via routing (just as with separate switches) in practice vendors sell combined
switches plus routers
Link Layer 5-74
VLANS spanning multiple switches
trunk port carries frames between VLANS defined over multiple physical switches frames forwarded within VLAN between switches canrsquot be vanilla
8021 frames (must carry VLAN ID info)
8021q protocol addsremoved additional header fields for frames forwarded between trunk ports
1
8
9
10 2
7
hellip
Electrical Engineering
(VLAN ports 1-8)
Computer Science
(VLAN ports 9-15)
15
hellip
2
7 3
Ports 235 belong to EE VLAN
Ports 4678 belong to CS VLAN
5
4 6 8 16
1
Link Layer 5-75
type
2-byte Tag Protocol Identifier
(value 81-00)
Tag Control Information (12 bit VLAN ID field
3 bit priority field like IP TOS)
Recomputed CRC
8021Q VLAN frame format
8021 frame
8021Q frame
dest address
source address
data (payload) CRC preamble
dest address
source address
preamble data (payload) CRC
type
Link Layer 5-76
Link layer LANs outline
51 introduction services
52 error detection correction
53 multiple access protocols
54 LANs addressing ARP
Ethernet
switches
VLANS
55 link virtualization MPLS
56 data center networking
57 a day in the life of a web request
Link Layer 5-77
Multiprotocol label switching (MPLS)
initial goal high-speed IP forwarding using fixed length label (instead of IP address) fast lookup using fixed length identifier (rather than
shortest prefix matching)
borrowing ideas from Virtual Circuit (VC) approach
but IP datagram still keeps IP address
PPP or Ethernet
header IP header remainder of link-layer frame MPLS header
label Exp S TTL
20 3 1 5
Link Layer 5-78
MPLS capable routers
aka label-switched router
forward packets to outgoing interface based only on label value (donrsquot inspect IP address) MPLS forwarding table distinct from IP forwarding tables
flexibility MPLS forwarding decisions can differ from those of IP use destination and source addresses to route flows to
same destination differently (traffic engineering)
re-route flows quickly if link fails pre-computed backup paths (useful for VoIP)
Link Layer 5-79
R2
D
R3 R5
A
R6
MPLS versus IP paths
IP router IP routing path to destination determined
by destination address alone
R4
Link Layer 5-80
R2
D
R3 R4
R5
A
R6
MPLS versus IP paths
IP-only router
IP routing path to destination determined by destination address alone
MPLS and IP router
MPLS routing path to destination can be based on source and dest address fast reroute precompute backup routes in
case of link failure
entry router (R4) can use different MPLS
routes to A based eg on source address
Link Layer 5-81
MPLS signaling
modify OSPF IS-IS link-state flooding protocols to carry info used by MPLS routing eg link bandwidth amount of ldquoreservedrdquo link bandwidth
D
R4
R5
A
R6
entry MPLS router uses RSVP-TE signaling protocol to set up MPLS forwarding at downstream routers
modified link state flooding
RSVP-TE
Link Layer 5-82
R1 R2
D
R3 R4 R5
0
1
0 0
A
R6
in out out
label label dest interface
6 - A 0
in out out
label label dest interface
10 6 A 1
12 9 D 0
in out out
label label dest interface
10 A 0
12 D 0
1
in out out
label label dest interface
8 6 A 0
0
8 A 1
MPLS forwarding tables
Link Layer 5-83
Link layer LANs outline
51 introduction services
52 error detection correction
53 multiple access protocols
54 LANs addressing ARP
Ethernet
switches
VLANS
55 link virtualization MPLS
56 data center networking
57 a day in the life of a web request
Link Layer 5-84
Data center networks
10rsquos to 100rsquos of thousands of hosts often closely coupled in close proximity e-business (eg Amazon)
content-servers (eg YouTube Akamai Apple Microsoft)
search engines data mining (eg Google)
challenges
multiple applications each serving massive numbers of clients
managingbalancing load avoiding processing networking data bottlenecks
Inside a 40-ft Microsoft container
Chicago data center
Link Layer 5-85
Server racks
TOR switches
Tier-1 switches
Tier-2 switches
Load balancer
Load balancer
B
1 2 3 4 5 6 7 8
A C
Border router
Access router
Internet
Data center networks
load balancer application-layer routing receives external client requests
directs workload within data center
returns results to external client (hiding data
center internals from client)
Server racks
TOR switches
Tier-1 switches
Tier-2 switches
1 2 3 4 5 6 7 8
Data center networks
rich interconnection among switches racks
increased throughput between racks (multiple routing
paths possible)
increased reliability via redundancy
Link Layer 5-87
Link layer LANs outline
51 introduction services
52 error detection correction
53 multiple access protocols
54 LANs addressing ARP
Ethernet
switches
VLANS
55 link virtualization MPLS
56 data center networking
57 a day in the life of a web request
Link Layer 5-88
Synthesis a day in the life of a web request
journey down protocol stack complete application transport network link
putting-it-all-together synthesis goal identify review understand protocols (at all
layers) involved in seemingly simple scenario requesting www page
scenario student attaches laptop to campus network requestsreceives wwwgooglecom
Link Layer 5-89
A day in the life scenario
Comcast network
68800013
Googlersquos network
64233160019 64233169105
web server
DNS server
school network
68802024
web page
browser
router
(runs DHCP)
Link Layer 5-90
A day in the lifehellip connecting to the Internet
connecting laptop needs to get its own IP address addr of first-hop router addr of DNS server use DHCP
DHCP
UDP
IP
Eth
Phy
DHCP
DHCP
DHCP
DHCP
DHCP
DHCP
UDP
IP
Eth
Phy
DHCP
DHCP
DHCP
DHCP DHCP
DHCP request encapsulated in UDP encapsulated in IP encapsulated in 8023 Ethernet
Ethernet frame broadcast
(dest FFFFFFFFFFFF) on LAN received at router running DHCP server
Ethernet demuxed to IP demuxed UDP demuxed to DHCP
router
(runs DHCP)
Link Layer 5-91
DHCP server formulates DHCP ACK containing clientrsquos IP address IP address of first-hop router for client name amp IP address of DNS server
DHCP
UDP
IP
Eth
Phy
DHCP
DHCP
DHCP
DHCP
DHCP
UDP
IP
Eth
Phy
DHCP
DHCP
DHCP
DHCP
DHCP
encapsulation at DHCP server frame forwarded (switch learning) through LAN demultiplexing at client
Client now has IP address knows name amp addr of DNS
server IP address of its first-hop router
DHCP client receives DHCP ACK reply
A day in the lifehellip connecting to the Internet
router
(runs DHCP)
Link Layer 5-92
A day in the lifehellip ARP (before DNS before HTTP)
before sending HTTP request need IP address of wwwgooglecom DNS
DNS
UDP
IP
Eth
Phy
DNS
DNS
DNS
DNS query created encapsulated in UDP encapsulated in IP encapsulated in Eth To send frame to router need MAC address of router interface ARP
ARP query broadcast received by
router which replies with ARP reply giving MAC address of router interface
client now knows MAC address of first hop router so can now send frame containing DNS query
ARP query
Eth
Phy
ARP
ARP
ARP reply
router
(runs DHCP)
Link Layer 5-93
DNS
UDP
IP
Eth
Phy
DNS
DNS
DNS
DNS
DNS
IP datagram containing DNS query forwarded via LAN switch from client to 1st hop router
IP datagram forwarded from campus network into comcast network routed (tables created by RIP OSPF IS-IS andor BGP routing protocols) to DNS server
demuxrsquoed to DNS server
DNS server replies to client with IP address of wwwgooglecom
Comcast network
68800013
DNS server
DNS
UDP
IP
Eth
Phy
DNS
DNS
DNS
DNS
A day in the lifehellip using DNS
router
(runs DHCP)
Link Layer 5-94
A day in the lifehellipTCP connection carrying HTTP
HTTP
TCP
IP
Eth
Phy
HTTP
to send HTTP request client first opens TCP socket to web server
TCP SYN segment (step 1 in 3-way handshake) inter-domain routed to web server
TCP connection established 64233169105
web server
SYN
SYN
SYN
SYN
TCP
IP
Eth
Phy
SYN
SYN
SYN
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
web server responds with TCP SYNACK (step 2 in 3-way handshake)
router
(runs DHCP)
Link Layer 5-95
A day in the lifehellip HTTP requestreply
HTTP
TCP
IP
Eth
Phy
HTTP
HTTP request sent into TCP socket
IP datagram containing HTTP request routed to wwwgooglecom
IP datagram containing HTTP reply routed back to client
64233169105
web server
HTTP
TCP
IP
Eth
Phy web server responds with
HTTP reply (containing web page)
HTTP
HTTP
HTTP HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
web page finally () displayed
Link Layer 5-96
Chapter 5 Summary
principles behind data link layer services error detection correction
sharing a broadcast channel multiple access
link layer addressing
instantiation and implementation of various link layer technologies Ethernet
switched LANS VLANs
virtualized networks as a link layer MPLS
synthesis a day in the life of a web request
Link Layer 5-97
Chapter 5 letrsquos take a breath
journey down protocol stack complete (except PHY)
solid understanding of networking principles practice
hellip could stop here hellip but lots of interesting topics wireless
multimedia
security
network management
Link Layer 5-55
Ethernet physical topology
bus popular through mid 90s all nodes in same collision domain (can collide with each
other)
star prevails today active switch in center
each ldquospokerdquo runs a (separate) Ethernet protocol (nodes
do not collide with each other)
switch
bus coaxial cable star
Link Layer 5-56
Ethernet frame structure
sending adapter encapsulates IP datagram (or other network layer protocol packet) in Ethernet frame
preamble
7 bytes with pattern 10101010 followed by one byte with pattern 10101011
used to synchronize receiver sender clock rates
dest address
source address
data (payload) CRC preamble
type
Link Layer 5-57
Ethernet frame structure (more)
addresses 6 byte source destination MAC addresses if adapter receives frame with matching destination
address or with broadcast address (eg ARP packet) it passes data in frame to network layer protocol
otherwise adapter discards frame
type indicates higher layer protocol (mostly IP but others possible eg Novell IPX AppleTalk)
CRC cyclic redundancy check at receiver error detected frame is dropped
dest address
source address
data (payload) CRC preamble
type
Link Layer 5-58
Ethernet unreliable connectionless
connectionless no handshaking between sending and receiving NICs
unreliable receiving NIC doesnt send acks or nacks to sending NIC
data in dropped frames recovered only if initial sender uses higher layer rdt (eg TCP) otherwise dropped data lost
Ethernetrsquos MAC protocol unslotted CSMACD wth binary backoff
Link Layer 5-59
8023 Ethernet standards link amp physical layers
many different Ethernet standards
common MAC protocol and frame format
different speeds 2 Mbps 10 Mbps 100 Mbps 1Gbps 10G bps
different physical layer media fiber cable
application
transport
network
link
physical
MAC protocol
and frame format
100BASE-TX
100BASE-T4
100BASE-FX 100BASE-T2
100BASE-SX 100BASE-BX
fiber physical layer copper (twister
pair) physical layer
Link Layer 5-60
Link layer LANs outline
51 introduction services
52 error detection correction
53 multiple access protocols
54 LANs addressing ARP
Ethernet
switches
VLANS
55 link virtualization MPLS
56 data center networking
57 a day in the life of a web request
Link Layer 5-61
Ethernet switch link-layer device takes an active role
store forward Ethernet frames
examine incoming framersquos MAC address selectively forward frame to one-or-more outgoing links when frame is to be forwarded on segment uses CSMACD to access segment
transparent
hosts are unaware of presence of switches
plug-and-play self-learning
switches do not need to be configured
Link Layer 5-62
Switch multiple simultaneous transmissions
hosts have dedicated direct connection to switch
switches buffer packets
Ethernet protocol used on each incoming link but no collisions full duplex
each link is its own collision domain
switching A-to-Arsquo and B-to-Brsquo can transmit simultaneously without collisions switch with six interfaces
(123456)
A
Arsquo
B
Brsquo C
Crsquo
1 2
3 4 5
6
Link Layer 5-63
Switch forwarding table
Q how does switch know Arsquo reachable via interface 4 Brsquo reachable via interface 5
switch with six interfaces
(123456)
A
Arsquo
B
Brsquo C
Crsquo
1 2
3 4 5
6 A each switch has a switch table each entry
(MAC address of host interface to
reach host time stamp)
looks like a routing table
Q how are entries created maintained in switch table
something like a routing protocol
A
Arsquo
B
Brsquo C
Crsquo
1 2
3 4 5
6
Link Layer 5-64
Switch self-learning
switch learns which hosts can be reached through which interfaces
when frame received switch ldquolearnsrdquo location of sender incoming LAN segment
records senderlocation pair in switch table
A Arsquo
Source A
Dest Arsquo
MAC addr interface TTL
Switch table
(initially empty) A 1 60
Link Layer 5-65
Switch frame filteringforwarding
when frame received at switch 1 record incoming link MAC address of sending host
2 index switch table using MAC destination address
3 if entry found for destination then
if destination on segment from which frame arrived then drop frame
else forward frame on interface indicated by entry
else flood forward on all interfaces except arriving
interface
A
Arsquo
B
Brsquo C
Crsquo
1 2
3 4 5
6
Link Layer 5-66
Self-learning forwarding example
A Arsquo
Source A
Dest Arsquo
MAC addr interface TTL
switch table
(initially empty) A 1 60
A Arsquo A Arsquo A Arsquo A Arsquo A Arsquo
frame destination Arsquo locaton unknown flood
Arsquo A
destination A location
known
Arsquo 4 60
selectively send
on just one link
Link Layer 5-67
Interconnecting switches
switches can be connected together
Q sending from A to G - how does S1 know to forward frame destined to F via S4 and S3
A self learning (works exactly the same as in single-switch case)
A
B
S1
C D
E
F
S2
S4
S3
H
I
G
Link Layer 5-68
Self-learning multi-switch example
Suppose C sends frame to I I responds to C
Q show switch tables and packet forwarding in S1 S2 S3 S4
A
B
S1
C D
E
F
S2
S4
S3
H
I
G
Link Layer 5-69
Institutional network
to external
network
router
IP subnet
mail server
web server
Link Layer 5-70
Switches vs routers
both are store-and-forward
routers network-layer devices (examine network-layer headers)
switches link-layer devices (examine link-layer headers)
both have forwarding tables
routers compute tables using routing algorithms IP addresses
switches learn forwarding table using flooding learning MAC addresses
application
transport
network
link
physical
network
link
physical
link
physical
switch
datagram
application
transport
network
link
physical
frame
frame
frame
datagram
Link Layer 5-71
VLANs motivation
consider CS user moves office to
EE but wants connect to CS switch
single broadcast domain
all layer-2 broadcast traffic (ARP DHCP unknown location of destination MAC address) must cross entire LAN
securityprivacy efficiency issues
Computer
Science Electrical
Engineering
Computer
Engineering
Link Layer 5-72
VLANs port-based VLAN switch ports
grouped (by switch management software) so that single physical switch helliphellip
switch(es) supporting
VLAN capabilities can
be configured to
define multiple virtual
LANS over single
physical LAN
infrastructure
Virtual Local
Area Network 1
8
9
16 10 2
7
hellip
Electrical Engineering
(VLAN ports 1-8)
Computer Science
(VLAN ports 9-15)
15
hellip
Electrical Engineering
(VLAN ports 1-8)
hellip
1
8 2
7 9
16 10
15
hellip
Computer Science
(VLAN ports 9-16)
hellip operates as multiple virtual switches
Link Layer 5-73
Port-based VLAN
1
8
9
16 10 2
7
hellip
Electrical Engineering
(VLAN ports 1-8)
Computer Science
(VLAN ports 9-15)
15
hellip
traffic isolation frames tofrom ports 1-8 can only reach ports 1-8 can also define VLAN based on
MAC addresses of endpoints rather than switch port
dynamic membership ports can be dynamically assigned among VLANs
router
forwarding between VLANS done via routing (just as with separate switches) in practice vendors sell combined
switches plus routers
Link Layer 5-74
VLANS spanning multiple switches
trunk port carries frames between VLANS defined over multiple physical switches frames forwarded within VLAN between switches canrsquot be vanilla
8021 frames (must carry VLAN ID info)
8021q protocol addsremoved additional header fields for frames forwarded between trunk ports
1
8
9
10 2
7
hellip
Electrical Engineering
(VLAN ports 1-8)
Computer Science
(VLAN ports 9-15)
15
hellip
2
7 3
Ports 235 belong to EE VLAN
Ports 4678 belong to CS VLAN
5
4 6 8 16
1
Link Layer 5-75
type
2-byte Tag Protocol Identifier
(value 81-00)
Tag Control Information (12 bit VLAN ID field
3 bit priority field like IP TOS)
Recomputed CRC
8021Q VLAN frame format
8021 frame
8021Q frame
dest address
source address
data (payload) CRC preamble
dest address
source address
preamble data (payload) CRC
type
Link Layer 5-76
Link layer LANs outline
51 introduction services
52 error detection correction
53 multiple access protocols
54 LANs addressing ARP
Ethernet
switches
VLANS
55 link virtualization MPLS
56 data center networking
57 a day in the life of a web request
Link Layer 5-77
Multiprotocol label switching (MPLS)
initial goal high-speed IP forwarding using fixed length label (instead of IP address) fast lookup using fixed length identifier (rather than
shortest prefix matching)
borrowing ideas from Virtual Circuit (VC) approach
but IP datagram still keeps IP address
PPP or Ethernet
header IP header remainder of link-layer frame MPLS header
label Exp S TTL
20 3 1 5
Link Layer 5-78
MPLS capable routers
aka label-switched router
forward packets to outgoing interface based only on label value (donrsquot inspect IP address) MPLS forwarding table distinct from IP forwarding tables
flexibility MPLS forwarding decisions can differ from those of IP use destination and source addresses to route flows to
same destination differently (traffic engineering)
re-route flows quickly if link fails pre-computed backup paths (useful for VoIP)
Link Layer 5-79
R2
D
R3 R5
A
R6
MPLS versus IP paths
IP router IP routing path to destination determined
by destination address alone
R4
Link Layer 5-80
R2
D
R3 R4
R5
A
R6
MPLS versus IP paths
IP-only router
IP routing path to destination determined by destination address alone
MPLS and IP router
MPLS routing path to destination can be based on source and dest address fast reroute precompute backup routes in
case of link failure
entry router (R4) can use different MPLS
routes to A based eg on source address
Link Layer 5-81
MPLS signaling
modify OSPF IS-IS link-state flooding protocols to carry info used by MPLS routing eg link bandwidth amount of ldquoreservedrdquo link bandwidth
D
R4
R5
A
R6
entry MPLS router uses RSVP-TE signaling protocol to set up MPLS forwarding at downstream routers
modified link state flooding
RSVP-TE
Link Layer 5-82
R1 R2
D
R3 R4 R5
0
1
0 0
A
R6
in out out
label label dest interface
6 - A 0
in out out
label label dest interface
10 6 A 1
12 9 D 0
in out out
label label dest interface
10 A 0
12 D 0
1
in out out
label label dest interface
8 6 A 0
0
8 A 1
MPLS forwarding tables
Link Layer 5-83
Link layer LANs outline
51 introduction services
52 error detection correction
53 multiple access protocols
54 LANs addressing ARP
Ethernet
switches
VLANS
55 link virtualization MPLS
56 data center networking
57 a day in the life of a web request
Link Layer 5-84
Data center networks
10rsquos to 100rsquos of thousands of hosts often closely coupled in close proximity e-business (eg Amazon)
content-servers (eg YouTube Akamai Apple Microsoft)
search engines data mining (eg Google)
challenges
multiple applications each serving massive numbers of clients
managingbalancing load avoiding processing networking data bottlenecks
Inside a 40-ft Microsoft container
Chicago data center
Link Layer 5-85
Server racks
TOR switches
Tier-1 switches
Tier-2 switches
Load balancer
Load balancer
B
1 2 3 4 5 6 7 8
A C
Border router
Access router
Internet
Data center networks
load balancer application-layer routing receives external client requests
directs workload within data center
returns results to external client (hiding data
center internals from client)
Server racks
TOR switches
Tier-1 switches
Tier-2 switches
1 2 3 4 5 6 7 8
Data center networks
rich interconnection among switches racks
increased throughput between racks (multiple routing
paths possible)
increased reliability via redundancy
Link Layer 5-87
Link layer LANs outline
51 introduction services
52 error detection correction
53 multiple access protocols
54 LANs addressing ARP
Ethernet
switches
VLANS
55 link virtualization MPLS
56 data center networking
57 a day in the life of a web request
Link Layer 5-88
Synthesis a day in the life of a web request
journey down protocol stack complete application transport network link
putting-it-all-together synthesis goal identify review understand protocols (at all
layers) involved in seemingly simple scenario requesting www page
scenario student attaches laptop to campus network requestsreceives wwwgooglecom
Link Layer 5-89
A day in the life scenario
Comcast network
68800013
Googlersquos network
64233160019 64233169105
web server
DNS server
school network
68802024
web page
browser
router
(runs DHCP)
Link Layer 5-90
A day in the lifehellip connecting to the Internet
connecting laptop needs to get its own IP address addr of first-hop router addr of DNS server use DHCP
DHCP
UDP
IP
Eth
Phy
DHCP
DHCP
DHCP
DHCP
DHCP
DHCP
UDP
IP
Eth
Phy
DHCP
DHCP
DHCP
DHCP DHCP
DHCP request encapsulated in UDP encapsulated in IP encapsulated in 8023 Ethernet
Ethernet frame broadcast
(dest FFFFFFFFFFFF) on LAN received at router running DHCP server
Ethernet demuxed to IP demuxed UDP demuxed to DHCP
router
(runs DHCP)
Link Layer 5-91
DHCP server formulates DHCP ACK containing clientrsquos IP address IP address of first-hop router for client name amp IP address of DNS server
DHCP
UDP
IP
Eth
Phy
DHCP
DHCP
DHCP
DHCP
DHCP
UDP
IP
Eth
Phy
DHCP
DHCP
DHCP
DHCP
DHCP
encapsulation at DHCP server frame forwarded (switch learning) through LAN demultiplexing at client
Client now has IP address knows name amp addr of DNS
server IP address of its first-hop router
DHCP client receives DHCP ACK reply
A day in the lifehellip connecting to the Internet
router
(runs DHCP)
Link Layer 5-92
A day in the lifehellip ARP (before DNS before HTTP)
before sending HTTP request need IP address of wwwgooglecom DNS
DNS
UDP
IP
Eth
Phy
DNS
DNS
DNS
DNS query created encapsulated in UDP encapsulated in IP encapsulated in Eth To send frame to router need MAC address of router interface ARP
ARP query broadcast received by
router which replies with ARP reply giving MAC address of router interface
client now knows MAC address of first hop router so can now send frame containing DNS query
ARP query
Eth
Phy
ARP
ARP
ARP reply
router
(runs DHCP)
Link Layer 5-93
DNS
UDP
IP
Eth
Phy
DNS
DNS
DNS
DNS
DNS
IP datagram containing DNS query forwarded via LAN switch from client to 1st hop router
IP datagram forwarded from campus network into comcast network routed (tables created by RIP OSPF IS-IS andor BGP routing protocols) to DNS server
demuxrsquoed to DNS server
DNS server replies to client with IP address of wwwgooglecom
Comcast network
68800013
DNS server
DNS
UDP
IP
Eth
Phy
DNS
DNS
DNS
DNS
A day in the lifehellip using DNS
router
(runs DHCP)
Link Layer 5-94
A day in the lifehellipTCP connection carrying HTTP
HTTP
TCP
IP
Eth
Phy
HTTP
to send HTTP request client first opens TCP socket to web server
TCP SYN segment (step 1 in 3-way handshake) inter-domain routed to web server
TCP connection established 64233169105
web server
SYN
SYN
SYN
SYN
TCP
IP
Eth
Phy
SYN
SYN
SYN
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
web server responds with TCP SYNACK (step 2 in 3-way handshake)
router
(runs DHCP)
Link Layer 5-95
A day in the lifehellip HTTP requestreply
HTTP
TCP
IP
Eth
Phy
HTTP
HTTP request sent into TCP socket
IP datagram containing HTTP request routed to wwwgooglecom
IP datagram containing HTTP reply routed back to client
64233169105
web server
HTTP
TCP
IP
Eth
Phy web server responds with
HTTP reply (containing web page)
HTTP
HTTP
HTTP HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
web page finally () displayed
Link Layer 5-96
Chapter 5 Summary
principles behind data link layer services error detection correction
sharing a broadcast channel multiple access
link layer addressing
instantiation and implementation of various link layer technologies Ethernet
switched LANS VLANs
virtualized networks as a link layer MPLS
synthesis a day in the life of a web request
Link Layer 5-97
Chapter 5 letrsquos take a breath
journey down protocol stack complete (except PHY)
solid understanding of networking principles practice
hellip could stop here hellip but lots of interesting topics wireless
multimedia
security
network management
Link Layer 5-56
Ethernet frame structure
sending adapter encapsulates IP datagram (or other network layer protocol packet) in Ethernet frame
preamble
7 bytes with pattern 10101010 followed by one byte with pattern 10101011
used to synchronize receiver sender clock rates
dest address
source address
data (payload) CRC preamble
type
Link Layer 5-57
Ethernet frame structure (more)
addresses 6 byte source destination MAC addresses if adapter receives frame with matching destination
address or with broadcast address (eg ARP packet) it passes data in frame to network layer protocol
otherwise adapter discards frame
type indicates higher layer protocol (mostly IP but others possible eg Novell IPX AppleTalk)
CRC cyclic redundancy check at receiver error detected frame is dropped
dest address
source address
data (payload) CRC preamble
type
Link Layer 5-58
Ethernet unreliable connectionless
connectionless no handshaking between sending and receiving NICs
unreliable receiving NIC doesnt send acks or nacks to sending NIC
data in dropped frames recovered only if initial sender uses higher layer rdt (eg TCP) otherwise dropped data lost
Ethernetrsquos MAC protocol unslotted CSMACD wth binary backoff
Link Layer 5-59
8023 Ethernet standards link amp physical layers
many different Ethernet standards
common MAC protocol and frame format
different speeds 2 Mbps 10 Mbps 100 Mbps 1Gbps 10G bps
different physical layer media fiber cable
application
transport
network
link
physical
MAC protocol
and frame format
100BASE-TX
100BASE-T4
100BASE-FX 100BASE-T2
100BASE-SX 100BASE-BX
fiber physical layer copper (twister
pair) physical layer
Link Layer 5-60
Link layer LANs outline
51 introduction services
52 error detection correction
53 multiple access protocols
54 LANs addressing ARP
Ethernet
switches
VLANS
55 link virtualization MPLS
56 data center networking
57 a day in the life of a web request
Link Layer 5-61
Ethernet switch link-layer device takes an active role
store forward Ethernet frames
examine incoming framersquos MAC address selectively forward frame to one-or-more outgoing links when frame is to be forwarded on segment uses CSMACD to access segment
transparent
hosts are unaware of presence of switches
plug-and-play self-learning
switches do not need to be configured
Link Layer 5-62
Switch multiple simultaneous transmissions
hosts have dedicated direct connection to switch
switches buffer packets
Ethernet protocol used on each incoming link but no collisions full duplex
each link is its own collision domain
switching A-to-Arsquo and B-to-Brsquo can transmit simultaneously without collisions switch with six interfaces
(123456)
A
Arsquo
B
Brsquo C
Crsquo
1 2
3 4 5
6
Link Layer 5-63
Switch forwarding table
Q how does switch know Arsquo reachable via interface 4 Brsquo reachable via interface 5
switch with six interfaces
(123456)
A
Arsquo
B
Brsquo C
Crsquo
1 2
3 4 5
6 A each switch has a switch table each entry
(MAC address of host interface to
reach host time stamp)
looks like a routing table
Q how are entries created maintained in switch table
something like a routing protocol
A
Arsquo
B
Brsquo C
Crsquo
1 2
3 4 5
6
Link Layer 5-64
Switch self-learning
switch learns which hosts can be reached through which interfaces
when frame received switch ldquolearnsrdquo location of sender incoming LAN segment
records senderlocation pair in switch table
A Arsquo
Source A
Dest Arsquo
MAC addr interface TTL
Switch table
(initially empty) A 1 60
Link Layer 5-65
Switch frame filteringforwarding
when frame received at switch 1 record incoming link MAC address of sending host
2 index switch table using MAC destination address
3 if entry found for destination then
if destination on segment from which frame arrived then drop frame
else forward frame on interface indicated by entry
else flood forward on all interfaces except arriving
interface
A
Arsquo
B
Brsquo C
Crsquo
1 2
3 4 5
6
Link Layer 5-66
Self-learning forwarding example
A Arsquo
Source A
Dest Arsquo
MAC addr interface TTL
switch table
(initially empty) A 1 60
A Arsquo A Arsquo A Arsquo A Arsquo A Arsquo
frame destination Arsquo locaton unknown flood
Arsquo A
destination A location
known
Arsquo 4 60
selectively send
on just one link
Link Layer 5-67
Interconnecting switches
switches can be connected together
Q sending from A to G - how does S1 know to forward frame destined to F via S4 and S3
A self learning (works exactly the same as in single-switch case)
A
B
S1
C D
E
F
S2
S4
S3
H
I
G
Link Layer 5-68
Self-learning multi-switch example
Suppose C sends frame to I I responds to C
Q show switch tables and packet forwarding in S1 S2 S3 S4
A
B
S1
C D
E
F
S2
S4
S3
H
I
G
Link Layer 5-69
Institutional network
to external
network
router
IP subnet
mail server
web server
Link Layer 5-70
Switches vs routers
both are store-and-forward
routers network-layer devices (examine network-layer headers)
switches link-layer devices (examine link-layer headers)
both have forwarding tables
routers compute tables using routing algorithms IP addresses
switches learn forwarding table using flooding learning MAC addresses
application
transport
network
link
physical
network
link
physical
link
physical
switch
datagram
application
transport
network
link
physical
frame
frame
frame
datagram
Link Layer 5-71
VLANs motivation
consider CS user moves office to
EE but wants connect to CS switch
single broadcast domain
all layer-2 broadcast traffic (ARP DHCP unknown location of destination MAC address) must cross entire LAN
securityprivacy efficiency issues
Computer
Science Electrical
Engineering
Computer
Engineering
Link Layer 5-72
VLANs port-based VLAN switch ports
grouped (by switch management software) so that single physical switch helliphellip
switch(es) supporting
VLAN capabilities can
be configured to
define multiple virtual
LANS over single
physical LAN
infrastructure
Virtual Local
Area Network 1
8
9
16 10 2
7
hellip
Electrical Engineering
(VLAN ports 1-8)
Computer Science
(VLAN ports 9-15)
15
hellip
Electrical Engineering
(VLAN ports 1-8)
hellip
1
8 2
7 9
16 10
15
hellip
Computer Science
(VLAN ports 9-16)
hellip operates as multiple virtual switches
Link Layer 5-73
Port-based VLAN
1
8
9
16 10 2
7
hellip
Electrical Engineering
(VLAN ports 1-8)
Computer Science
(VLAN ports 9-15)
15
hellip
traffic isolation frames tofrom ports 1-8 can only reach ports 1-8 can also define VLAN based on
MAC addresses of endpoints rather than switch port
dynamic membership ports can be dynamically assigned among VLANs
router
forwarding between VLANS done via routing (just as with separate switches) in practice vendors sell combined
switches plus routers
Link Layer 5-74
VLANS spanning multiple switches
trunk port carries frames between VLANS defined over multiple physical switches frames forwarded within VLAN between switches canrsquot be vanilla
8021 frames (must carry VLAN ID info)
8021q protocol addsremoved additional header fields for frames forwarded between trunk ports
1
8
9
10 2
7
hellip
Electrical Engineering
(VLAN ports 1-8)
Computer Science
(VLAN ports 9-15)
15
hellip
2
7 3
Ports 235 belong to EE VLAN
Ports 4678 belong to CS VLAN
5
4 6 8 16
1
Link Layer 5-75
type
2-byte Tag Protocol Identifier
(value 81-00)
Tag Control Information (12 bit VLAN ID field
3 bit priority field like IP TOS)
Recomputed CRC
8021Q VLAN frame format
8021 frame
8021Q frame
dest address
source address
data (payload) CRC preamble
dest address
source address
preamble data (payload) CRC
type
Link Layer 5-76
Link layer LANs outline
51 introduction services
52 error detection correction
53 multiple access protocols
54 LANs addressing ARP
Ethernet
switches
VLANS
55 link virtualization MPLS
56 data center networking
57 a day in the life of a web request
Link Layer 5-77
Multiprotocol label switching (MPLS)
initial goal high-speed IP forwarding using fixed length label (instead of IP address) fast lookup using fixed length identifier (rather than
shortest prefix matching)
borrowing ideas from Virtual Circuit (VC) approach
but IP datagram still keeps IP address
PPP or Ethernet
header IP header remainder of link-layer frame MPLS header
label Exp S TTL
20 3 1 5
Link Layer 5-78
MPLS capable routers
aka label-switched router
forward packets to outgoing interface based only on label value (donrsquot inspect IP address) MPLS forwarding table distinct from IP forwarding tables
flexibility MPLS forwarding decisions can differ from those of IP use destination and source addresses to route flows to
same destination differently (traffic engineering)
re-route flows quickly if link fails pre-computed backup paths (useful for VoIP)
Link Layer 5-79
R2
D
R3 R5
A
R6
MPLS versus IP paths
IP router IP routing path to destination determined
by destination address alone
R4
Link Layer 5-80
R2
D
R3 R4
R5
A
R6
MPLS versus IP paths
IP-only router
IP routing path to destination determined by destination address alone
MPLS and IP router
MPLS routing path to destination can be based on source and dest address fast reroute precompute backup routes in
case of link failure
entry router (R4) can use different MPLS
routes to A based eg on source address
Link Layer 5-81
MPLS signaling
modify OSPF IS-IS link-state flooding protocols to carry info used by MPLS routing eg link bandwidth amount of ldquoreservedrdquo link bandwidth
D
R4
R5
A
R6
entry MPLS router uses RSVP-TE signaling protocol to set up MPLS forwarding at downstream routers
modified link state flooding
RSVP-TE
Link Layer 5-82
R1 R2
D
R3 R4 R5
0
1
0 0
A
R6
in out out
label label dest interface
6 - A 0
in out out
label label dest interface
10 6 A 1
12 9 D 0
in out out
label label dest interface
10 A 0
12 D 0
1
in out out
label label dest interface
8 6 A 0
0
8 A 1
MPLS forwarding tables
Link Layer 5-83
Link layer LANs outline
51 introduction services
52 error detection correction
53 multiple access protocols
54 LANs addressing ARP
Ethernet
switches
VLANS
55 link virtualization MPLS
56 data center networking
57 a day in the life of a web request
Link Layer 5-84
Data center networks
10rsquos to 100rsquos of thousands of hosts often closely coupled in close proximity e-business (eg Amazon)
content-servers (eg YouTube Akamai Apple Microsoft)
search engines data mining (eg Google)
challenges
multiple applications each serving massive numbers of clients
managingbalancing load avoiding processing networking data bottlenecks
Inside a 40-ft Microsoft container
Chicago data center
Link Layer 5-85
Server racks
TOR switches
Tier-1 switches
Tier-2 switches
Load balancer
Load balancer
B
1 2 3 4 5 6 7 8
A C
Border router
Access router
Internet
Data center networks
load balancer application-layer routing receives external client requests
directs workload within data center
returns results to external client (hiding data
center internals from client)
Server racks
TOR switches
Tier-1 switches
Tier-2 switches
1 2 3 4 5 6 7 8
Data center networks
rich interconnection among switches racks
increased throughput between racks (multiple routing
paths possible)
increased reliability via redundancy
Link Layer 5-87
Link layer LANs outline
51 introduction services
52 error detection correction
53 multiple access protocols
54 LANs addressing ARP
Ethernet
switches
VLANS
55 link virtualization MPLS
56 data center networking
57 a day in the life of a web request
Link Layer 5-88
Synthesis a day in the life of a web request
journey down protocol stack complete application transport network link
putting-it-all-together synthesis goal identify review understand protocols (at all
layers) involved in seemingly simple scenario requesting www page
scenario student attaches laptop to campus network requestsreceives wwwgooglecom
Link Layer 5-89
A day in the life scenario
Comcast network
68800013
Googlersquos network
64233160019 64233169105
web server
DNS server
school network
68802024
web page
browser
router
(runs DHCP)
Link Layer 5-90
A day in the lifehellip connecting to the Internet
connecting laptop needs to get its own IP address addr of first-hop router addr of DNS server use DHCP
DHCP
UDP
IP
Eth
Phy
DHCP
DHCP
DHCP
DHCP
DHCP
DHCP
UDP
IP
Eth
Phy
DHCP
DHCP
DHCP
DHCP DHCP
DHCP request encapsulated in UDP encapsulated in IP encapsulated in 8023 Ethernet
Ethernet frame broadcast
(dest FFFFFFFFFFFF) on LAN received at router running DHCP server
Ethernet demuxed to IP demuxed UDP demuxed to DHCP
router
(runs DHCP)
Link Layer 5-91
DHCP server formulates DHCP ACK containing clientrsquos IP address IP address of first-hop router for client name amp IP address of DNS server
DHCP
UDP
IP
Eth
Phy
DHCP
DHCP
DHCP
DHCP
DHCP
UDP
IP
Eth
Phy
DHCP
DHCP
DHCP
DHCP
DHCP
encapsulation at DHCP server frame forwarded (switch learning) through LAN demultiplexing at client
Client now has IP address knows name amp addr of DNS
server IP address of its first-hop router
DHCP client receives DHCP ACK reply
A day in the lifehellip connecting to the Internet
router
(runs DHCP)
Link Layer 5-92
A day in the lifehellip ARP (before DNS before HTTP)
before sending HTTP request need IP address of wwwgooglecom DNS
DNS
UDP
IP
Eth
Phy
DNS
DNS
DNS
DNS query created encapsulated in UDP encapsulated in IP encapsulated in Eth To send frame to router need MAC address of router interface ARP
ARP query broadcast received by
router which replies with ARP reply giving MAC address of router interface
client now knows MAC address of first hop router so can now send frame containing DNS query
ARP query
Eth
Phy
ARP
ARP
ARP reply
router
(runs DHCP)
Link Layer 5-93
DNS
UDP
IP
Eth
Phy
DNS
DNS
DNS
DNS
DNS
IP datagram containing DNS query forwarded via LAN switch from client to 1st hop router
IP datagram forwarded from campus network into comcast network routed (tables created by RIP OSPF IS-IS andor BGP routing protocols) to DNS server
demuxrsquoed to DNS server
DNS server replies to client with IP address of wwwgooglecom
Comcast network
68800013
DNS server
DNS
UDP
IP
Eth
Phy
DNS
DNS
DNS
DNS
A day in the lifehellip using DNS
router
(runs DHCP)
Link Layer 5-94
A day in the lifehellipTCP connection carrying HTTP
HTTP
TCP
IP
Eth
Phy
HTTP
to send HTTP request client first opens TCP socket to web server
TCP SYN segment (step 1 in 3-way handshake) inter-domain routed to web server
TCP connection established 64233169105
web server
SYN
SYN
SYN
SYN
TCP
IP
Eth
Phy
SYN
SYN
SYN
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
web server responds with TCP SYNACK (step 2 in 3-way handshake)
router
(runs DHCP)
Link Layer 5-95
A day in the lifehellip HTTP requestreply
HTTP
TCP
IP
Eth
Phy
HTTP
HTTP request sent into TCP socket
IP datagram containing HTTP request routed to wwwgooglecom
IP datagram containing HTTP reply routed back to client
64233169105
web server
HTTP
TCP
IP
Eth
Phy web server responds with
HTTP reply (containing web page)
HTTP
HTTP
HTTP HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
web page finally () displayed
Link Layer 5-96
Chapter 5 Summary
principles behind data link layer services error detection correction
sharing a broadcast channel multiple access
link layer addressing
instantiation and implementation of various link layer technologies Ethernet
switched LANS VLANs
virtualized networks as a link layer MPLS
synthesis a day in the life of a web request
Link Layer 5-97
Chapter 5 letrsquos take a breath
journey down protocol stack complete (except PHY)
solid understanding of networking principles practice
hellip could stop here hellip but lots of interesting topics wireless
multimedia
security
network management
Link Layer 5-57
Ethernet frame structure (more)
addresses 6 byte source destination MAC addresses if adapter receives frame with matching destination
address or with broadcast address (eg ARP packet) it passes data in frame to network layer protocol
otherwise adapter discards frame
type indicates higher layer protocol (mostly IP but others possible eg Novell IPX AppleTalk)
CRC cyclic redundancy check at receiver error detected frame is dropped
dest address
source address
data (payload) CRC preamble
type
Link Layer 5-58
Ethernet unreliable connectionless
connectionless no handshaking between sending and receiving NICs
unreliable receiving NIC doesnt send acks or nacks to sending NIC
data in dropped frames recovered only if initial sender uses higher layer rdt (eg TCP) otherwise dropped data lost
Ethernetrsquos MAC protocol unslotted CSMACD wth binary backoff
Link Layer 5-59
8023 Ethernet standards link amp physical layers
many different Ethernet standards
common MAC protocol and frame format
different speeds 2 Mbps 10 Mbps 100 Mbps 1Gbps 10G bps
different physical layer media fiber cable
application
transport
network
link
physical
MAC protocol
and frame format
100BASE-TX
100BASE-T4
100BASE-FX 100BASE-T2
100BASE-SX 100BASE-BX
fiber physical layer copper (twister
pair) physical layer
Link Layer 5-60
Link layer LANs outline
51 introduction services
52 error detection correction
53 multiple access protocols
54 LANs addressing ARP
Ethernet
switches
VLANS
55 link virtualization MPLS
56 data center networking
57 a day in the life of a web request
Link Layer 5-61
Ethernet switch link-layer device takes an active role
store forward Ethernet frames
examine incoming framersquos MAC address selectively forward frame to one-or-more outgoing links when frame is to be forwarded on segment uses CSMACD to access segment
transparent
hosts are unaware of presence of switches
plug-and-play self-learning
switches do not need to be configured
Link Layer 5-62
Switch multiple simultaneous transmissions
hosts have dedicated direct connection to switch
switches buffer packets
Ethernet protocol used on each incoming link but no collisions full duplex
each link is its own collision domain
switching A-to-Arsquo and B-to-Brsquo can transmit simultaneously without collisions switch with six interfaces
(123456)
A
Arsquo
B
Brsquo C
Crsquo
1 2
3 4 5
6
Link Layer 5-63
Switch forwarding table
Q how does switch know Arsquo reachable via interface 4 Brsquo reachable via interface 5
switch with six interfaces
(123456)
A
Arsquo
B
Brsquo C
Crsquo
1 2
3 4 5
6 A each switch has a switch table each entry
(MAC address of host interface to
reach host time stamp)
looks like a routing table
Q how are entries created maintained in switch table
something like a routing protocol
A
Arsquo
B
Brsquo C
Crsquo
1 2
3 4 5
6
Link Layer 5-64
Switch self-learning
switch learns which hosts can be reached through which interfaces
when frame received switch ldquolearnsrdquo location of sender incoming LAN segment
records senderlocation pair in switch table
A Arsquo
Source A
Dest Arsquo
MAC addr interface TTL
Switch table
(initially empty) A 1 60
Link Layer 5-65
Switch frame filteringforwarding
when frame received at switch 1 record incoming link MAC address of sending host
2 index switch table using MAC destination address
3 if entry found for destination then
if destination on segment from which frame arrived then drop frame
else forward frame on interface indicated by entry
else flood forward on all interfaces except arriving
interface
A
Arsquo
B
Brsquo C
Crsquo
1 2
3 4 5
6
Link Layer 5-66
Self-learning forwarding example
A Arsquo
Source A
Dest Arsquo
MAC addr interface TTL
switch table
(initially empty) A 1 60
A Arsquo A Arsquo A Arsquo A Arsquo A Arsquo
frame destination Arsquo locaton unknown flood
Arsquo A
destination A location
known
Arsquo 4 60
selectively send
on just one link
Link Layer 5-67
Interconnecting switches
switches can be connected together
Q sending from A to G - how does S1 know to forward frame destined to F via S4 and S3
A self learning (works exactly the same as in single-switch case)
A
B
S1
C D
E
F
S2
S4
S3
H
I
G
Link Layer 5-68
Self-learning multi-switch example
Suppose C sends frame to I I responds to C
Q show switch tables and packet forwarding in S1 S2 S3 S4
A
B
S1
C D
E
F
S2
S4
S3
H
I
G
Link Layer 5-69
Institutional network
to external
network
router
IP subnet
mail server
web server
Link Layer 5-70
Switches vs routers
both are store-and-forward
routers network-layer devices (examine network-layer headers)
switches link-layer devices (examine link-layer headers)
both have forwarding tables
routers compute tables using routing algorithms IP addresses
switches learn forwarding table using flooding learning MAC addresses
application
transport
network
link
physical
network
link
physical
link
physical
switch
datagram
application
transport
network
link
physical
frame
frame
frame
datagram
Link Layer 5-71
VLANs motivation
consider CS user moves office to
EE but wants connect to CS switch
single broadcast domain
all layer-2 broadcast traffic (ARP DHCP unknown location of destination MAC address) must cross entire LAN
securityprivacy efficiency issues
Computer
Science Electrical
Engineering
Computer
Engineering
Link Layer 5-72
VLANs port-based VLAN switch ports
grouped (by switch management software) so that single physical switch helliphellip
switch(es) supporting
VLAN capabilities can
be configured to
define multiple virtual
LANS over single
physical LAN
infrastructure
Virtual Local
Area Network 1
8
9
16 10 2
7
hellip
Electrical Engineering
(VLAN ports 1-8)
Computer Science
(VLAN ports 9-15)
15
hellip
Electrical Engineering
(VLAN ports 1-8)
hellip
1
8 2
7 9
16 10
15
hellip
Computer Science
(VLAN ports 9-16)
hellip operates as multiple virtual switches
Link Layer 5-73
Port-based VLAN
1
8
9
16 10 2
7
hellip
Electrical Engineering
(VLAN ports 1-8)
Computer Science
(VLAN ports 9-15)
15
hellip
traffic isolation frames tofrom ports 1-8 can only reach ports 1-8 can also define VLAN based on
MAC addresses of endpoints rather than switch port
dynamic membership ports can be dynamically assigned among VLANs
router
forwarding between VLANS done via routing (just as with separate switches) in practice vendors sell combined
switches plus routers
Link Layer 5-74
VLANS spanning multiple switches
trunk port carries frames between VLANS defined over multiple physical switches frames forwarded within VLAN between switches canrsquot be vanilla
8021 frames (must carry VLAN ID info)
8021q protocol addsremoved additional header fields for frames forwarded between trunk ports
1
8
9
10 2
7
hellip
Electrical Engineering
(VLAN ports 1-8)
Computer Science
(VLAN ports 9-15)
15
hellip
2
7 3
Ports 235 belong to EE VLAN
Ports 4678 belong to CS VLAN
5
4 6 8 16
1
Link Layer 5-75
type
2-byte Tag Protocol Identifier
(value 81-00)
Tag Control Information (12 bit VLAN ID field
3 bit priority field like IP TOS)
Recomputed CRC
8021Q VLAN frame format
8021 frame
8021Q frame
dest address
source address
data (payload) CRC preamble
dest address
source address
preamble data (payload) CRC
type
Link Layer 5-76
Link layer LANs outline
51 introduction services
52 error detection correction
53 multiple access protocols
54 LANs addressing ARP
Ethernet
switches
VLANS
55 link virtualization MPLS
56 data center networking
57 a day in the life of a web request
Link Layer 5-77
Multiprotocol label switching (MPLS)
initial goal high-speed IP forwarding using fixed length label (instead of IP address) fast lookup using fixed length identifier (rather than
shortest prefix matching)
borrowing ideas from Virtual Circuit (VC) approach
but IP datagram still keeps IP address
PPP or Ethernet
header IP header remainder of link-layer frame MPLS header
label Exp S TTL
20 3 1 5
Link Layer 5-78
MPLS capable routers
aka label-switched router
forward packets to outgoing interface based only on label value (donrsquot inspect IP address) MPLS forwarding table distinct from IP forwarding tables
flexibility MPLS forwarding decisions can differ from those of IP use destination and source addresses to route flows to
same destination differently (traffic engineering)
re-route flows quickly if link fails pre-computed backup paths (useful for VoIP)
Link Layer 5-79
R2
D
R3 R5
A
R6
MPLS versus IP paths
IP router IP routing path to destination determined
by destination address alone
R4
Link Layer 5-80
R2
D
R3 R4
R5
A
R6
MPLS versus IP paths
IP-only router
IP routing path to destination determined by destination address alone
MPLS and IP router
MPLS routing path to destination can be based on source and dest address fast reroute precompute backup routes in
case of link failure
entry router (R4) can use different MPLS
routes to A based eg on source address
Link Layer 5-81
MPLS signaling
modify OSPF IS-IS link-state flooding protocols to carry info used by MPLS routing eg link bandwidth amount of ldquoreservedrdquo link bandwidth
D
R4
R5
A
R6
entry MPLS router uses RSVP-TE signaling protocol to set up MPLS forwarding at downstream routers
modified link state flooding
RSVP-TE
Link Layer 5-82
R1 R2
D
R3 R4 R5
0
1
0 0
A
R6
in out out
label label dest interface
6 - A 0
in out out
label label dest interface
10 6 A 1
12 9 D 0
in out out
label label dest interface
10 A 0
12 D 0
1
in out out
label label dest interface
8 6 A 0
0
8 A 1
MPLS forwarding tables
Link Layer 5-83
Link layer LANs outline
51 introduction services
52 error detection correction
53 multiple access protocols
54 LANs addressing ARP
Ethernet
switches
VLANS
55 link virtualization MPLS
56 data center networking
57 a day in the life of a web request
Link Layer 5-84
Data center networks
10rsquos to 100rsquos of thousands of hosts often closely coupled in close proximity e-business (eg Amazon)
content-servers (eg YouTube Akamai Apple Microsoft)
search engines data mining (eg Google)
challenges
multiple applications each serving massive numbers of clients
managingbalancing load avoiding processing networking data bottlenecks
Inside a 40-ft Microsoft container
Chicago data center
Link Layer 5-85
Server racks
TOR switches
Tier-1 switches
Tier-2 switches
Load balancer
Load balancer
B
1 2 3 4 5 6 7 8
A C
Border router
Access router
Internet
Data center networks
load balancer application-layer routing receives external client requests
directs workload within data center
returns results to external client (hiding data
center internals from client)
Server racks
TOR switches
Tier-1 switches
Tier-2 switches
1 2 3 4 5 6 7 8
Data center networks
rich interconnection among switches racks
increased throughput between racks (multiple routing
paths possible)
increased reliability via redundancy
Link Layer 5-87
Link layer LANs outline
51 introduction services
52 error detection correction
53 multiple access protocols
54 LANs addressing ARP
Ethernet
switches
VLANS
55 link virtualization MPLS
56 data center networking
57 a day in the life of a web request
Link Layer 5-88
Synthesis a day in the life of a web request
journey down protocol stack complete application transport network link
putting-it-all-together synthesis goal identify review understand protocols (at all
layers) involved in seemingly simple scenario requesting www page
scenario student attaches laptop to campus network requestsreceives wwwgooglecom
Link Layer 5-89
A day in the life scenario
Comcast network
68800013
Googlersquos network
64233160019 64233169105
web server
DNS server
school network
68802024
web page
browser
router
(runs DHCP)
Link Layer 5-90
A day in the lifehellip connecting to the Internet
connecting laptop needs to get its own IP address addr of first-hop router addr of DNS server use DHCP
DHCP
UDP
IP
Eth
Phy
DHCP
DHCP
DHCP
DHCP
DHCP
DHCP
UDP
IP
Eth
Phy
DHCP
DHCP
DHCP
DHCP DHCP
DHCP request encapsulated in UDP encapsulated in IP encapsulated in 8023 Ethernet
Ethernet frame broadcast
(dest FFFFFFFFFFFF) on LAN received at router running DHCP server
Ethernet demuxed to IP demuxed UDP demuxed to DHCP
router
(runs DHCP)
Link Layer 5-91
DHCP server formulates DHCP ACK containing clientrsquos IP address IP address of first-hop router for client name amp IP address of DNS server
DHCP
UDP
IP
Eth
Phy
DHCP
DHCP
DHCP
DHCP
DHCP
UDP
IP
Eth
Phy
DHCP
DHCP
DHCP
DHCP
DHCP
encapsulation at DHCP server frame forwarded (switch learning) through LAN demultiplexing at client
Client now has IP address knows name amp addr of DNS
server IP address of its first-hop router
DHCP client receives DHCP ACK reply
A day in the lifehellip connecting to the Internet
router
(runs DHCP)
Link Layer 5-92
A day in the lifehellip ARP (before DNS before HTTP)
before sending HTTP request need IP address of wwwgooglecom DNS
DNS
UDP
IP
Eth
Phy
DNS
DNS
DNS
DNS query created encapsulated in UDP encapsulated in IP encapsulated in Eth To send frame to router need MAC address of router interface ARP
ARP query broadcast received by
router which replies with ARP reply giving MAC address of router interface
client now knows MAC address of first hop router so can now send frame containing DNS query
ARP query
Eth
Phy
ARP
ARP
ARP reply
router
(runs DHCP)
Link Layer 5-93
DNS
UDP
IP
Eth
Phy
DNS
DNS
DNS
DNS
DNS
IP datagram containing DNS query forwarded via LAN switch from client to 1st hop router
IP datagram forwarded from campus network into comcast network routed (tables created by RIP OSPF IS-IS andor BGP routing protocols) to DNS server
demuxrsquoed to DNS server
DNS server replies to client with IP address of wwwgooglecom
Comcast network
68800013
DNS server
DNS
UDP
IP
Eth
Phy
DNS
DNS
DNS
DNS
A day in the lifehellip using DNS
router
(runs DHCP)
Link Layer 5-94
A day in the lifehellipTCP connection carrying HTTP
HTTP
TCP
IP
Eth
Phy
HTTP
to send HTTP request client first opens TCP socket to web server
TCP SYN segment (step 1 in 3-way handshake) inter-domain routed to web server
TCP connection established 64233169105
web server
SYN
SYN
SYN
SYN
TCP
IP
Eth
Phy
SYN
SYN
SYN
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
web server responds with TCP SYNACK (step 2 in 3-way handshake)
router
(runs DHCP)
Link Layer 5-95
A day in the lifehellip HTTP requestreply
HTTP
TCP
IP
Eth
Phy
HTTP
HTTP request sent into TCP socket
IP datagram containing HTTP request routed to wwwgooglecom
IP datagram containing HTTP reply routed back to client
64233169105
web server
HTTP
TCP
IP
Eth
Phy web server responds with
HTTP reply (containing web page)
HTTP
HTTP
HTTP HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
web page finally () displayed
Link Layer 5-96
Chapter 5 Summary
principles behind data link layer services error detection correction
sharing a broadcast channel multiple access
link layer addressing
instantiation and implementation of various link layer technologies Ethernet
switched LANS VLANs
virtualized networks as a link layer MPLS
synthesis a day in the life of a web request
Link Layer 5-97
Chapter 5 letrsquos take a breath
journey down protocol stack complete (except PHY)
solid understanding of networking principles practice
hellip could stop here hellip but lots of interesting topics wireless
multimedia
security
network management
Link Layer 5-58
Ethernet unreliable connectionless
connectionless no handshaking between sending and receiving NICs
unreliable receiving NIC doesnt send acks or nacks to sending NIC
data in dropped frames recovered only if initial sender uses higher layer rdt (eg TCP) otherwise dropped data lost
Ethernetrsquos MAC protocol unslotted CSMACD wth binary backoff
Link Layer 5-59
8023 Ethernet standards link amp physical layers
many different Ethernet standards
common MAC protocol and frame format
different speeds 2 Mbps 10 Mbps 100 Mbps 1Gbps 10G bps
different physical layer media fiber cable
application
transport
network
link
physical
MAC protocol
and frame format
100BASE-TX
100BASE-T4
100BASE-FX 100BASE-T2
100BASE-SX 100BASE-BX
fiber physical layer copper (twister
pair) physical layer
Link Layer 5-60
Link layer LANs outline
51 introduction services
52 error detection correction
53 multiple access protocols
54 LANs addressing ARP
Ethernet
switches
VLANS
55 link virtualization MPLS
56 data center networking
57 a day in the life of a web request
Link Layer 5-61
Ethernet switch link-layer device takes an active role
store forward Ethernet frames
examine incoming framersquos MAC address selectively forward frame to one-or-more outgoing links when frame is to be forwarded on segment uses CSMACD to access segment
transparent
hosts are unaware of presence of switches
plug-and-play self-learning
switches do not need to be configured
Link Layer 5-62
Switch multiple simultaneous transmissions
hosts have dedicated direct connection to switch
switches buffer packets
Ethernet protocol used on each incoming link but no collisions full duplex
each link is its own collision domain
switching A-to-Arsquo and B-to-Brsquo can transmit simultaneously without collisions switch with six interfaces
(123456)
A
Arsquo
B
Brsquo C
Crsquo
1 2
3 4 5
6
Link Layer 5-63
Switch forwarding table
Q how does switch know Arsquo reachable via interface 4 Brsquo reachable via interface 5
switch with six interfaces
(123456)
A
Arsquo
B
Brsquo C
Crsquo
1 2
3 4 5
6 A each switch has a switch table each entry
(MAC address of host interface to
reach host time stamp)
looks like a routing table
Q how are entries created maintained in switch table
something like a routing protocol
A
Arsquo
B
Brsquo C
Crsquo
1 2
3 4 5
6
Link Layer 5-64
Switch self-learning
switch learns which hosts can be reached through which interfaces
when frame received switch ldquolearnsrdquo location of sender incoming LAN segment
records senderlocation pair in switch table
A Arsquo
Source A
Dest Arsquo
MAC addr interface TTL
Switch table
(initially empty) A 1 60
Link Layer 5-65
Switch frame filteringforwarding
when frame received at switch 1 record incoming link MAC address of sending host
2 index switch table using MAC destination address
3 if entry found for destination then
if destination on segment from which frame arrived then drop frame
else forward frame on interface indicated by entry
else flood forward on all interfaces except arriving
interface
A
Arsquo
B
Brsquo C
Crsquo
1 2
3 4 5
6
Link Layer 5-66
Self-learning forwarding example
A Arsquo
Source A
Dest Arsquo
MAC addr interface TTL
switch table
(initially empty) A 1 60
A Arsquo A Arsquo A Arsquo A Arsquo A Arsquo
frame destination Arsquo locaton unknown flood
Arsquo A
destination A location
known
Arsquo 4 60
selectively send
on just one link
Link Layer 5-67
Interconnecting switches
switches can be connected together
Q sending from A to G - how does S1 know to forward frame destined to F via S4 and S3
A self learning (works exactly the same as in single-switch case)
A
B
S1
C D
E
F
S2
S4
S3
H
I
G
Link Layer 5-68
Self-learning multi-switch example
Suppose C sends frame to I I responds to C
Q show switch tables and packet forwarding in S1 S2 S3 S4
A
B
S1
C D
E
F
S2
S4
S3
H
I
G
Link Layer 5-69
Institutional network
to external
network
router
IP subnet
mail server
web server
Link Layer 5-70
Switches vs routers
both are store-and-forward
routers network-layer devices (examine network-layer headers)
switches link-layer devices (examine link-layer headers)
both have forwarding tables
routers compute tables using routing algorithms IP addresses
switches learn forwarding table using flooding learning MAC addresses
application
transport
network
link
physical
network
link
physical
link
physical
switch
datagram
application
transport
network
link
physical
frame
frame
frame
datagram
Link Layer 5-71
VLANs motivation
consider CS user moves office to
EE but wants connect to CS switch
single broadcast domain
all layer-2 broadcast traffic (ARP DHCP unknown location of destination MAC address) must cross entire LAN
securityprivacy efficiency issues
Computer
Science Electrical
Engineering
Computer
Engineering
Link Layer 5-72
VLANs port-based VLAN switch ports
grouped (by switch management software) so that single physical switch helliphellip
switch(es) supporting
VLAN capabilities can
be configured to
define multiple virtual
LANS over single
physical LAN
infrastructure
Virtual Local
Area Network 1
8
9
16 10 2
7
hellip
Electrical Engineering
(VLAN ports 1-8)
Computer Science
(VLAN ports 9-15)
15
hellip
Electrical Engineering
(VLAN ports 1-8)
hellip
1
8 2
7 9
16 10
15
hellip
Computer Science
(VLAN ports 9-16)
hellip operates as multiple virtual switches
Link Layer 5-73
Port-based VLAN
1
8
9
16 10 2
7
hellip
Electrical Engineering
(VLAN ports 1-8)
Computer Science
(VLAN ports 9-15)
15
hellip
traffic isolation frames tofrom ports 1-8 can only reach ports 1-8 can also define VLAN based on
MAC addresses of endpoints rather than switch port
dynamic membership ports can be dynamically assigned among VLANs
router
forwarding between VLANS done via routing (just as with separate switches) in practice vendors sell combined
switches plus routers
Link Layer 5-74
VLANS spanning multiple switches
trunk port carries frames between VLANS defined over multiple physical switches frames forwarded within VLAN between switches canrsquot be vanilla
8021 frames (must carry VLAN ID info)
8021q protocol addsremoved additional header fields for frames forwarded between trunk ports
1
8
9
10 2
7
hellip
Electrical Engineering
(VLAN ports 1-8)
Computer Science
(VLAN ports 9-15)
15
hellip
2
7 3
Ports 235 belong to EE VLAN
Ports 4678 belong to CS VLAN
5
4 6 8 16
1
Link Layer 5-75
type
2-byte Tag Protocol Identifier
(value 81-00)
Tag Control Information (12 bit VLAN ID field
3 bit priority field like IP TOS)
Recomputed CRC
8021Q VLAN frame format
8021 frame
8021Q frame
dest address
source address
data (payload) CRC preamble
dest address
source address
preamble data (payload) CRC
type
Link Layer 5-76
Link layer LANs outline
51 introduction services
52 error detection correction
53 multiple access protocols
54 LANs addressing ARP
Ethernet
switches
VLANS
55 link virtualization MPLS
56 data center networking
57 a day in the life of a web request
Link Layer 5-77
Multiprotocol label switching (MPLS)
initial goal high-speed IP forwarding using fixed length label (instead of IP address) fast lookup using fixed length identifier (rather than
shortest prefix matching)
borrowing ideas from Virtual Circuit (VC) approach
but IP datagram still keeps IP address
PPP or Ethernet
header IP header remainder of link-layer frame MPLS header
label Exp S TTL
20 3 1 5
Link Layer 5-78
MPLS capable routers
aka label-switched router
forward packets to outgoing interface based only on label value (donrsquot inspect IP address) MPLS forwarding table distinct from IP forwarding tables
flexibility MPLS forwarding decisions can differ from those of IP use destination and source addresses to route flows to
same destination differently (traffic engineering)
re-route flows quickly if link fails pre-computed backup paths (useful for VoIP)
Link Layer 5-79
R2
D
R3 R5
A
R6
MPLS versus IP paths
IP router IP routing path to destination determined
by destination address alone
R4
Link Layer 5-80
R2
D
R3 R4
R5
A
R6
MPLS versus IP paths
IP-only router
IP routing path to destination determined by destination address alone
MPLS and IP router
MPLS routing path to destination can be based on source and dest address fast reroute precompute backup routes in
case of link failure
entry router (R4) can use different MPLS
routes to A based eg on source address
Link Layer 5-81
MPLS signaling
modify OSPF IS-IS link-state flooding protocols to carry info used by MPLS routing eg link bandwidth amount of ldquoreservedrdquo link bandwidth
D
R4
R5
A
R6
entry MPLS router uses RSVP-TE signaling protocol to set up MPLS forwarding at downstream routers
modified link state flooding
RSVP-TE
Link Layer 5-82
R1 R2
D
R3 R4 R5
0
1
0 0
A
R6
in out out
label label dest interface
6 - A 0
in out out
label label dest interface
10 6 A 1
12 9 D 0
in out out
label label dest interface
10 A 0
12 D 0
1
in out out
label label dest interface
8 6 A 0
0
8 A 1
MPLS forwarding tables
Link Layer 5-83
Link layer LANs outline
51 introduction services
52 error detection correction
53 multiple access protocols
54 LANs addressing ARP
Ethernet
switches
VLANS
55 link virtualization MPLS
56 data center networking
57 a day in the life of a web request
Link Layer 5-84
Data center networks
10rsquos to 100rsquos of thousands of hosts often closely coupled in close proximity e-business (eg Amazon)
content-servers (eg YouTube Akamai Apple Microsoft)
search engines data mining (eg Google)
challenges
multiple applications each serving massive numbers of clients
managingbalancing load avoiding processing networking data bottlenecks
Inside a 40-ft Microsoft container
Chicago data center
Link Layer 5-85
Server racks
TOR switches
Tier-1 switches
Tier-2 switches
Load balancer
Load balancer
B
1 2 3 4 5 6 7 8
A C
Border router
Access router
Internet
Data center networks
load balancer application-layer routing receives external client requests
directs workload within data center
returns results to external client (hiding data
center internals from client)
Server racks
TOR switches
Tier-1 switches
Tier-2 switches
1 2 3 4 5 6 7 8
Data center networks
rich interconnection among switches racks
increased throughput between racks (multiple routing
paths possible)
increased reliability via redundancy
Link Layer 5-87
Link layer LANs outline
51 introduction services
52 error detection correction
53 multiple access protocols
54 LANs addressing ARP
Ethernet
switches
VLANS
55 link virtualization MPLS
56 data center networking
57 a day in the life of a web request
Link Layer 5-88
Synthesis a day in the life of a web request
journey down protocol stack complete application transport network link
putting-it-all-together synthesis goal identify review understand protocols (at all
layers) involved in seemingly simple scenario requesting www page
scenario student attaches laptop to campus network requestsreceives wwwgooglecom
Link Layer 5-89
A day in the life scenario
Comcast network
68800013
Googlersquos network
64233160019 64233169105
web server
DNS server
school network
68802024
web page
browser
router
(runs DHCP)
Link Layer 5-90
A day in the lifehellip connecting to the Internet
connecting laptop needs to get its own IP address addr of first-hop router addr of DNS server use DHCP
DHCP
UDP
IP
Eth
Phy
DHCP
DHCP
DHCP
DHCP
DHCP
DHCP
UDP
IP
Eth
Phy
DHCP
DHCP
DHCP
DHCP DHCP
DHCP request encapsulated in UDP encapsulated in IP encapsulated in 8023 Ethernet
Ethernet frame broadcast
(dest FFFFFFFFFFFF) on LAN received at router running DHCP server
Ethernet demuxed to IP demuxed UDP demuxed to DHCP
router
(runs DHCP)
Link Layer 5-91
DHCP server formulates DHCP ACK containing clientrsquos IP address IP address of first-hop router for client name amp IP address of DNS server
DHCP
UDP
IP
Eth
Phy
DHCP
DHCP
DHCP
DHCP
DHCP
UDP
IP
Eth
Phy
DHCP
DHCP
DHCP
DHCP
DHCP
encapsulation at DHCP server frame forwarded (switch learning) through LAN demultiplexing at client
Client now has IP address knows name amp addr of DNS
server IP address of its first-hop router
DHCP client receives DHCP ACK reply
A day in the lifehellip connecting to the Internet
router
(runs DHCP)
Link Layer 5-92
A day in the lifehellip ARP (before DNS before HTTP)
before sending HTTP request need IP address of wwwgooglecom DNS
DNS
UDP
IP
Eth
Phy
DNS
DNS
DNS
DNS query created encapsulated in UDP encapsulated in IP encapsulated in Eth To send frame to router need MAC address of router interface ARP
ARP query broadcast received by
router which replies with ARP reply giving MAC address of router interface
client now knows MAC address of first hop router so can now send frame containing DNS query
ARP query
Eth
Phy
ARP
ARP
ARP reply
router
(runs DHCP)
Link Layer 5-93
DNS
UDP
IP
Eth
Phy
DNS
DNS
DNS
DNS
DNS
IP datagram containing DNS query forwarded via LAN switch from client to 1st hop router
IP datagram forwarded from campus network into comcast network routed (tables created by RIP OSPF IS-IS andor BGP routing protocols) to DNS server
demuxrsquoed to DNS server
DNS server replies to client with IP address of wwwgooglecom
Comcast network
68800013
DNS server
DNS
UDP
IP
Eth
Phy
DNS
DNS
DNS
DNS
A day in the lifehellip using DNS
router
(runs DHCP)
Link Layer 5-94
A day in the lifehellipTCP connection carrying HTTP
HTTP
TCP
IP
Eth
Phy
HTTP
to send HTTP request client first opens TCP socket to web server
TCP SYN segment (step 1 in 3-way handshake) inter-domain routed to web server
TCP connection established 64233169105
web server
SYN
SYN
SYN
SYN
TCP
IP
Eth
Phy
SYN
SYN
SYN
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
web server responds with TCP SYNACK (step 2 in 3-way handshake)
router
(runs DHCP)
Link Layer 5-95
A day in the lifehellip HTTP requestreply
HTTP
TCP
IP
Eth
Phy
HTTP
HTTP request sent into TCP socket
IP datagram containing HTTP request routed to wwwgooglecom
IP datagram containing HTTP reply routed back to client
64233169105
web server
HTTP
TCP
IP
Eth
Phy web server responds with
HTTP reply (containing web page)
HTTP
HTTP
HTTP HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
web page finally () displayed
Link Layer 5-96
Chapter 5 Summary
principles behind data link layer services error detection correction
sharing a broadcast channel multiple access
link layer addressing
instantiation and implementation of various link layer technologies Ethernet
switched LANS VLANs
virtualized networks as a link layer MPLS
synthesis a day in the life of a web request
Link Layer 5-97
Chapter 5 letrsquos take a breath
journey down protocol stack complete (except PHY)
solid understanding of networking principles practice
hellip could stop here hellip but lots of interesting topics wireless
multimedia
security
network management
Link Layer 5-59
8023 Ethernet standards link amp physical layers
many different Ethernet standards
common MAC protocol and frame format
different speeds 2 Mbps 10 Mbps 100 Mbps 1Gbps 10G bps
different physical layer media fiber cable
application
transport
network
link
physical
MAC protocol
and frame format
100BASE-TX
100BASE-T4
100BASE-FX 100BASE-T2
100BASE-SX 100BASE-BX
fiber physical layer copper (twister
pair) physical layer
Link Layer 5-60
Link layer LANs outline
51 introduction services
52 error detection correction
53 multiple access protocols
54 LANs addressing ARP
Ethernet
switches
VLANS
55 link virtualization MPLS
56 data center networking
57 a day in the life of a web request
Link Layer 5-61
Ethernet switch link-layer device takes an active role
store forward Ethernet frames
examine incoming framersquos MAC address selectively forward frame to one-or-more outgoing links when frame is to be forwarded on segment uses CSMACD to access segment
transparent
hosts are unaware of presence of switches
plug-and-play self-learning
switches do not need to be configured
Link Layer 5-62
Switch multiple simultaneous transmissions
hosts have dedicated direct connection to switch
switches buffer packets
Ethernet protocol used on each incoming link but no collisions full duplex
each link is its own collision domain
switching A-to-Arsquo and B-to-Brsquo can transmit simultaneously without collisions switch with six interfaces
(123456)
A
Arsquo
B
Brsquo C
Crsquo
1 2
3 4 5
6
Link Layer 5-63
Switch forwarding table
Q how does switch know Arsquo reachable via interface 4 Brsquo reachable via interface 5
switch with six interfaces
(123456)
A
Arsquo
B
Brsquo C
Crsquo
1 2
3 4 5
6 A each switch has a switch table each entry
(MAC address of host interface to
reach host time stamp)
looks like a routing table
Q how are entries created maintained in switch table
something like a routing protocol
A
Arsquo
B
Brsquo C
Crsquo
1 2
3 4 5
6
Link Layer 5-64
Switch self-learning
switch learns which hosts can be reached through which interfaces
when frame received switch ldquolearnsrdquo location of sender incoming LAN segment
records senderlocation pair in switch table
A Arsquo
Source A
Dest Arsquo
MAC addr interface TTL
Switch table
(initially empty) A 1 60
Link Layer 5-65
Switch frame filteringforwarding
when frame received at switch 1 record incoming link MAC address of sending host
2 index switch table using MAC destination address
3 if entry found for destination then
if destination on segment from which frame arrived then drop frame
else forward frame on interface indicated by entry
else flood forward on all interfaces except arriving
interface
A
Arsquo
B
Brsquo C
Crsquo
1 2
3 4 5
6
Link Layer 5-66
Self-learning forwarding example
A Arsquo
Source A
Dest Arsquo
MAC addr interface TTL
switch table
(initially empty) A 1 60
A Arsquo A Arsquo A Arsquo A Arsquo A Arsquo
frame destination Arsquo locaton unknown flood
Arsquo A
destination A location
known
Arsquo 4 60
selectively send
on just one link
Link Layer 5-67
Interconnecting switches
switches can be connected together
Q sending from A to G - how does S1 know to forward frame destined to F via S4 and S3
A self learning (works exactly the same as in single-switch case)
A
B
S1
C D
E
F
S2
S4
S3
H
I
G
Link Layer 5-68
Self-learning multi-switch example
Suppose C sends frame to I I responds to C
Q show switch tables and packet forwarding in S1 S2 S3 S4
A
B
S1
C D
E
F
S2
S4
S3
H
I
G
Link Layer 5-69
Institutional network
to external
network
router
IP subnet
mail server
web server
Link Layer 5-70
Switches vs routers
both are store-and-forward
routers network-layer devices (examine network-layer headers)
switches link-layer devices (examine link-layer headers)
both have forwarding tables
routers compute tables using routing algorithms IP addresses
switches learn forwarding table using flooding learning MAC addresses
application
transport
network
link
physical
network
link
physical
link
physical
switch
datagram
application
transport
network
link
physical
frame
frame
frame
datagram
Link Layer 5-71
VLANs motivation
consider CS user moves office to
EE but wants connect to CS switch
single broadcast domain
all layer-2 broadcast traffic (ARP DHCP unknown location of destination MAC address) must cross entire LAN
securityprivacy efficiency issues
Computer
Science Electrical
Engineering
Computer
Engineering
Link Layer 5-72
VLANs port-based VLAN switch ports
grouped (by switch management software) so that single physical switch helliphellip
switch(es) supporting
VLAN capabilities can
be configured to
define multiple virtual
LANS over single
physical LAN
infrastructure
Virtual Local
Area Network 1
8
9
16 10 2
7
hellip
Electrical Engineering
(VLAN ports 1-8)
Computer Science
(VLAN ports 9-15)
15
hellip
Electrical Engineering
(VLAN ports 1-8)
hellip
1
8 2
7 9
16 10
15
hellip
Computer Science
(VLAN ports 9-16)
hellip operates as multiple virtual switches
Link Layer 5-73
Port-based VLAN
1
8
9
16 10 2
7
hellip
Electrical Engineering
(VLAN ports 1-8)
Computer Science
(VLAN ports 9-15)
15
hellip
traffic isolation frames tofrom ports 1-8 can only reach ports 1-8 can also define VLAN based on
MAC addresses of endpoints rather than switch port
dynamic membership ports can be dynamically assigned among VLANs
router
forwarding between VLANS done via routing (just as with separate switches) in practice vendors sell combined
switches plus routers
Link Layer 5-74
VLANS spanning multiple switches
trunk port carries frames between VLANS defined over multiple physical switches frames forwarded within VLAN between switches canrsquot be vanilla
8021 frames (must carry VLAN ID info)
8021q protocol addsremoved additional header fields for frames forwarded between trunk ports
1
8
9
10 2
7
hellip
Electrical Engineering
(VLAN ports 1-8)
Computer Science
(VLAN ports 9-15)
15
hellip
2
7 3
Ports 235 belong to EE VLAN
Ports 4678 belong to CS VLAN
5
4 6 8 16
1
Link Layer 5-75
type
2-byte Tag Protocol Identifier
(value 81-00)
Tag Control Information (12 bit VLAN ID field
3 bit priority field like IP TOS)
Recomputed CRC
8021Q VLAN frame format
8021 frame
8021Q frame
dest address
source address
data (payload) CRC preamble
dest address
source address
preamble data (payload) CRC
type
Link Layer 5-76
Link layer LANs outline
51 introduction services
52 error detection correction
53 multiple access protocols
54 LANs addressing ARP
Ethernet
switches
VLANS
55 link virtualization MPLS
56 data center networking
57 a day in the life of a web request
Link Layer 5-77
Multiprotocol label switching (MPLS)
initial goal high-speed IP forwarding using fixed length label (instead of IP address) fast lookup using fixed length identifier (rather than
shortest prefix matching)
borrowing ideas from Virtual Circuit (VC) approach
but IP datagram still keeps IP address
PPP or Ethernet
header IP header remainder of link-layer frame MPLS header
label Exp S TTL
20 3 1 5
Link Layer 5-78
MPLS capable routers
aka label-switched router
forward packets to outgoing interface based only on label value (donrsquot inspect IP address) MPLS forwarding table distinct from IP forwarding tables
flexibility MPLS forwarding decisions can differ from those of IP use destination and source addresses to route flows to
same destination differently (traffic engineering)
re-route flows quickly if link fails pre-computed backup paths (useful for VoIP)
Link Layer 5-79
R2
D
R3 R5
A
R6
MPLS versus IP paths
IP router IP routing path to destination determined
by destination address alone
R4
Link Layer 5-80
R2
D
R3 R4
R5
A
R6
MPLS versus IP paths
IP-only router
IP routing path to destination determined by destination address alone
MPLS and IP router
MPLS routing path to destination can be based on source and dest address fast reroute precompute backup routes in
case of link failure
entry router (R4) can use different MPLS
routes to A based eg on source address
Link Layer 5-81
MPLS signaling
modify OSPF IS-IS link-state flooding protocols to carry info used by MPLS routing eg link bandwidth amount of ldquoreservedrdquo link bandwidth
D
R4
R5
A
R6
entry MPLS router uses RSVP-TE signaling protocol to set up MPLS forwarding at downstream routers
modified link state flooding
RSVP-TE
Link Layer 5-82
R1 R2
D
R3 R4 R5
0
1
0 0
A
R6
in out out
label label dest interface
6 - A 0
in out out
label label dest interface
10 6 A 1
12 9 D 0
in out out
label label dest interface
10 A 0
12 D 0
1
in out out
label label dest interface
8 6 A 0
0
8 A 1
MPLS forwarding tables
Link Layer 5-83
Link layer LANs outline
51 introduction services
52 error detection correction
53 multiple access protocols
54 LANs addressing ARP
Ethernet
switches
VLANS
55 link virtualization MPLS
56 data center networking
57 a day in the life of a web request
Link Layer 5-84
Data center networks
10rsquos to 100rsquos of thousands of hosts often closely coupled in close proximity e-business (eg Amazon)
content-servers (eg YouTube Akamai Apple Microsoft)
search engines data mining (eg Google)
challenges
multiple applications each serving massive numbers of clients
managingbalancing load avoiding processing networking data bottlenecks
Inside a 40-ft Microsoft container
Chicago data center
Link Layer 5-85
Server racks
TOR switches
Tier-1 switches
Tier-2 switches
Load balancer
Load balancer
B
1 2 3 4 5 6 7 8
A C
Border router
Access router
Internet
Data center networks
load balancer application-layer routing receives external client requests
directs workload within data center
returns results to external client (hiding data
center internals from client)
Server racks
TOR switches
Tier-1 switches
Tier-2 switches
1 2 3 4 5 6 7 8
Data center networks
rich interconnection among switches racks
increased throughput between racks (multiple routing
paths possible)
increased reliability via redundancy
Link Layer 5-87
Link layer LANs outline
51 introduction services
52 error detection correction
53 multiple access protocols
54 LANs addressing ARP
Ethernet
switches
VLANS
55 link virtualization MPLS
56 data center networking
57 a day in the life of a web request
Link Layer 5-88
Synthesis a day in the life of a web request
journey down protocol stack complete application transport network link
putting-it-all-together synthesis goal identify review understand protocols (at all
layers) involved in seemingly simple scenario requesting www page
scenario student attaches laptop to campus network requestsreceives wwwgooglecom
Link Layer 5-89
A day in the life scenario
Comcast network
68800013
Googlersquos network
64233160019 64233169105
web server
DNS server
school network
68802024
web page
browser
router
(runs DHCP)
Link Layer 5-90
A day in the lifehellip connecting to the Internet
connecting laptop needs to get its own IP address addr of first-hop router addr of DNS server use DHCP
DHCP
UDP
IP
Eth
Phy
DHCP
DHCP
DHCP
DHCP
DHCP
DHCP
UDP
IP
Eth
Phy
DHCP
DHCP
DHCP
DHCP DHCP
DHCP request encapsulated in UDP encapsulated in IP encapsulated in 8023 Ethernet
Ethernet frame broadcast
(dest FFFFFFFFFFFF) on LAN received at router running DHCP server
Ethernet demuxed to IP demuxed UDP demuxed to DHCP
router
(runs DHCP)
Link Layer 5-91
DHCP server formulates DHCP ACK containing clientrsquos IP address IP address of first-hop router for client name amp IP address of DNS server
DHCP
UDP
IP
Eth
Phy
DHCP
DHCP
DHCP
DHCP
DHCP
UDP
IP
Eth
Phy
DHCP
DHCP
DHCP
DHCP
DHCP
encapsulation at DHCP server frame forwarded (switch learning) through LAN demultiplexing at client
Client now has IP address knows name amp addr of DNS
server IP address of its first-hop router
DHCP client receives DHCP ACK reply
A day in the lifehellip connecting to the Internet
router
(runs DHCP)
Link Layer 5-92
A day in the lifehellip ARP (before DNS before HTTP)
before sending HTTP request need IP address of wwwgooglecom DNS
DNS
UDP
IP
Eth
Phy
DNS
DNS
DNS
DNS query created encapsulated in UDP encapsulated in IP encapsulated in Eth To send frame to router need MAC address of router interface ARP
ARP query broadcast received by
router which replies with ARP reply giving MAC address of router interface
client now knows MAC address of first hop router so can now send frame containing DNS query
ARP query
Eth
Phy
ARP
ARP
ARP reply
router
(runs DHCP)
Link Layer 5-93
DNS
UDP
IP
Eth
Phy
DNS
DNS
DNS
DNS
DNS
IP datagram containing DNS query forwarded via LAN switch from client to 1st hop router
IP datagram forwarded from campus network into comcast network routed (tables created by RIP OSPF IS-IS andor BGP routing protocols) to DNS server
demuxrsquoed to DNS server
DNS server replies to client with IP address of wwwgooglecom
Comcast network
68800013
DNS server
DNS
UDP
IP
Eth
Phy
DNS
DNS
DNS
DNS
A day in the lifehellip using DNS
router
(runs DHCP)
Link Layer 5-94
A day in the lifehellipTCP connection carrying HTTP
HTTP
TCP
IP
Eth
Phy
HTTP
to send HTTP request client first opens TCP socket to web server
TCP SYN segment (step 1 in 3-way handshake) inter-domain routed to web server
TCP connection established 64233169105
web server
SYN
SYN
SYN
SYN
TCP
IP
Eth
Phy
SYN
SYN
SYN
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
web server responds with TCP SYNACK (step 2 in 3-way handshake)
router
(runs DHCP)
Link Layer 5-95
A day in the lifehellip HTTP requestreply
HTTP
TCP
IP
Eth
Phy
HTTP
HTTP request sent into TCP socket
IP datagram containing HTTP request routed to wwwgooglecom
IP datagram containing HTTP reply routed back to client
64233169105
web server
HTTP
TCP
IP
Eth
Phy web server responds with
HTTP reply (containing web page)
HTTP
HTTP
HTTP HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
web page finally () displayed
Link Layer 5-96
Chapter 5 Summary
principles behind data link layer services error detection correction
sharing a broadcast channel multiple access
link layer addressing
instantiation and implementation of various link layer technologies Ethernet
switched LANS VLANs
virtualized networks as a link layer MPLS
synthesis a day in the life of a web request
Link Layer 5-97
Chapter 5 letrsquos take a breath
journey down protocol stack complete (except PHY)
solid understanding of networking principles practice
hellip could stop here hellip but lots of interesting topics wireless
multimedia
security
network management
Link Layer 5-60
Link layer LANs outline
51 introduction services
52 error detection correction
53 multiple access protocols
54 LANs addressing ARP
Ethernet
switches
VLANS
55 link virtualization MPLS
56 data center networking
57 a day in the life of a web request
Link Layer 5-61
Ethernet switch link-layer device takes an active role
store forward Ethernet frames
examine incoming framersquos MAC address selectively forward frame to one-or-more outgoing links when frame is to be forwarded on segment uses CSMACD to access segment
transparent
hosts are unaware of presence of switches
plug-and-play self-learning
switches do not need to be configured
Link Layer 5-62
Switch multiple simultaneous transmissions
hosts have dedicated direct connection to switch
switches buffer packets
Ethernet protocol used on each incoming link but no collisions full duplex
each link is its own collision domain
switching A-to-Arsquo and B-to-Brsquo can transmit simultaneously without collisions switch with six interfaces
(123456)
A
Arsquo
B
Brsquo C
Crsquo
1 2
3 4 5
6
Link Layer 5-63
Switch forwarding table
Q how does switch know Arsquo reachable via interface 4 Brsquo reachable via interface 5
switch with six interfaces
(123456)
A
Arsquo
B
Brsquo C
Crsquo
1 2
3 4 5
6 A each switch has a switch table each entry
(MAC address of host interface to
reach host time stamp)
looks like a routing table
Q how are entries created maintained in switch table
something like a routing protocol
A
Arsquo
B
Brsquo C
Crsquo
1 2
3 4 5
6
Link Layer 5-64
Switch self-learning
switch learns which hosts can be reached through which interfaces
when frame received switch ldquolearnsrdquo location of sender incoming LAN segment
records senderlocation pair in switch table
A Arsquo
Source A
Dest Arsquo
MAC addr interface TTL
Switch table
(initially empty) A 1 60
Link Layer 5-65
Switch frame filteringforwarding
when frame received at switch 1 record incoming link MAC address of sending host
2 index switch table using MAC destination address
3 if entry found for destination then
if destination on segment from which frame arrived then drop frame
else forward frame on interface indicated by entry
else flood forward on all interfaces except arriving
interface
A
Arsquo
B
Brsquo C
Crsquo
1 2
3 4 5
6
Link Layer 5-66
Self-learning forwarding example
A Arsquo
Source A
Dest Arsquo
MAC addr interface TTL
switch table
(initially empty) A 1 60
A Arsquo A Arsquo A Arsquo A Arsquo A Arsquo
frame destination Arsquo locaton unknown flood
Arsquo A
destination A location
known
Arsquo 4 60
selectively send
on just one link
Link Layer 5-67
Interconnecting switches
switches can be connected together
Q sending from A to G - how does S1 know to forward frame destined to F via S4 and S3
A self learning (works exactly the same as in single-switch case)
A
B
S1
C D
E
F
S2
S4
S3
H
I
G
Link Layer 5-68
Self-learning multi-switch example
Suppose C sends frame to I I responds to C
Q show switch tables and packet forwarding in S1 S2 S3 S4
A
B
S1
C D
E
F
S2
S4
S3
H
I
G
Link Layer 5-69
Institutional network
to external
network
router
IP subnet
mail server
web server
Link Layer 5-70
Switches vs routers
both are store-and-forward
routers network-layer devices (examine network-layer headers)
switches link-layer devices (examine link-layer headers)
both have forwarding tables
routers compute tables using routing algorithms IP addresses
switches learn forwarding table using flooding learning MAC addresses
application
transport
network
link
physical
network
link
physical
link
physical
switch
datagram
application
transport
network
link
physical
frame
frame
frame
datagram
Link Layer 5-71
VLANs motivation
consider CS user moves office to
EE but wants connect to CS switch
single broadcast domain
all layer-2 broadcast traffic (ARP DHCP unknown location of destination MAC address) must cross entire LAN
securityprivacy efficiency issues
Computer
Science Electrical
Engineering
Computer
Engineering
Link Layer 5-72
VLANs port-based VLAN switch ports
grouped (by switch management software) so that single physical switch helliphellip
switch(es) supporting
VLAN capabilities can
be configured to
define multiple virtual
LANS over single
physical LAN
infrastructure
Virtual Local
Area Network 1
8
9
16 10 2
7
hellip
Electrical Engineering
(VLAN ports 1-8)
Computer Science
(VLAN ports 9-15)
15
hellip
Electrical Engineering
(VLAN ports 1-8)
hellip
1
8 2
7 9
16 10
15
hellip
Computer Science
(VLAN ports 9-16)
hellip operates as multiple virtual switches
Link Layer 5-73
Port-based VLAN
1
8
9
16 10 2
7
hellip
Electrical Engineering
(VLAN ports 1-8)
Computer Science
(VLAN ports 9-15)
15
hellip
traffic isolation frames tofrom ports 1-8 can only reach ports 1-8 can also define VLAN based on
MAC addresses of endpoints rather than switch port
dynamic membership ports can be dynamically assigned among VLANs
router
forwarding between VLANS done via routing (just as with separate switches) in practice vendors sell combined
switches plus routers
Link Layer 5-74
VLANS spanning multiple switches
trunk port carries frames between VLANS defined over multiple physical switches frames forwarded within VLAN between switches canrsquot be vanilla
8021 frames (must carry VLAN ID info)
8021q protocol addsremoved additional header fields for frames forwarded between trunk ports
1
8
9
10 2
7
hellip
Electrical Engineering
(VLAN ports 1-8)
Computer Science
(VLAN ports 9-15)
15
hellip
2
7 3
Ports 235 belong to EE VLAN
Ports 4678 belong to CS VLAN
5
4 6 8 16
1
Link Layer 5-75
type
2-byte Tag Protocol Identifier
(value 81-00)
Tag Control Information (12 bit VLAN ID field
3 bit priority field like IP TOS)
Recomputed CRC
8021Q VLAN frame format
8021 frame
8021Q frame
dest address
source address
data (payload) CRC preamble
dest address
source address
preamble data (payload) CRC
type
Link Layer 5-76
Link layer LANs outline
51 introduction services
52 error detection correction
53 multiple access protocols
54 LANs addressing ARP
Ethernet
switches
VLANS
55 link virtualization MPLS
56 data center networking
57 a day in the life of a web request
Link Layer 5-77
Multiprotocol label switching (MPLS)
initial goal high-speed IP forwarding using fixed length label (instead of IP address) fast lookup using fixed length identifier (rather than
shortest prefix matching)
borrowing ideas from Virtual Circuit (VC) approach
but IP datagram still keeps IP address
PPP or Ethernet
header IP header remainder of link-layer frame MPLS header
label Exp S TTL
20 3 1 5
Link Layer 5-78
MPLS capable routers
aka label-switched router
forward packets to outgoing interface based only on label value (donrsquot inspect IP address) MPLS forwarding table distinct from IP forwarding tables
flexibility MPLS forwarding decisions can differ from those of IP use destination and source addresses to route flows to
same destination differently (traffic engineering)
re-route flows quickly if link fails pre-computed backup paths (useful for VoIP)
Link Layer 5-79
R2
D
R3 R5
A
R6
MPLS versus IP paths
IP router IP routing path to destination determined
by destination address alone
R4
Link Layer 5-80
R2
D
R3 R4
R5
A
R6
MPLS versus IP paths
IP-only router
IP routing path to destination determined by destination address alone
MPLS and IP router
MPLS routing path to destination can be based on source and dest address fast reroute precompute backup routes in
case of link failure
entry router (R4) can use different MPLS
routes to A based eg on source address
Link Layer 5-81
MPLS signaling
modify OSPF IS-IS link-state flooding protocols to carry info used by MPLS routing eg link bandwidth amount of ldquoreservedrdquo link bandwidth
D
R4
R5
A
R6
entry MPLS router uses RSVP-TE signaling protocol to set up MPLS forwarding at downstream routers
modified link state flooding
RSVP-TE
Link Layer 5-82
R1 R2
D
R3 R4 R5
0
1
0 0
A
R6
in out out
label label dest interface
6 - A 0
in out out
label label dest interface
10 6 A 1
12 9 D 0
in out out
label label dest interface
10 A 0
12 D 0
1
in out out
label label dest interface
8 6 A 0
0
8 A 1
MPLS forwarding tables
Link Layer 5-83
Link layer LANs outline
51 introduction services
52 error detection correction
53 multiple access protocols
54 LANs addressing ARP
Ethernet
switches
VLANS
55 link virtualization MPLS
56 data center networking
57 a day in the life of a web request
Link Layer 5-84
Data center networks
10rsquos to 100rsquos of thousands of hosts often closely coupled in close proximity e-business (eg Amazon)
content-servers (eg YouTube Akamai Apple Microsoft)
search engines data mining (eg Google)
challenges
multiple applications each serving massive numbers of clients
managingbalancing load avoiding processing networking data bottlenecks
Inside a 40-ft Microsoft container
Chicago data center
Link Layer 5-85
Server racks
TOR switches
Tier-1 switches
Tier-2 switches
Load balancer
Load balancer
B
1 2 3 4 5 6 7 8
A C
Border router
Access router
Internet
Data center networks
load balancer application-layer routing receives external client requests
directs workload within data center
returns results to external client (hiding data
center internals from client)
Server racks
TOR switches
Tier-1 switches
Tier-2 switches
1 2 3 4 5 6 7 8
Data center networks
rich interconnection among switches racks
increased throughput between racks (multiple routing
paths possible)
increased reliability via redundancy
Link Layer 5-87
Link layer LANs outline
51 introduction services
52 error detection correction
53 multiple access protocols
54 LANs addressing ARP
Ethernet
switches
VLANS
55 link virtualization MPLS
56 data center networking
57 a day in the life of a web request
Link Layer 5-88
Synthesis a day in the life of a web request
journey down protocol stack complete application transport network link
putting-it-all-together synthesis goal identify review understand protocols (at all
layers) involved in seemingly simple scenario requesting www page
scenario student attaches laptop to campus network requestsreceives wwwgooglecom
Link Layer 5-89
A day in the life scenario
Comcast network
68800013
Googlersquos network
64233160019 64233169105
web server
DNS server
school network
68802024
web page
browser
router
(runs DHCP)
Link Layer 5-90
A day in the lifehellip connecting to the Internet
connecting laptop needs to get its own IP address addr of first-hop router addr of DNS server use DHCP
DHCP
UDP
IP
Eth
Phy
DHCP
DHCP
DHCP
DHCP
DHCP
DHCP
UDP
IP
Eth
Phy
DHCP
DHCP
DHCP
DHCP DHCP
DHCP request encapsulated in UDP encapsulated in IP encapsulated in 8023 Ethernet
Ethernet frame broadcast
(dest FFFFFFFFFFFF) on LAN received at router running DHCP server
Ethernet demuxed to IP demuxed UDP demuxed to DHCP
router
(runs DHCP)
Link Layer 5-91
DHCP server formulates DHCP ACK containing clientrsquos IP address IP address of first-hop router for client name amp IP address of DNS server
DHCP
UDP
IP
Eth
Phy
DHCP
DHCP
DHCP
DHCP
DHCP
UDP
IP
Eth
Phy
DHCP
DHCP
DHCP
DHCP
DHCP
encapsulation at DHCP server frame forwarded (switch learning) through LAN demultiplexing at client
Client now has IP address knows name amp addr of DNS
server IP address of its first-hop router
DHCP client receives DHCP ACK reply
A day in the lifehellip connecting to the Internet
router
(runs DHCP)
Link Layer 5-92
A day in the lifehellip ARP (before DNS before HTTP)
before sending HTTP request need IP address of wwwgooglecom DNS
DNS
UDP
IP
Eth
Phy
DNS
DNS
DNS
DNS query created encapsulated in UDP encapsulated in IP encapsulated in Eth To send frame to router need MAC address of router interface ARP
ARP query broadcast received by
router which replies with ARP reply giving MAC address of router interface
client now knows MAC address of first hop router so can now send frame containing DNS query
ARP query
Eth
Phy
ARP
ARP
ARP reply
router
(runs DHCP)
Link Layer 5-93
DNS
UDP
IP
Eth
Phy
DNS
DNS
DNS
DNS
DNS
IP datagram containing DNS query forwarded via LAN switch from client to 1st hop router
IP datagram forwarded from campus network into comcast network routed (tables created by RIP OSPF IS-IS andor BGP routing protocols) to DNS server
demuxrsquoed to DNS server
DNS server replies to client with IP address of wwwgooglecom
Comcast network
68800013
DNS server
DNS
UDP
IP
Eth
Phy
DNS
DNS
DNS
DNS
A day in the lifehellip using DNS
router
(runs DHCP)
Link Layer 5-94
A day in the lifehellipTCP connection carrying HTTP
HTTP
TCP
IP
Eth
Phy
HTTP
to send HTTP request client first opens TCP socket to web server
TCP SYN segment (step 1 in 3-way handshake) inter-domain routed to web server
TCP connection established 64233169105
web server
SYN
SYN
SYN
SYN
TCP
IP
Eth
Phy
SYN
SYN
SYN
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
web server responds with TCP SYNACK (step 2 in 3-way handshake)
router
(runs DHCP)
Link Layer 5-95
A day in the lifehellip HTTP requestreply
HTTP
TCP
IP
Eth
Phy
HTTP
HTTP request sent into TCP socket
IP datagram containing HTTP request routed to wwwgooglecom
IP datagram containing HTTP reply routed back to client
64233169105
web server
HTTP
TCP
IP
Eth
Phy web server responds with
HTTP reply (containing web page)
HTTP
HTTP
HTTP HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
web page finally () displayed
Link Layer 5-96
Chapter 5 Summary
principles behind data link layer services error detection correction
sharing a broadcast channel multiple access
link layer addressing
instantiation and implementation of various link layer technologies Ethernet
switched LANS VLANs
virtualized networks as a link layer MPLS
synthesis a day in the life of a web request
Link Layer 5-97
Chapter 5 letrsquos take a breath
journey down protocol stack complete (except PHY)
solid understanding of networking principles practice
hellip could stop here hellip but lots of interesting topics wireless
multimedia
security
network management
Link Layer 5-61
Ethernet switch link-layer device takes an active role
store forward Ethernet frames
examine incoming framersquos MAC address selectively forward frame to one-or-more outgoing links when frame is to be forwarded on segment uses CSMACD to access segment
transparent
hosts are unaware of presence of switches
plug-and-play self-learning
switches do not need to be configured
Link Layer 5-62
Switch multiple simultaneous transmissions
hosts have dedicated direct connection to switch
switches buffer packets
Ethernet protocol used on each incoming link but no collisions full duplex
each link is its own collision domain
switching A-to-Arsquo and B-to-Brsquo can transmit simultaneously without collisions switch with six interfaces
(123456)
A
Arsquo
B
Brsquo C
Crsquo
1 2
3 4 5
6
Link Layer 5-63
Switch forwarding table
Q how does switch know Arsquo reachable via interface 4 Brsquo reachable via interface 5
switch with six interfaces
(123456)
A
Arsquo
B
Brsquo C
Crsquo
1 2
3 4 5
6 A each switch has a switch table each entry
(MAC address of host interface to
reach host time stamp)
looks like a routing table
Q how are entries created maintained in switch table
something like a routing protocol
A
Arsquo
B
Brsquo C
Crsquo
1 2
3 4 5
6
Link Layer 5-64
Switch self-learning
switch learns which hosts can be reached through which interfaces
when frame received switch ldquolearnsrdquo location of sender incoming LAN segment
records senderlocation pair in switch table
A Arsquo
Source A
Dest Arsquo
MAC addr interface TTL
Switch table
(initially empty) A 1 60
Link Layer 5-65
Switch frame filteringforwarding
when frame received at switch 1 record incoming link MAC address of sending host
2 index switch table using MAC destination address
3 if entry found for destination then
if destination on segment from which frame arrived then drop frame
else forward frame on interface indicated by entry
else flood forward on all interfaces except arriving
interface
A
Arsquo
B
Brsquo C
Crsquo
1 2
3 4 5
6
Link Layer 5-66
Self-learning forwarding example
A Arsquo
Source A
Dest Arsquo
MAC addr interface TTL
switch table
(initially empty) A 1 60
A Arsquo A Arsquo A Arsquo A Arsquo A Arsquo
frame destination Arsquo locaton unknown flood
Arsquo A
destination A location
known
Arsquo 4 60
selectively send
on just one link
Link Layer 5-67
Interconnecting switches
switches can be connected together
Q sending from A to G - how does S1 know to forward frame destined to F via S4 and S3
A self learning (works exactly the same as in single-switch case)
A
B
S1
C D
E
F
S2
S4
S3
H
I
G
Link Layer 5-68
Self-learning multi-switch example
Suppose C sends frame to I I responds to C
Q show switch tables and packet forwarding in S1 S2 S3 S4
A
B
S1
C D
E
F
S2
S4
S3
H
I
G
Link Layer 5-69
Institutional network
to external
network
router
IP subnet
mail server
web server
Link Layer 5-70
Switches vs routers
both are store-and-forward
routers network-layer devices (examine network-layer headers)
switches link-layer devices (examine link-layer headers)
both have forwarding tables
routers compute tables using routing algorithms IP addresses
switches learn forwarding table using flooding learning MAC addresses
application
transport
network
link
physical
network
link
physical
link
physical
switch
datagram
application
transport
network
link
physical
frame
frame
frame
datagram
Link Layer 5-71
VLANs motivation
consider CS user moves office to
EE but wants connect to CS switch
single broadcast domain
all layer-2 broadcast traffic (ARP DHCP unknown location of destination MAC address) must cross entire LAN
securityprivacy efficiency issues
Computer
Science Electrical
Engineering
Computer
Engineering
Link Layer 5-72
VLANs port-based VLAN switch ports
grouped (by switch management software) so that single physical switch helliphellip
switch(es) supporting
VLAN capabilities can
be configured to
define multiple virtual
LANS over single
physical LAN
infrastructure
Virtual Local
Area Network 1
8
9
16 10 2
7
hellip
Electrical Engineering
(VLAN ports 1-8)
Computer Science
(VLAN ports 9-15)
15
hellip
Electrical Engineering
(VLAN ports 1-8)
hellip
1
8 2
7 9
16 10
15
hellip
Computer Science
(VLAN ports 9-16)
hellip operates as multiple virtual switches
Link Layer 5-73
Port-based VLAN
1
8
9
16 10 2
7
hellip
Electrical Engineering
(VLAN ports 1-8)
Computer Science
(VLAN ports 9-15)
15
hellip
traffic isolation frames tofrom ports 1-8 can only reach ports 1-8 can also define VLAN based on
MAC addresses of endpoints rather than switch port
dynamic membership ports can be dynamically assigned among VLANs
router
forwarding between VLANS done via routing (just as with separate switches) in practice vendors sell combined
switches plus routers
Link Layer 5-74
VLANS spanning multiple switches
trunk port carries frames between VLANS defined over multiple physical switches frames forwarded within VLAN between switches canrsquot be vanilla
8021 frames (must carry VLAN ID info)
8021q protocol addsremoved additional header fields for frames forwarded between trunk ports
1
8
9
10 2
7
hellip
Electrical Engineering
(VLAN ports 1-8)
Computer Science
(VLAN ports 9-15)
15
hellip
2
7 3
Ports 235 belong to EE VLAN
Ports 4678 belong to CS VLAN
5
4 6 8 16
1
Link Layer 5-75
type
2-byte Tag Protocol Identifier
(value 81-00)
Tag Control Information (12 bit VLAN ID field
3 bit priority field like IP TOS)
Recomputed CRC
8021Q VLAN frame format
8021 frame
8021Q frame
dest address
source address
data (payload) CRC preamble
dest address
source address
preamble data (payload) CRC
type
Link Layer 5-76
Link layer LANs outline
51 introduction services
52 error detection correction
53 multiple access protocols
54 LANs addressing ARP
Ethernet
switches
VLANS
55 link virtualization MPLS
56 data center networking
57 a day in the life of a web request
Link Layer 5-77
Multiprotocol label switching (MPLS)
initial goal high-speed IP forwarding using fixed length label (instead of IP address) fast lookup using fixed length identifier (rather than
shortest prefix matching)
borrowing ideas from Virtual Circuit (VC) approach
but IP datagram still keeps IP address
PPP or Ethernet
header IP header remainder of link-layer frame MPLS header
label Exp S TTL
20 3 1 5
Link Layer 5-78
MPLS capable routers
aka label-switched router
forward packets to outgoing interface based only on label value (donrsquot inspect IP address) MPLS forwarding table distinct from IP forwarding tables
flexibility MPLS forwarding decisions can differ from those of IP use destination and source addresses to route flows to
same destination differently (traffic engineering)
re-route flows quickly if link fails pre-computed backup paths (useful for VoIP)
Link Layer 5-79
R2
D
R3 R5
A
R6
MPLS versus IP paths
IP router IP routing path to destination determined
by destination address alone
R4
Link Layer 5-80
R2
D
R3 R4
R5
A
R6
MPLS versus IP paths
IP-only router
IP routing path to destination determined by destination address alone
MPLS and IP router
MPLS routing path to destination can be based on source and dest address fast reroute precompute backup routes in
case of link failure
entry router (R4) can use different MPLS
routes to A based eg on source address
Link Layer 5-81
MPLS signaling
modify OSPF IS-IS link-state flooding protocols to carry info used by MPLS routing eg link bandwidth amount of ldquoreservedrdquo link bandwidth
D
R4
R5
A
R6
entry MPLS router uses RSVP-TE signaling protocol to set up MPLS forwarding at downstream routers
modified link state flooding
RSVP-TE
Link Layer 5-82
R1 R2
D
R3 R4 R5
0
1
0 0
A
R6
in out out
label label dest interface
6 - A 0
in out out
label label dest interface
10 6 A 1
12 9 D 0
in out out
label label dest interface
10 A 0
12 D 0
1
in out out
label label dest interface
8 6 A 0
0
8 A 1
MPLS forwarding tables
Link Layer 5-83
Link layer LANs outline
51 introduction services
52 error detection correction
53 multiple access protocols
54 LANs addressing ARP
Ethernet
switches
VLANS
55 link virtualization MPLS
56 data center networking
57 a day in the life of a web request
Link Layer 5-84
Data center networks
10rsquos to 100rsquos of thousands of hosts often closely coupled in close proximity e-business (eg Amazon)
content-servers (eg YouTube Akamai Apple Microsoft)
search engines data mining (eg Google)
challenges
multiple applications each serving massive numbers of clients
managingbalancing load avoiding processing networking data bottlenecks
Inside a 40-ft Microsoft container
Chicago data center
Link Layer 5-85
Server racks
TOR switches
Tier-1 switches
Tier-2 switches
Load balancer
Load balancer
B
1 2 3 4 5 6 7 8
A C
Border router
Access router
Internet
Data center networks
load balancer application-layer routing receives external client requests
directs workload within data center
returns results to external client (hiding data
center internals from client)
Server racks
TOR switches
Tier-1 switches
Tier-2 switches
1 2 3 4 5 6 7 8
Data center networks
rich interconnection among switches racks
increased throughput between racks (multiple routing
paths possible)
increased reliability via redundancy
Link Layer 5-87
Link layer LANs outline
51 introduction services
52 error detection correction
53 multiple access protocols
54 LANs addressing ARP
Ethernet
switches
VLANS
55 link virtualization MPLS
56 data center networking
57 a day in the life of a web request
Link Layer 5-88
Synthesis a day in the life of a web request
journey down protocol stack complete application transport network link
putting-it-all-together synthesis goal identify review understand protocols (at all
layers) involved in seemingly simple scenario requesting www page
scenario student attaches laptop to campus network requestsreceives wwwgooglecom
Link Layer 5-89
A day in the life scenario
Comcast network
68800013
Googlersquos network
64233160019 64233169105
web server
DNS server
school network
68802024
web page
browser
router
(runs DHCP)
Link Layer 5-90
A day in the lifehellip connecting to the Internet
connecting laptop needs to get its own IP address addr of first-hop router addr of DNS server use DHCP
DHCP
UDP
IP
Eth
Phy
DHCP
DHCP
DHCP
DHCP
DHCP
DHCP
UDP
IP
Eth
Phy
DHCP
DHCP
DHCP
DHCP DHCP
DHCP request encapsulated in UDP encapsulated in IP encapsulated in 8023 Ethernet
Ethernet frame broadcast
(dest FFFFFFFFFFFF) on LAN received at router running DHCP server
Ethernet demuxed to IP demuxed UDP demuxed to DHCP
router
(runs DHCP)
Link Layer 5-91
DHCP server formulates DHCP ACK containing clientrsquos IP address IP address of first-hop router for client name amp IP address of DNS server
DHCP
UDP
IP
Eth
Phy
DHCP
DHCP
DHCP
DHCP
DHCP
UDP
IP
Eth
Phy
DHCP
DHCP
DHCP
DHCP
DHCP
encapsulation at DHCP server frame forwarded (switch learning) through LAN demultiplexing at client
Client now has IP address knows name amp addr of DNS
server IP address of its first-hop router
DHCP client receives DHCP ACK reply
A day in the lifehellip connecting to the Internet
router
(runs DHCP)
Link Layer 5-92
A day in the lifehellip ARP (before DNS before HTTP)
before sending HTTP request need IP address of wwwgooglecom DNS
DNS
UDP
IP
Eth
Phy
DNS
DNS
DNS
DNS query created encapsulated in UDP encapsulated in IP encapsulated in Eth To send frame to router need MAC address of router interface ARP
ARP query broadcast received by
router which replies with ARP reply giving MAC address of router interface
client now knows MAC address of first hop router so can now send frame containing DNS query
ARP query
Eth
Phy
ARP
ARP
ARP reply
router
(runs DHCP)
Link Layer 5-93
DNS
UDP
IP
Eth
Phy
DNS
DNS
DNS
DNS
DNS
IP datagram containing DNS query forwarded via LAN switch from client to 1st hop router
IP datagram forwarded from campus network into comcast network routed (tables created by RIP OSPF IS-IS andor BGP routing protocols) to DNS server
demuxrsquoed to DNS server
DNS server replies to client with IP address of wwwgooglecom
Comcast network
68800013
DNS server
DNS
UDP
IP
Eth
Phy
DNS
DNS
DNS
DNS
A day in the lifehellip using DNS
router
(runs DHCP)
Link Layer 5-94
A day in the lifehellipTCP connection carrying HTTP
HTTP
TCP
IP
Eth
Phy
HTTP
to send HTTP request client first opens TCP socket to web server
TCP SYN segment (step 1 in 3-way handshake) inter-domain routed to web server
TCP connection established 64233169105
web server
SYN
SYN
SYN
SYN
TCP
IP
Eth
Phy
SYN
SYN
SYN
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
web server responds with TCP SYNACK (step 2 in 3-way handshake)
router
(runs DHCP)
Link Layer 5-95
A day in the lifehellip HTTP requestreply
HTTP
TCP
IP
Eth
Phy
HTTP
HTTP request sent into TCP socket
IP datagram containing HTTP request routed to wwwgooglecom
IP datagram containing HTTP reply routed back to client
64233169105
web server
HTTP
TCP
IP
Eth
Phy web server responds with
HTTP reply (containing web page)
HTTP
HTTP
HTTP HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
web page finally () displayed
Link Layer 5-96
Chapter 5 Summary
principles behind data link layer services error detection correction
sharing a broadcast channel multiple access
link layer addressing
instantiation and implementation of various link layer technologies Ethernet
switched LANS VLANs
virtualized networks as a link layer MPLS
synthesis a day in the life of a web request
Link Layer 5-97
Chapter 5 letrsquos take a breath
journey down protocol stack complete (except PHY)
solid understanding of networking principles practice
hellip could stop here hellip but lots of interesting topics wireless
multimedia
security
network management
Link Layer 5-62
Switch multiple simultaneous transmissions
hosts have dedicated direct connection to switch
switches buffer packets
Ethernet protocol used on each incoming link but no collisions full duplex
each link is its own collision domain
switching A-to-Arsquo and B-to-Brsquo can transmit simultaneously without collisions switch with six interfaces
(123456)
A
Arsquo
B
Brsquo C
Crsquo
1 2
3 4 5
6
Link Layer 5-63
Switch forwarding table
Q how does switch know Arsquo reachable via interface 4 Brsquo reachable via interface 5
switch with six interfaces
(123456)
A
Arsquo
B
Brsquo C
Crsquo
1 2
3 4 5
6 A each switch has a switch table each entry
(MAC address of host interface to
reach host time stamp)
looks like a routing table
Q how are entries created maintained in switch table
something like a routing protocol
A
Arsquo
B
Brsquo C
Crsquo
1 2
3 4 5
6
Link Layer 5-64
Switch self-learning
switch learns which hosts can be reached through which interfaces
when frame received switch ldquolearnsrdquo location of sender incoming LAN segment
records senderlocation pair in switch table
A Arsquo
Source A
Dest Arsquo
MAC addr interface TTL
Switch table
(initially empty) A 1 60
Link Layer 5-65
Switch frame filteringforwarding
when frame received at switch 1 record incoming link MAC address of sending host
2 index switch table using MAC destination address
3 if entry found for destination then
if destination on segment from which frame arrived then drop frame
else forward frame on interface indicated by entry
else flood forward on all interfaces except arriving
interface
A
Arsquo
B
Brsquo C
Crsquo
1 2
3 4 5
6
Link Layer 5-66
Self-learning forwarding example
A Arsquo
Source A
Dest Arsquo
MAC addr interface TTL
switch table
(initially empty) A 1 60
A Arsquo A Arsquo A Arsquo A Arsquo A Arsquo
frame destination Arsquo locaton unknown flood
Arsquo A
destination A location
known
Arsquo 4 60
selectively send
on just one link
Link Layer 5-67
Interconnecting switches
switches can be connected together
Q sending from A to G - how does S1 know to forward frame destined to F via S4 and S3
A self learning (works exactly the same as in single-switch case)
A
B
S1
C D
E
F
S2
S4
S3
H
I
G
Link Layer 5-68
Self-learning multi-switch example
Suppose C sends frame to I I responds to C
Q show switch tables and packet forwarding in S1 S2 S3 S4
A
B
S1
C D
E
F
S2
S4
S3
H
I
G
Link Layer 5-69
Institutional network
to external
network
router
IP subnet
mail server
web server
Link Layer 5-70
Switches vs routers
both are store-and-forward
routers network-layer devices (examine network-layer headers)
switches link-layer devices (examine link-layer headers)
both have forwarding tables
routers compute tables using routing algorithms IP addresses
switches learn forwarding table using flooding learning MAC addresses
application
transport
network
link
physical
network
link
physical
link
physical
switch
datagram
application
transport
network
link
physical
frame
frame
frame
datagram
Link Layer 5-71
VLANs motivation
consider CS user moves office to
EE but wants connect to CS switch
single broadcast domain
all layer-2 broadcast traffic (ARP DHCP unknown location of destination MAC address) must cross entire LAN
securityprivacy efficiency issues
Computer
Science Electrical
Engineering
Computer
Engineering
Link Layer 5-72
VLANs port-based VLAN switch ports
grouped (by switch management software) so that single physical switch helliphellip
switch(es) supporting
VLAN capabilities can
be configured to
define multiple virtual
LANS over single
physical LAN
infrastructure
Virtual Local
Area Network 1
8
9
16 10 2
7
hellip
Electrical Engineering
(VLAN ports 1-8)
Computer Science
(VLAN ports 9-15)
15
hellip
Electrical Engineering
(VLAN ports 1-8)
hellip
1
8 2
7 9
16 10
15
hellip
Computer Science
(VLAN ports 9-16)
hellip operates as multiple virtual switches
Link Layer 5-73
Port-based VLAN
1
8
9
16 10 2
7
hellip
Electrical Engineering
(VLAN ports 1-8)
Computer Science
(VLAN ports 9-15)
15
hellip
traffic isolation frames tofrom ports 1-8 can only reach ports 1-8 can also define VLAN based on
MAC addresses of endpoints rather than switch port
dynamic membership ports can be dynamically assigned among VLANs
router
forwarding between VLANS done via routing (just as with separate switches) in practice vendors sell combined
switches plus routers
Link Layer 5-74
VLANS spanning multiple switches
trunk port carries frames between VLANS defined over multiple physical switches frames forwarded within VLAN between switches canrsquot be vanilla
8021 frames (must carry VLAN ID info)
8021q protocol addsremoved additional header fields for frames forwarded between trunk ports
1
8
9
10 2
7
hellip
Electrical Engineering
(VLAN ports 1-8)
Computer Science
(VLAN ports 9-15)
15
hellip
2
7 3
Ports 235 belong to EE VLAN
Ports 4678 belong to CS VLAN
5
4 6 8 16
1
Link Layer 5-75
type
2-byte Tag Protocol Identifier
(value 81-00)
Tag Control Information (12 bit VLAN ID field
3 bit priority field like IP TOS)
Recomputed CRC
8021Q VLAN frame format
8021 frame
8021Q frame
dest address
source address
data (payload) CRC preamble
dest address
source address
preamble data (payload) CRC
type
Link Layer 5-76
Link layer LANs outline
51 introduction services
52 error detection correction
53 multiple access protocols
54 LANs addressing ARP
Ethernet
switches
VLANS
55 link virtualization MPLS
56 data center networking
57 a day in the life of a web request
Link Layer 5-77
Multiprotocol label switching (MPLS)
initial goal high-speed IP forwarding using fixed length label (instead of IP address) fast lookup using fixed length identifier (rather than
shortest prefix matching)
borrowing ideas from Virtual Circuit (VC) approach
but IP datagram still keeps IP address
PPP or Ethernet
header IP header remainder of link-layer frame MPLS header
label Exp S TTL
20 3 1 5
Link Layer 5-78
MPLS capable routers
aka label-switched router
forward packets to outgoing interface based only on label value (donrsquot inspect IP address) MPLS forwarding table distinct from IP forwarding tables
flexibility MPLS forwarding decisions can differ from those of IP use destination and source addresses to route flows to
same destination differently (traffic engineering)
re-route flows quickly if link fails pre-computed backup paths (useful for VoIP)
Link Layer 5-79
R2
D
R3 R5
A
R6
MPLS versus IP paths
IP router IP routing path to destination determined
by destination address alone
R4
Link Layer 5-80
R2
D
R3 R4
R5
A
R6
MPLS versus IP paths
IP-only router
IP routing path to destination determined by destination address alone
MPLS and IP router
MPLS routing path to destination can be based on source and dest address fast reroute precompute backup routes in
case of link failure
entry router (R4) can use different MPLS
routes to A based eg on source address
Link Layer 5-81
MPLS signaling
modify OSPF IS-IS link-state flooding protocols to carry info used by MPLS routing eg link bandwidth amount of ldquoreservedrdquo link bandwidth
D
R4
R5
A
R6
entry MPLS router uses RSVP-TE signaling protocol to set up MPLS forwarding at downstream routers
modified link state flooding
RSVP-TE
Link Layer 5-82
R1 R2
D
R3 R4 R5
0
1
0 0
A
R6
in out out
label label dest interface
6 - A 0
in out out
label label dest interface
10 6 A 1
12 9 D 0
in out out
label label dest interface
10 A 0
12 D 0
1
in out out
label label dest interface
8 6 A 0
0
8 A 1
MPLS forwarding tables
Link Layer 5-83
Link layer LANs outline
51 introduction services
52 error detection correction
53 multiple access protocols
54 LANs addressing ARP
Ethernet
switches
VLANS
55 link virtualization MPLS
56 data center networking
57 a day in the life of a web request
Link Layer 5-84
Data center networks
10rsquos to 100rsquos of thousands of hosts often closely coupled in close proximity e-business (eg Amazon)
content-servers (eg YouTube Akamai Apple Microsoft)
search engines data mining (eg Google)
challenges
multiple applications each serving massive numbers of clients
managingbalancing load avoiding processing networking data bottlenecks
Inside a 40-ft Microsoft container
Chicago data center
Link Layer 5-85
Server racks
TOR switches
Tier-1 switches
Tier-2 switches
Load balancer
Load balancer
B
1 2 3 4 5 6 7 8
A C
Border router
Access router
Internet
Data center networks
load balancer application-layer routing receives external client requests
directs workload within data center
returns results to external client (hiding data
center internals from client)
Server racks
TOR switches
Tier-1 switches
Tier-2 switches
1 2 3 4 5 6 7 8
Data center networks
rich interconnection among switches racks
increased throughput between racks (multiple routing
paths possible)
increased reliability via redundancy
Link Layer 5-87
Link layer LANs outline
51 introduction services
52 error detection correction
53 multiple access protocols
54 LANs addressing ARP
Ethernet
switches
VLANS
55 link virtualization MPLS
56 data center networking
57 a day in the life of a web request
Link Layer 5-88
Synthesis a day in the life of a web request
journey down protocol stack complete application transport network link
putting-it-all-together synthesis goal identify review understand protocols (at all
layers) involved in seemingly simple scenario requesting www page
scenario student attaches laptop to campus network requestsreceives wwwgooglecom
Link Layer 5-89
A day in the life scenario
Comcast network
68800013
Googlersquos network
64233160019 64233169105
web server
DNS server
school network
68802024
web page
browser
router
(runs DHCP)
Link Layer 5-90
A day in the lifehellip connecting to the Internet
connecting laptop needs to get its own IP address addr of first-hop router addr of DNS server use DHCP
DHCP
UDP
IP
Eth
Phy
DHCP
DHCP
DHCP
DHCP
DHCP
DHCP
UDP
IP
Eth
Phy
DHCP
DHCP
DHCP
DHCP DHCP
DHCP request encapsulated in UDP encapsulated in IP encapsulated in 8023 Ethernet
Ethernet frame broadcast
(dest FFFFFFFFFFFF) on LAN received at router running DHCP server
Ethernet demuxed to IP demuxed UDP demuxed to DHCP
router
(runs DHCP)
Link Layer 5-91
DHCP server formulates DHCP ACK containing clientrsquos IP address IP address of first-hop router for client name amp IP address of DNS server
DHCP
UDP
IP
Eth
Phy
DHCP
DHCP
DHCP
DHCP
DHCP
UDP
IP
Eth
Phy
DHCP
DHCP
DHCP
DHCP
DHCP
encapsulation at DHCP server frame forwarded (switch learning) through LAN demultiplexing at client
Client now has IP address knows name amp addr of DNS
server IP address of its first-hop router
DHCP client receives DHCP ACK reply
A day in the lifehellip connecting to the Internet
router
(runs DHCP)
Link Layer 5-92
A day in the lifehellip ARP (before DNS before HTTP)
before sending HTTP request need IP address of wwwgooglecom DNS
DNS
UDP
IP
Eth
Phy
DNS
DNS
DNS
DNS query created encapsulated in UDP encapsulated in IP encapsulated in Eth To send frame to router need MAC address of router interface ARP
ARP query broadcast received by
router which replies with ARP reply giving MAC address of router interface
client now knows MAC address of first hop router so can now send frame containing DNS query
ARP query
Eth
Phy
ARP
ARP
ARP reply
router
(runs DHCP)
Link Layer 5-93
DNS
UDP
IP
Eth
Phy
DNS
DNS
DNS
DNS
DNS
IP datagram containing DNS query forwarded via LAN switch from client to 1st hop router
IP datagram forwarded from campus network into comcast network routed (tables created by RIP OSPF IS-IS andor BGP routing protocols) to DNS server
demuxrsquoed to DNS server
DNS server replies to client with IP address of wwwgooglecom
Comcast network
68800013
DNS server
DNS
UDP
IP
Eth
Phy
DNS
DNS
DNS
DNS
A day in the lifehellip using DNS
router
(runs DHCP)
Link Layer 5-94
A day in the lifehellipTCP connection carrying HTTP
HTTP
TCP
IP
Eth
Phy
HTTP
to send HTTP request client first opens TCP socket to web server
TCP SYN segment (step 1 in 3-way handshake) inter-domain routed to web server
TCP connection established 64233169105
web server
SYN
SYN
SYN
SYN
TCP
IP
Eth
Phy
SYN
SYN
SYN
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
web server responds with TCP SYNACK (step 2 in 3-way handshake)
router
(runs DHCP)
Link Layer 5-95
A day in the lifehellip HTTP requestreply
HTTP
TCP
IP
Eth
Phy
HTTP
HTTP request sent into TCP socket
IP datagram containing HTTP request routed to wwwgooglecom
IP datagram containing HTTP reply routed back to client
64233169105
web server
HTTP
TCP
IP
Eth
Phy web server responds with
HTTP reply (containing web page)
HTTP
HTTP
HTTP HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
web page finally () displayed
Link Layer 5-96
Chapter 5 Summary
principles behind data link layer services error detection correction
sharing a broadcast channel multiple access
link layer addressing
instantiation and implementation of various link layer technologies Ethernet
switched LANS VLANs
virtualized networks as a link layer MPLS
synthesis a day in the life of a web request
Link Layer 5-97
Chapter 5 letrsquos take a breath
journey down protocol stack complete (except PHY)
solid understanding of networking principles practice
hellip could stop here hellip but lots of interesting topics wireless
multimedia
security
network management
Link Layer 5-63
Switch forwarding table
Q how does switch know Arsquo reachable via interface 4 Brsquo reachable via interface 5
switch with six interfaces
(123456)
A
Arsquo
B
Brsquo C
Crsquo
1 2
3 4 5
6 A each switch has a switch table each entry
(MAC address of host interface to
reach host time stamp)
looks like a routing table
Q how are entries created maintained in switch table
something like a routing protocol
A
Arsquo
B
Brsquo C
Crsquo
1 2
3 4 5
6
Link Layer 5-64
Switch self-learning
switch learns which hosts can be reached through which interfaces
when frame received switch ldquolearnsrdquo location of sender incoming LAN segment
records senderlocation pair in switch table
A Arsquo
Source A
Dest Arsquo
MAC addr interface TTL
Switch table
(initially empty) A 1 60
Link Layer 5-65
Switch frame filteringforwarding
when frame received at switch 1 record incoming link MAC address of sending host
2 index switch table using MAC destination address
3 if entry found for destination then
if destination on segment from which frame arrived then drop frame
else forward frame on interface indicated by entry
else flood forward on all interfaces except arriving
interface
A
Arsquo
B
Brsquo C
Crsquo
1 2
3 4 5
6
Link Layer 5-66
Self-learning forwarding example
A Arsquo
Source A
Dest Arsquo
MAC addr interface TTL
switch table
(initially empty) A 1 60
A Arsquo A Arsquo A Arsquo A Arsquo A Arsquo
frame destination Arsquo locaton unknown flood
Arsquo A
destination A location
known
Arsquo 4 60
selectively send
on just one link
Link Layer 5-67
Interconnecting switches
switches can be connected together
Q sending from A to G - how does S1 know to forward frame destined to F via S4 and S3
A self learning (works exactly the same as in single-switch case)
A
B
S1
C D
E
F
S2
S4
S3
H
I
G
Link Layer 5-68
Self-learning multi-switch example
Suppose C sends frame to I I responds to C
Q show switch tables and packet forwarding in S1 S2 S3 S4
A
B
S1
C D
E
F
S2
S4
S3
H
I
G
Link Layer 5-69
Institutional network
to external
network
router
IP subnet
mail server
web server
Link Layer 5-70
Switches vs routers
both are store-and-forward
routers network-layer devices (examine network-layer headers)
switches link-layer devices (examine link-layer headers)
both have forwarding tables
routers compute tables using routing algorithms IP addresses
switches learn forwarding table using flooding learning MAC addresses
application
transport
network
link
physical
network
link
physical
link
physical
switch
datagram
application
transport
network
link
physical
frame
frame
frame
datagram
Link Layer 5-71
VLANs motivation
consider CS user moves office to
EE but wants connect to CS switch
single broadcast domain
all layer-2 broadcast traffic (ARP DHCP unknown location of destination MAC address) must cross entire LAN
securityprivacy efficiency issues
Computer
Science Electrical
Engineering
Computer
Engineering
Link Layer 5-72
VLANs port-based VLAN switch ports
grouped (by switch management software) so that single physical switch helliphellip
switch(es) supporting
VLAN capabilities can
be configured to
define multiple virtual
LANS over single
physical LAN
infrastructure
Virtual Local
Area Network 1
8
9
16 10 2
7
hellip
Electrical Engineering
(VLAN ports 1-8)
Computer Science
(VLAN ports 9-15)
15
hellip
Electrical Engineering
(VLAN ports 1-8)
hellip
1
8 2
7 9
16 10
15
hellip
Computer Science
(VLAN ports 9-16)
hellip operates as multiple virtual switches
Link Layer 5-73
Port-based VLAN
1
8
9
16 10 2
7
hellip
Electrical Engineering
(VLAN ports 1-8)
Computer Science
(VLAN ports 9-15)
15
hellip
traffic isolation frames tofrom ports 1-8 can only reach ports 1-8 can also define VLAN based on
MAC addresses of endpoints rather than switch port
dynamic membership ports can be dynamically assigned among VLANs
router
forwarding between VLANS done via routing (just as with separate switches) in practice vendors sell combined
switches plus routers
Link Layer 5-74
VLANS spanning multiple switches
trunk port carries frames between VLANS defined over multiple physical switches frames forwarded within VLAN between switches canrsquot be vanilla
8021 frames (must carry VLAN ID info)
8021q protocol addsremoved additional header fields for frames forwarded between trunk ports
1
8
9
10 2
7
hellip
Electrical Engineering
(VLAN ports 1-8)
Computer Science
(VLAN ports 9-15)
15
hellip
2
7 3
Ports 235 belong to EE VLAN
Ports 4678 belong to CS VLAN
5
4 6 8 16
1
Link Layer 5-75
type
2-byte Tag Protocol Identifier
(value 81-00)
Tag Control Information (12 bit VLAN ID field
3 bit priority field like IP TOS)
Recomputed CRC
8021Q VLAN frame format
8021 frame
8021Q frame
dest address
source address
data (payload) CRC preamble
dest address
source address
preamble data (payload) CRC
type
Link Layer 5-76
Link layer LANs outline
51 introduction services
52 error detection correction
53 multiple access protocols
54 LANs addressing ARP
Ethernet
switches
VLANS
55 link virtualization MPLS
56 data center networking
57 a day in the life of a web request
Link Layer 5-77
Multiprotocol label switching (MPLS)
initial goal high-speed IP forwarding using fixed length label (instead of IP address) fast lookup using fixed length identifier (rather than
shortest prefix matching)
borrowing ideas from Virtual Circuit (VC) approach
but IP datagram still keeps IP address
PPP or Ethernet
header IP header remainder of link-layer frame MPLS header
label Exp S TTL
20 3 1 5
Link Layer 5-78
MPLS capable routers
aka label-switched router
forward packets to outgoing interface based only on label value (donrsquot inspect IP address) MPLS forwarding table distinct from IP forwarding tables
flexibility MPLS forwarding decisions can differ from those of IP use destination and source addresses to route flows to
same destination differently (traffic engineering)
re-route flows quickly if link fails pre-computed backup paths (useful for VoIP)
Link Layer 5-79
R2
D
R3 R5
A
R6
MPLS versus IP paths
IP router IP routing path to destination determined
by destination address alone
R4
Link Layer 5-80
R2
D
R3 R4
R5
A
R6
MPLS versus IP paths
IP-only router
IP routing path to destination determined by destination address alone
MPLS and IP router
MPLS routing path to destination can be based on source and dest address fast reroute precompute backup routes in
case of link failure
entry router (R4) can use different MPLS
routes to A based eg on source address
Link Layer 5-81
MPLS signaling
modify OSPF IS-IS link-state flooding protocols to carry info used by MPLS routing eg link bandwidth amount of ldquoreservedrdquo link bandwidth
D
R4
R5
A
R6
entry MPLS router uses RSVP-TE signaling protocol to set up MPLS forwarding at downstream routers
modified link state flooding
RSVP-TE
Link Layer 5-82
R1 R2
D
R3 R4 R5
0
1
0 0
A
R6
in out out
label label dest interface
6 - A 0
in out out
label label dest interface
10 6 A 1
12 9 D 0
in out out
label label dest interface
10 A 0
12 D 0
1
in out out
label label dest interface
8 6 A 0
0
8 A 1
MPLS forwarding tables
Link Layer 5-83
Link layer LANs outline
51 introduction services
52 error detection correction
53 multiple access protocols
54 LANs addressing ARP
Ethernet
switches
VLANS
55 link virtualization MPLS
56 data center networking
57 a day in the life of a web request
Link Layer 5-84
Data center networks
10rsquos to 100rsquos of thousands of hosts often closely coupled in close proximity e-business (eg Amazon)
content-servers (eg YouTube Akamai Apple Microsoft)
search engines data mining (eg Google)
challenges
multiple applications each serving massive numbers of clients
managingbalancing load avoiding processing networking data bottlenecks
Inside a 40-ft Microsoft container
Chicago data center
Link Layer 5-85
Server racks
TOR switches
Tier-1 switches
Tier-2 switches
Load balancer
Load balancer
B
1 2 3 4 5 6 7 8
A C
Border router
Access router
Internet
Data center networks
load balancer application-layer routing receives external client requests
directs workload within data center
returns results to external client (hiding data
center internals from client)
Server racks
TOR switches
Tier-1 switches
Tier-2 switches
1 2 3 4 5 6 7 8
Data center networks
rich interconnection among switches racks
increased throughput between racks (multiple routing
paths possible)
increased reliability via redundancy
Link Layer 5-87
Link layer LANs outline
51 introduction services
52 error detection correction
53 multiple access protocols
54 LANs addressing ARP
Ethernet
switches
VLANS
55 link virtualization MPLS
56 data center networking
57 a day in the life of a web request
Link Layer 5-88
Synthesis a day in the life of a web request
journey down protocol stack complete application transport network link
putting-it-all-together synthesis goal identify review understand protocols (at all
layers) involved in seemingly simple scenario requesting www page
scenario student attaches laptop to campus network requestsreceives wwwgooglecom
Link Layer 5-89
A day in the life scenario
Comcast network
68800013
Googlersquos network
64233160019 64233169105
web server
DNS server
school network
68802024
web page
browser
router
(runs DHCP)
Link Layer 5-90
A day in the lifehellip connecting to the Internet
connecting laptop needs to get its own IP address addr of first-hop router addr of DNS server use DHCP
DHCP
UDP
IP
Eth
Phy
DHCP
DHCP
DHCP
DHCP
DHCP
DHCP
UDP
IP
Eth
Phy
DHCP
DHCP
DHCP
DHCP DHCP
DHCP request encapsulated in UDP encapsulated in IP encapsulated in 8023 Ethernet
Ethernet frame broadcast
(dest FFFFFFFFFFFF) on LAN received at router running DHCP server
Ethernet demuxed to IP demuxed UDP demuxed to DHCP
router
(runs DHCP)
Link Layer 5-91
DHCP server formulates DHCP ACK containing clientrsquos IP address IP address of first-hop router for client name amp IP address of DNS server
DHCP
UDP
IP
Eth
Phy
DHCP
DHCP
DHCP
DHCP
DHCP
UDP
IP
Eth
Phy
DHCP
DHCP
DHCP
DHCP
DHCP
encapsulation at DHCP server frame forwarded (switch learning) through LAN demultiplexing at client
Client now has IP address knows name amp addr of DNS
server IP address of its first-hop router
DHCP client receives DHCP ACK reply
A day in the lifehellip connecting to the Internet
router
(runs DHCP)
Link Layer 5-92
A day in the lifehellip ARP (before DNS before HTTP)
before sending HTTP request need IP address of wwwgooglecom DNS
DNS
UDP
IP
Eth
Phy
DNS
DNS
DNS
DNS query created encapsulated in UDP encapsulated in IP encapsulated in Eth To send frame to router need MAC address of router interface ARP
ARP query broadcast received by
router which replies with ARP reply giving MAC address of router interface
client now knows MAC address of first hop router so can now send frame containing DNS query
ARP query
Eth
Phy
ARP
ARP
ARP reply
router
(runs DHCP)
Link Layer 5-93
DNS
UDP
IP
Eth
Phy
DNS
DNS
DNS
DNS
DNS
IP datagram containing DNS query forwarded via LAN switch from client to 1st hop router
IP datagram forwarded from campus network into comcast network routed (tables created by RIP OSPF IS-IS andor BGP routing protocols) to DNS server
demuxrsquoed to DNS server
DNS server replies to client with IP address of wwwgooglecom
Comcast network
68800013
DNS server
DNS
UDP
IP
Eth
Phy
DNS
DNS
DNS
DNS
A day in the lifehellip using DNS
router
(runs DHCP)
Link Layer 5-94
A day in the lifehellipTCP connection carrying HTTP
HTTP
TCP
IP
Eth
Phy
HTTP
to send HTTP request client first opens TCP socket to web server
TCP SYN segment (step 1 in 3-way handshake) inter-domain routed to web server
TCP connection established 64233169105
web server
SYN
SYN
SYN
SYN
TCP
IP
Eth
Phy
SYN
SYN
SYN
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
web server responds with TCP SYNACK (step 2 in 3-way handshake)
router
(runs DHCP)
Link Layer 5-95
A day in the lifehellip HTTP requestreply
HTTP
TCP
IP
Eth
Phy
HTTP
HTTP request sent into TCP socket
IP datagram containing HTTP request routed to wwwgooglecom
IP datagram containing HTTP reply routed back to client
64233169105
web server
HTTP
TCP
IP
Eth
Phy web server responds with
HTTP reply (containing web page)
HTTP
HTTP
HTTP HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
web page finally () displayed
Link Layer 5-96
Chapter 5 Summary
principles behind data link layer services error detection correction
sharing a broadcast channel multiple access
link layer addressing
instantiation and implementation of various link layer technologies Ethernet
switched LANS VLANs
virtualized networks as a link layer MPLS
synthesis a day in the life of a web request
Link Layer 5-97
Chapter 5 letrsquos take a breath
journey down protocol stack complete (except PHY)
solid understanding of networking principles practice
hellip could stop here hellip but lots of interesting topics wireless
multimedia
security
network management
A
Arsquo
B
Brsquo C
Crsquo
1 2
3 4 5
6
Link Layer 5-64
Switch self-learning
switch learns which hosts can be reached through which interfaces
when frame received switch ldquolearnsrdquo location of sender incoming LAN segment
records senderlocation pair in switch table
A Arsquo
Source A
Dest Arsquo
MAC addr interface TTL
Switch table
(initially empty) A 1 60
Link Layer 5-65
Switch frame filteringforwarding
when frame received at switch 1 record incoming link MAC address of sending host
2 index switch table using MAC destination address
3 if entry found for destination then
if destination on segment from which frame arrived then drop frame
else forward frame on interface indicated by entry
else flood forward on all interfaces except arriving
interface
A
Arsquo
B
Brsquo C
Crsquo
1 2
3 4 5
6
Link Layer 5-66
Self-learning forwarding example
A Arsquo
Source A
Dest Arsquo
MAC addr interface TTL
switch table
(initially empty) A 1 60
A Arsquo A Arsquo A Arsquo A Arsquo A Arsquo
frame destination Arsquo locaton unknown flood
Arsquo A
destination A location
known
Arsquo 4 60
selectively send
on just one link
Link Layer 5-67
Interconnecting switches
switches can be connected together
Q sending from A to G - how does S1 know to forward frame destined to F via S4 and S3
A self learning (works exactly the same as in single-switch case)
A
B
S1
C D
E
F
S2
S4
S3
H
I
G
Link Layer 5-68
Self-learning multi-switch example
Suppose C sends frame to I I responds to C
Q show switch tables and packet forwarding in S1 S2 S3 S4
A
B
S1
C D
E
F
S2
S4
S3
H
I
G
Link Layer 5-69
Institutional network
to external
network
router
IP subnet
mail server
web server
Link Layer 5-70
Switches vs routers
both are store-and-forward
routers network-layer devices (examine network-layer headers)
switches link-layer devices (examine link-layer headers)
both have forwarding tables
routers compute tables using routing algorithms IP addresses
switches learn forwarding table using flooding learning MAC addresses
application
transport
network
link
physical
network
link
physical
link
physical
switch
datagram
application
transport
network
link
physical
frame
frame
frame
datagram
Link Layer 5-71
VLANs motivation
consider CS user moves office to
EE but wants connect to CS switch
single broadcast domain
all layer-2 broadcast traffic (ARP DHCP unknown location of destination MAC address) must cross entire LAN
securityprivacy efficiency issues
Computer
Science Electrical
Engineering
Computer
Engineering
Link Layer 5-72
VLANs port-based VLAN switch ports
grouped (by switch management software) so that single physical switch helliphellip
switch(es) supporting
VLAN capabilities can
be configured to
define multiple virtual
LANS over single
physical LAN
infrastructure
Virtual Local
Area Network 1
8
9
16 10 2
7
hellip
Electrical Engineering
(VLAN ports 1-8)
Computer Science
(VLAN ports 9-15)
15
hellip
Electrical Engineering
(VLAN ports 1-8)
hellip
1
8 2
7 9
16 10
15
hellip
Computer Science
(VLAN ports 9-16)
hellip operates as multiple virtual switches
Link Layer 5-73
Port-based VLAN
1
8
9
16 10 2
7
hellip
Electrical Engineering
(VLAN ports 1-8)
Computer Science
(VLAN ports 9-15)
15
hellip
traffic isolation frames tofrom ports 1-8 can only reach ports 1-8 can also define VLAN based on
MAC addresses of endpoints rather than switch port
dynamic membership ports can be dynamically assigned among VLANs
router
forwarding between VLANS done via routing (just as with separate switches) in practice vendors sell combined
switches plus routers
Link Layer 5-74
VLANS spanning multiple switches
trunk port carries frames between VLANS defined over multiple physical switches frames forwarded within VLAN between switches canrsquot be vanilla
8021 frames (must carry VLAN ID info)
8021q protocol addsremoved additional header fields for frames forwarded between trunk ports
1
8
9
10 2
7
hellip
Electrical Engineering
(VLAN ports 1-8)
Computer Science
(VLAN ports 9-15)
15
hellip
2
7 3
Ports 235 belong to EE VLAN
Ports 4678 belong to CS VLAN
5
4 6 8 16
1
Link Layer 5-75
type
2-byte Tag Protocol Identifier
(value 81-00)
Tag Control Information (12 bit VLAN ID field
3 bit priority field like IP TOS)
Recomputed CRC
8021Q VLAN frame format
8021 frame
8021Q frame
dest address
source address
data (payload) CRC preamble
dest address
source address
preamble data (payload) CRC
type
Link Layer 5-76
Link layer LANs outline
51 introduction services
52 error detection correction
53 multiple access protocols
54 LANs addressing ARP
Ethernet
switches
VLANS
55 link virtualization MPLS
56 data center networking
57 a day in the life of a web request
Link Layer 5-77
Multiprotocol label switching (MPLS)
initial goal high-speed IP forwarding using fixed length label (instead of IP address) fast lookup using fixed length identifier (rather than
shortest prefix matching)
borrowing ideas from Virtual Circuit (VC) approach
but IP datagram still keeps IP address
PPP or Ethernet
header IP header remainder of link-layer frame MPLS header
label Exp S TTL
20 3 1 5
Link Layer 5-78
MPLS capable routers
aka label-switched router
forward packets to outgoing interface based only on label value (donrsquot inspect IP address) MPLS forwarding table distinct from IP forwarding tables
flexibility MPLS forwarding decisions can differ from those of IP use destination and source addresses to route flows to
same destination differently (traffic engineering)
re-route flows quickly if link fails pre-computed backup paths (useful for VoIP)
Link Layer 5-79
R2
D
R3 R5
A
R6
MPLS versus IP paths
IP router IP routing path to destination determined
by destination address alone
R4
Link Layer 5-80
R2
D
R3 R4
R5
A
R6
MPLS versus IP paths
IP-only router
IP routing path to destination determined by destination address alone
MPLS and IP router
MPLS routing path to destination can be based on source and dest address fast reroute precompute backup routes in
case of link failure
entry router (R4) can use different MPLS
routes to A based eg on source address
Link Layer 5-81
MPLS signaling
modify OSPF IS-IS link-state flooding protocols to carry info used by MPLS routing eg link bandwidth amount of ldquoreservedrdquo link bandwidth
D
R4
R5
A
R6
entry MPLS router uses RSVP-TE signaling protocol to set up MPLS forwarding at downstream routers
modified link state flooding
RSVP-TE
Link Layer 5-82
R1 R2
D
R3 R4 R5
0
1
0 0
A
R6
in out out
label label dest interface
6 - A 0
in out out
label label dest interface
10 6 A 1
12 9 D 0
in out out
label label dest interface
10 A 0
12 D 0
1
in out out
label label dest interface
8 6 A 0
0
8 A 1
MPLS forwarding tables
Link Layer 5-83
Link layer LANs outline
51 introduction services
52 error detection correction
53 multiple access protocols
54 LANs addressing ARP
Ethernet
switches
VLANS
55 link virtualization MPLS
56 data center networking
57 a day in the life of a web request
Link Layer 5-84
Data center networks
10rsquos to 100rsquos of thousands of hosts often closely coupled in close proximity e-business (eg Amazon)
content-servers (eg YouTube Akamai Apple Microsoft)
search engines data mining (eg Google)
challenges
multiple applications each serving massive numbers of clients
managingbalancing load avoiding processing networking data bottlenecks
Inside a 40-ft Microsoft container
Chicago data center
Link Layer 5-85
Server racks
TOR switches
Tier-1 switches
Tier-2 switches
Load balancer
Load balancer
B
1 2 3 4 5 6 7 8
A C
Border router
Access router
Internet
Data center networks
load balancer application-layer routing receives external client requests
directs workload within data center
returns results to external client (hiding data
center internals from client)
Server racks
TOR switches
Tier-1 switches
Tier-2 switches
1 2 3 4 5 6 7 8
Data center networks
rich interconnection among switches racks
increased throughput between racks (multiple routing
paths possible)
increased reliability via redundancy
Link Layer 5-87
Link layer LANs outline
51 introduction services
52 error detection correction
53 multiple access protocols
54 LANs addressing ARP
Ethernet
switches
VLANS
55 link virtualization MPLS
56 data center networking
57 a day in the life of a web request
Link Layer 5-88
Synthesis a day in the life of a web request
journey down protocol stack complete application transport network link
putting-it-all-together synthesis goal identify review understand protocols (at all
layers) involved in seemingly simple scenario requesting www page
scenario student attaches laptop to campus network requestsreceives wwwgooglecom
Link Layer 5-89
A day in the life scenario
Comcast network
68800013
Googlersquos network
64233160019 64233169105
web server
DNS server
school network
68802024
web page
browser
router
(runs DHCP)
Link Layer 5-90
A day in the lifehellip connecting to the Internet
connecting laptop needs to get its own IP address addr of first-hop router addr of DNS server use DHCP
DHCP
UDP
IP
Eth
Phy
DHCP
DHCP
DHCP
DHCP
DHCP
DHCP
UDP
IP
Eth
Phy
DHCP
DHCP
DHCP
DHCP DHCP
DHCP request encapsulated in UDP encapsulated in IP encapsulated in 8023 Ethernet
Ethernet frame broadcast
(dest FFFFFFFFFFFF) on LAN received at router running DHCP server
Ethernet demuxed to IP demuxed UDP demuxed to DHCP
router
(runs DHCP)
Link Layer 5-91
DHCP server formulates DHCP ACK containing clientrsquos IP address IP address of first-hop router for client name amp IP address of DNS server
DHCP
UDP
IP
Eth
Phy
DHCP
DHCP
DHCP
DHCP
DHCP
UDP
IP
Eth
Phy
DHCP
DHCP
DHCP
DHCP
DHCP
encapsulation at DHCP server frame forwarded (switch learning) through LAN demultiplexing at client
Client now has IP address knows name amp addr of DNS
server IP address of its first-hop router
DHCP client receives DHCP ACK reply
A day in the lifehellip connecting to the Internet
router
(runs DHCP)
Link Layer 5-92
A day in the lifehellip ARP (before DNS before HTTP)
before sending HTTP request need IP address of wwwgooglecom DNS
DNS
UDP
IP
Eth
Phy
DNS
DNS
DNS
DNS query created encapsulated in UDP encapsulated in IP encapsulated in Eth To send frame to router need MAC address of router interface ARP
ARP query broadcast received by
router which replies with ARP reply giving MAC address of router interface
client now knows MAC address of first hop router so can now send frame containing DNS query
ARP query
Eth
Phy
ARP
ARP
ARP reply
router
(runs DHCP)
Link Layer 5-93
DNS
UDP
IP
Eth
Phy
DNS
DNS
DNS
DNS
DNS
IP datagram containing DNS query forwarded via LAN switch from client to 1st hop router
IP datagram forwarded from campus network into comcast network routed (tables created by RIP OSPF IS-IS andor BGP routing protocols) to DNS server
demuxrsquoed to DNS server
DNS server replies to client with IP address of wwwgooglecom
Comcast network
68800013
DNS server
DNS
UDP
IP
Eth
Phy
DNS
DNS
DNS
DNS
A day in the lifehellip using DNS
router
(runs DHCP)
Link Layer 5-94
A day in the lifehellipTCP connection carrying HTTP
HTTP
TCP
IP
Eth
Phy
HTTP
to send HTTP request client first opens TCP socket to web server
TCP SYN segment (step 1 in 3-way handshake) inter-domain routed to web server
TCP connection established 64233169105
web server
SYN
SYN
SYN
SYN
TCP
IP
Eth
Phy
SYN
SYN
SYN
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
web server responds with TCP SYNACK (step 2 in 3-way handshake)
router
(runs DHCP)
Link Layer 5-95
A day in the lifehellip HTTP requestreply
HTTP
TCP
IP
Eth
Phy
HTTP
HTTP request sent into TCP socket
IP datagram containing HTTP request routed to wwwgooglecom
IP datagram containing HTTP reply routed back to client
64233169105
web server
HTTP
TCP
IP
Eth
Phy web server responds with
HTTP reply (containing web page)
HTTP
HTTP
HTTP HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
web page finally () displayed
Link Layer 5-96
Chapter 5 Summary
principles behind data link layer services error detection correction
sharing a broadcast channel multiple access
link layer addressing
instantiation and implementation of various link layer technologies Ethernet
switched LANS VLANs
virtualized networks as a link layer MPLS
synthesis a day in the life of a web request
Link Layer 5-97
Chapter 5 letrsquos take a breath
journey down protocol stack complete (except PHY)
solid understanding of networking principles practice
hellip could stop here hellip but lots of interesting topics wireless
multimedia
security
network management
Link Layer 5-65
Switch frame filteringforwarding
when frame received at switch 1 record incoming link MAC address of sending host
2 index switch table using MAC destination address
3 if entry found for destination then
if destination on segment from which frame arrived then drop frame
else forward frame on interface indicated by entry
else flood forward on all interfaces except arriving
interface
A
Arsquo
B
Brsquo C
Crsquo
1 2
3 4 5
6
Link Layer 5-66
Self-learning forwarding example
A Arsquo
Source A
Dest Arsquo
MAC addr interface TTL
switch table
(initially empty) A 1 60
A Arsquo A Arsquo A Arsquo A Arsquo A Arsquo
frame destination Arsquo locaton unknown flood
Arsquo A
destination A location
known
Arsquo 4 60
selectively send
on just one link
Link Layer 5-67
Interconnecting switches
switches can be connected together
Q sending from A to G - how does S1 know to forward frame destined to F via S4 and S3
A self learning (works exactly the same as in single-switch case)
A
B
S1
C D
E
F
S2
S4
S3
H
I
G
Link Layer 5-68
Self-learning multi-switch example
Suppose C sends frame to I I responds to C
Q show switch tables and packet forwarding in S1 S2 S3 S4
A
B
S1
C D
E
F
S2
S4
S3
H
I
G
Link Layer 5-69
Institutional network
to external
network
router
IP subnet
mail server
web server
Link Layer 5-70
Switches vs routers
both are store-and-forward
routers network-layer devices (examine network-layer headers)
switches link-layer devices (examine link-layer headers)
both have forwarding tables
routers compute tables using routing algorithms IP addresses
switches learn forwarding table using flooding learning MAC addresses
application
transport
network
link
physical
network
link
physical
link
physical
switch
datagram
application
transport
network
link
physical
frame
frame
frame
datagram
Link Layer 5-71
VLANs motivation
consider CS user moves office to
EE but wants connect to CS switch
single broadcast domain
all layer-2 broadcast traffic (ARP DHCP unknown location of destination MAC address) must cross entire LAN
securityprivacy efficiency issues
Computer
Science Electrical
Engineering
Computer
Engineering
Link Layer 5-72
VLANs port-based VLAN switch ports
grouped (by switch management software) so that single physical switch helliphellip
switch(es) supporting
VLAN capabilities can
be configured to
define multiple virtual
LANS over single
physical LAN
infrastructure
Virtual Local
Area Network 1
8
9
16 10 2
7
hellip
Electrical Engineering
(VLAN ports 1-8)
Computer Science
(VLAN ports 9-15)
15
hellip
Electrical Engineering
(VLAN ports 1-8)
hellip
1
8 2
7 9
16 10
15
hellip
Computer Science
(VLAN ports 9-16)
hellip operates as multiple virtual switches
Link Layer 5-73
Port-based VLAN
1
8
9
16 10 2
7
hellip
Electrical Engineering
(VLAN ports 1-8)
Computer Science
(VLAN ports 9-15)
15
hellip
traffic isolation frames tofrom ports 1-8 can only reach ports 1-8 can also define VLAN based on
MAC addresses of endpoints rather than switch port
dynamic membership ports can be dynamically assigned among VLANs
router
forwarding between VLANS done via routing (just as with separate switches) in practice vendors sell combined
switches plus routers
Link Layer 5-74
VLANS spanning multiple switches
trunk port carries frames between VLANS defined over multiple physical switches frames forwarded within VLAN between switches canrsquot be vanilla
8021 frames (must carry VLAN ID info)
8021q protocol addsremoved additional header fields for frames forwarded between trunk ports
1
8
9
10 2
7
hellip
Electrical Engineering
(VLAN ports 1-8)
Computer Science
(VLAN ports 9-15)
15
hellip
2
7 3
Ports 235 belong to EE VLAN
Ports 4678 belong to CS VLAN
5
4 6 8 16
1
Link Layer 5-75
type
2-byte Tag Protocol Identifier
(value 81-00)
Tag Control Information (12 bit VLAN ID field
3 bit priority field like IP TOS)
Recomputed CRC
8021Q VLAN frame format
8021 frame
8021Q frame
dest address
source address
data (payload) CRC preamble
dest address
source address
preamble data (payload) CRC
type
Link Layer 5-76
Link layer LANs outline
51 introduction services
52 error detection correction
53 multiple access protocols
54 LANs addressing ARP
Ethernet
switches
VLANS
55 link virtualization MPLS
56 data center networking
57 a day in the life of a web request
Link Layer 5-77
Multiprotocol label switching (MPLS)
initial goal high-speed IP forwarding using fixed length label (instead of IP address) fast lookup using fixed length identifier (rather than
shortest prefix matching)
borrowing ideas from Virtual Circuit (VC) approach
but IP datagram still keeps IP address
PPP or Ethernet
header IP header remainder of link-layer frame MPLS header
label Exp S TTL
20 3 1 5
Link Layer 5-78
MPLS capable routers
aka label-switched router
forward packets to outgoing interface based only on label value (donrsquot inspect IP address) MPLS forwarding table distinct from IP forwarding tables
flexibility MPLS forwarding decisions can differ from those of IP use destination and source addresses to route flows to
same destination differently (traffic engineering)
re-route flows quickly if link fails pre-computed backup paths (useful for VoIP)
Link Layer 5-79
R2
D
R3 R5
A
R6
MPLS versus IP paths
IP router IP routing path to destination determined
by destination address alone
R4
Link Layer 5-80
R2
D
R3 R4
R5
A
R6
MPLS versus IP paths
IP-only router
IP routing path to destination determined by destination address alone
MPLS and IP router
MPLS routing path to destination can be based on source and dest address fast reroute precompute backup routes in
case of link failure
entry router (R4) can use different MPLS
routes to A based eg on source address
Link Layer 5-81
MPLS signaling
modify OSPF IS-IS link-state flooding protocols to carry info used by MPLS routing eg link bandwidth amount of ldquoreservedrdquo link bandwidth
D
R4
R5
A
R6
entry MPLS router uses RSVP-TE signaling protocol to set up MPLS forwarding at downstream routers
modified link state flooding
RSVP-TE
Link Layer 5-82
R1 R2
D
R3 R4 R5
0
1
0 0
A
R6
in out out
label label dest interface
6 - A 0
in out out
label label dest interface
10 6 A 1
12 9 D 0
in out out
label label dest interface
10 A 0
12 D 0
1
in out out
label label dest interface
8 6 A 0
0
8 A 1
MPLS forwarding tables
Link Layer 5-83
Link layer LANs outline
51 introduction services
52 error detection correction
53 multiple access protocols
54 LANs addressing ARP
Ethernet
switches
VLANS
55 link virtualization MPLS
56 data center networking
57 a day in the life of a web request
Link Layer 5-84
Data center networks
10rsquos to 100rsquos of thousands of hosts often closely coupled in close proximity e-business (eg Amazon)
content-servers (eg YouTube Akamai Apple Microsoft)
search engines data mining (eg Google)
challenges
multiple applications each serving massive numbers of clients
managingbalancing load avoiding processing networking data bottlenecks
Inside a 40-ft Microsoft container
Chicago data center
Link Layer 5-85
Server racks
TOR switches
Tier-1 switches
Tier-2 switches
Load balancer
Load balancer
B
1 2 3 4 5 6 7 8
A C
Border router
Access router
Internet
Data center networks
load balancer application-layer routing receives external client requests
directs workload within data center
returns results to external client (hiding data
center internals from client)
Server racks
TOR switches
Tier-1 switches
Tier-2 switches
1 2 3 4 5 6 7 8
Data center networks
rich interconnection among switches racks
increased throughput between racks (multiple routing
paths possible)
increased reliability via redundancy
Link Layer 5-87
Link layer LANs outline
51 introduction services
52 error detection correction
53 multiple access protocols
54 LANs addressing ARP
Ethernet
switches
VLANS
55 link virtualization MPLS
56 data center networking
57 a day in the life of a web request
Link Layer 5-88
Synthesis a day in the life of a web request
journey down protocol stack complete application transport network link
putting-it-all-together synthesis goal identify review understand protocols (at all
layers) involved in seemingly simple scenario requesting www page
scenario student attaches laptop to campus network requestsreceives wwwgooglecom
Link Layer 5-89
A day in the life scenario
Comcast network
68800013
Googlersquos network
64233160019 64233169105
web server
DNS server
school network
68802024
web page
browser
router
(runs DHCP)
Link Layer 5-90
A day in the lifehellip connecting to the Internet
connecting laptop needs to get its own IP address addr of first-hop router addr of DNS server use DHCP
DHCP
UDP
IP
Eth
Phy
DHCP
DHCP
DHCP
DHCP
DHCP
DHCP
UDP
IP
Eth
Phy
DHCP
DHCP
DHCP
DHCP DHCP
DHCP request encapsulated in UDP encapsulated in IP encapsulated in 8023 Ethernet
Ethernet frame broadcast
(dest FFFFFFFFFFFF) on LAN received at router running DHCP server
Ethernet demuxed to IP demuxed UDP demuxed to DHCP
router
(runs DHCP)
Link Layer 5-91
DHCP server formulates DHCP ACK containing clientrsquos IP address IP address of first-hop router for client name amp IP address of DNS server
DHCP
UDP
IP
Eth
Phy
DHCP
DHCP
DHCP
DHCP
DHCP
UDP
IP
Eth
Phy
DHCP
DHCP
DHCP
DHCP
DHCP
encapsulation at DHCP server frame forwarded (switch learning) through LAN demultiplexing at client
Client now has IP address knows name amp addr of DNS
server IP address of its first-hop router
DHCP client receives DHCP ACK reply
A day in the lifehellip connecting to the Internet
router
(runs DHCP)
Link Layer 5-92
A day in the lifehellip ARP (before DNS before HTTP)
before sending HTTP request need IP address of wwwgooglecom DNS
DNS
UDP
IP
Eth
Phy
DNS
DNS
DNS
DNS query created encapsulated in UDP encapsulated in IP encapsulated in Eth To send frame to router need MAC address of router interface ARP
ARP query broadcast received by
router which replies with ARP reply giving MAC address of router interface
client now knows MAC address of first hop router so can now send frame containing DNS query
ARP query
Eth
Phy
ARP
ARP
ARP reply
router
(runs DHCP)
Link Layer 5-93
DNS
UDP
IP
Eth
Phy
DNS
DNS
DNS
DNS
DNS
IP datagram containing DNS query forwarded via LAN switch from client to 1st hop router
IP datagram forwarded from campus network into comcast network routed (tables created by RIP OSPF IS-IS andor BGP routing protocols) to DNS server
demuxrsquoed to DNS server
DNS server replies to client with IP address of wwwgooglecom
Comcast network
68800013
DNS server
DNS
UDP
IP
Eth
Phy
DNS
DNS
DNS
DNS
A day in the lifehellip using DNS
router
(runs DHCP)
Link Layer 5-94
A day in the lifehellipTCP connection carrying HTTP
HTTP
TCP
IP
Eth
Phy
HTTP
to send HTTP request client first opens TCP socket to web server
TCP SYN segment (step 1 in 3-way handshake) inter-domain routed to web server
TCP connection established 64233169105
web server
SYN
SYN
SYN
SYN
TCP
IP
Eth
Phy
SYN
SYN
SYN
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
web server responds with TCP SYNACK (step 2 in 3-way handshake)
router
(runs DHCP)
Link Layer 5-95
A day in the lifehellip HTTP requestreply
HTTP
TCP
IP
Eth
Phy
HTTP
HTTP request sent into TCP socket
IP datagram containing HTTP request routed to wwwgooglecom
IP datagram containing HTTP reply routed back to client
64233169105
web server
HTTP
TCP
IP
Eth
Phy web server responds with
HTTP reply (containing web page)
HTTP
HTTP
HTTP HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
web page finally () displayed
Link Layer 5-96
Chapter 5 Summary
principles behind data link layer services error detection correction
sharing a broadcast channel multiple access
link layer addressing
instantiation and implementation of various link layer technologies Ethernet
switched LANS VLANs
virtualized networks as a link layer MPLS
synthesis a day in the life of a web request
Link Layer 5-97
Chapter 5 letrsquos take a breath
journey down protocol stack complete (except PHY)
solid understanding of networking principles practice
hellip could stop here hellip but lots of interesting topics wireless
multimedia
security
network management
A
Arsquo
B
Brsquo C
Crsquo
1 2
3 4 5
6
Link Layer 5-66
Self-learning forwarding example
A Arsquo
Source A
Dest Arsquo
MAC addr interface TTL
switch table
(initially empty) A 1 60
A Arsquo A Arsquo A Arsquo A Arsquo A Arsquo
frame destination Arsquo locaton unknown flood
Arsquo A
destination A location
known
Arsquo 4 60
selectively send
on just one link
Link Layer 5-67
Interconnecting switches
switches can be connected together
Q sending from A to G - how does S1 know to forward frame destined to F via S4 and S3
A self learning (works exactly the same as in single-switch case)
A
B
S1
C D
E
F
S2
S4
S3
H
I
G
Link Layer 5-68
Self-learning multi-switch example
Suppose C sends frame to I I responds to C
Q show switch tables and packet forwarding in S1 S2 S3 S4
A
B
S1
C D
E
F
S2
S4
S3
H
I
G
Link Layer 5-69
Institutional network
to external
network
router
IP subnet
mail server
web server
Link Layer 5-70
Switches vs routers
both are store-and-forward
routers network-layer devices (examine network-layer headers)
switches link-layer devices (examine link-layer headers)
both have forwarding tables
routers compute tables using routing algorithms IP addresses
switches learn forwarding table using flooding learning MAC addresses
application
transport
network
link
physical
network
link
physical
link
physical
switch
datagram
application
transport
network
link
physical
frame
frame
frame
datagram
Link Layer 5-71
VLANs motivation
consider CS user moves office to
EE but wants connect to CS switch
single broadcast domain
all layer-2 broadcast traffic (ARP DHCP unknown location of destination MAC address) must cross entire LAN
securityprivacy efficiency issues
Computer
Science Electrical
Engineering
Computer
Engineering
Link Layer 5-72
VLANs port-based VLAN switch ports
grouped (by switch management software) so that single physical switch helliphellip
switch(es) supporting
VLAN capabilities can
be configured to
define multiple virtual
LANS over single
physical LAN
infrastructure
Virtual Local
Area Network 1
8
9
16 10 2
7
hellip
Electrical Engineering
(VLAN ports 1-8)
Computer Science
(VLAN ports 9-15)
15
hellip
Electrical Engineering
(VLAN ports 1-8)
hellip
1
8 2
7 9
16 10
15
hellip
Computer Science
(VLAN ports 9-16)
hellip operates as multiple virtual switches
Link Layer 5-73
Port-based VLAN
1
8
9
16 10 2
7
hellip
Electrical Engineering
(VLAN ports 1-8)
Computer Science
(VLAN ports 9-15)
15
hellip
traffic isolation frames tofrom ports 1-8 can only reach ports 1-8 can also define VLAN based on
MAC addresses of endpoints rather than switch port
dynamic membership ports can be dynamically assigned among VLANs
router
forwarding between VLANS done via routing (just as with separate switches) in practice vendors sell combined
switches plus routers
Link Layer 5-74
VLANS spanning multiple switches
trunk port carries frames between VLANS defined over multiple physical switches frames forwarded within VLAN between switches canrsquot be vanilla
8021 frames (must carry VLAN ID info)
8021q protocol addsremoved additional header fields for frames forwarded between trunk ports
1
8
9
10 2
7
hellip
Electrical Engineering
(VLAN ports 1-8)
Computer Science
(VLAN ports 9-15)
15
hellip
2
7 3
Ports 235 belong to EE VLAN
Ports 4678 belong to CS VLAN
5
4 6 8 16
1
Link Layer 5-75
type
2-byte Tag Protocol Identifier
(value 81-00)
Tag Control Information (12 bit VLAN ID field
3 bit priority field like IP TOS)
Recomputed CRC
8021Q VLAN frame format
8021 frame
8021Q frame
dest address
source address
data (payload) CRC preamble
dest address
source address
preamble data (payload) CRC
type
Link Layer 5-76
Link layer LANs outline
51 introduction services
52 error detection correction
53 multiple access protocols
54 LANs addressing ARP
Ethernet
switches
VLANS
55 link virtualization MPLS
56 data center networking
57 a day in the life of a web request
Link Layer 5-77
Multiprotocol label switching (MPLS)
initial goal high-speed IP forwarding using fixed length label (instead of IP address) fast lookup using fixed length identifier (rather than
shortest prefix matching)
borrowing ideas from Virtual Circuit (VC) approach
but IP datagram still keeps IP address
PPP or Ethernet
header IP header remainder of link-layer frame MPLS header
label Exp S TTL
20 3 1 5
Link Layer 5-78
MPLS capable routers
aka label-switched router
forward packets to outgoing interface based only on label value (donrsquot inspect IP address) MPLS forwarding table distinct from IP forwarding tables
flexibility MPLS forwarding decisions can differ from those of IP use destination and source addresses to route flows to
same destination differently (traffic engineering)
re-route flows quickly if link fails pre-computed backup paths (useful for VoIP)
Link Layer 5-79
R2
D
R3 R5
A
R6
MPLS versus IP paths
IP router IP routing path to destination determined
by destination address alone
R4
Link Layer 5-80
R2
D
R3 R4
R5
A
R6
MPLS versus IP paths
IP-only router
IP routing path to destination determined by destination address alone
MPLS and IP router
MPLS routing path to destination can be based on source and dest address fast reroute precompute backup routes in
case of link failure
entry router (R4) can use different MPLS
routes to A based eg on source address
Link Layer 5-81
MPLS signaling
modify OSPF IS-IS link-state flooding protocols to carry info used by MPLS routing eg link bandwidth amount of ldquoreservedrdquo link bandwidth
D
R4
R5
A
R6
entry MPLS router uses RSVP-TE signaling protocol to set up MPLS forwarding at downstream routers
modified link state flooding
RSVP-TE
Link Layer 5-82
R1 R2
D
R3 R4 R5
0
1
0 0
A
R6
in out out
label label dest interface
6 - A 0
in out out
label label dest interface
10 6 A 1
12 9 D 0
in out out
label label dest interface
10 A 0
12 D 0
1
in out out
label label dest interface
8 6 A 0
0
8 A 1
MPLS forwarding tables
Link Layer 5-83
Link layer LANs outline
51 introduction services
52 error detection correction
53 multiple access protocols
54 LANs addressing ARP
Ethernet
switches
VLANS
55 link virtualization MPLS
56 data center networking
57 a day in the life of a web request
Link Layer 5-84
Data center networks
10rsquos to 100rsquos of thousands of hosts often closely coupled in close proximity e-business (eg Amazon)
content-servers (eg YouTube Akamai Apple Microsoft)
search engines data mining (eg Google)
challenges
multiple applications each serving massive numbers of clients
managingbalancing load avoiding processing networking data bottlenecks
Inside a 40-ft Microsoft container
Chicago data center
Link Layer 5-85
Server racks
TOR switches
Tier-1 switches
Tier-2 switches
Load balancer
Load balancer
B
1 2 3 4 5 6 7 8
A C
Border router
Access router
Internet
Data center networks
load balancer application-layer routing receives external client requests
directs workload within data center
returns results to external client (hiding data
center internals from client)
Server racks
TOR switches
Tier-1 switches
Tier-2 switches
1 2 3 4 5 6 7 8
Data center networks
rich interconnection among switches racks
increased throughput between racks (multiple routing
paths possible)
increased reliability via redundancy
Link Layer 5-87
Link layer LANs outline
51 introduction services
52 error detection correction
53 multiple access protocols
54 LANs addressing ARP
Ethernet
switches
VLANS
55 link virtualization MPLS
56 data center networking
57 a day in the life of a web request
Link Layer 5-88
Synthesis a day in the life of a web request
journey down protocol stack complete application transport network link
putting-it-all-together synthesis goal identify review understand protocols (at all
layers) involved in seemingly simple scenario requesting www page
scenario student attaches laptop to campus network requestsreceives wwwgooglecom
Link Layer 5-89
A day in the life scenario
Comcast network
68800013
Googlersquos network
64233160019 64233169105
web server
DNS server
school network
68802024
web page
browser
router
(runs DHCP)
Link Layer 5-90
A day in the lifehellip connecting to the Internet
connecting laptop needs to get its own IP address addr of first-hop router addr of DNS server use DHCP
DHCP
UDP
IP
Eth
Phy
DHCP
DHCP
DHCP
DHCP
DHCP
DHCP
UDP
IP
Eth
Phy
DHCP
DHCP
DHCP
DHCP DHCP
DHCP request encapsulated in UDP encapsulated in IP encapsulated in 8023 Ethernet
Ethernet frame broadcast
(dest FFFFFFFFFFFF) on LAN received at router running DHCP server
Ethernet demuxed to IP demuxed UDP demuxed to DHCP
router
(runs DHCP)
Link Layer 5-91
DHCP server formulates DHCP ACK containing clientrsquos IP address IP address of first-hop router for client name amp IP address of DNS server
DHCP
UDP
IP
Eth
Phy
DHCP
DHCP
DHCP
DHCP
DHCP
UDP
IP
Eth
Phy
DHCP
DHCP
DHCP
DHCP
DHCP
encapsulation at DHCP server frame forwarded (switch learning) through LAN demultiplexing at client
Client now has IP address knows name amp addr of DNS
server IP address of its first-hop router
DHCP client receives DHCP ACK reply
A day in the lifehellip connecting to the Internet
router
(runs DHCP)
Link Layer 5-92
A day in the lifehellip ARP (before DNS before HTTP)
before sending HTTP request need IP address of wwwgooglecom DNS
DNS
UDP
IP
Eth
Phy
DNS
DNS
DNS
DNS query created encapsulated in UDP encapsulated in IP encapsulated in Eth To send frame to router need MAC address of router interface ARP
ARP query broadcast received by
router which replies with ARP reply giving MAC address of router interface
client now knows MAC address of first hop router so can now send frame containing DNS query
ARP query
Eth
Phy
ARP
ARP
ARP reply
router
(runs DHCP)
Link Layer 5-93
DNS
UDP
IP
Eth
Phy
DNS
DNS
DNS
DNS
DNS
IP datagram containing DNS query forwarded via LAN switch from client to 1st hop router
IP datagram forwarded from campus network into comcast network routed (tables created by RIP OSPF IS-IS andor BGP routing protocols) to DNS server
demuxrsquoed to DNS server
DNS server replies to client with IP address of wwwgooglecom
Comcast network
68800013
DNS server
DNS
UDP
IP
Eth
Phy
DNS
DNS
DNS
DNS
A day in the lifehellip using DNS
router
(runs DHCP)
Link Layer 5-94
A day in the lifehellipTCP connection carrying HTTP
HTTP
TCP
IP
Eth
Phy
HTTP
to send HTTP request client first opens TCP socket to web server
TCP SYN segment (step 1 in 3-way handshake) inter-domain routed to web server
TCP connection established 64233169105
web server
SYN
SYN
SYN
SYN
TCP
IP
Eth
Phy
SYN
SYN
SYN
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
web server responds with TCP SYNACK (step 2 in 3-way handshake)
router
(runs DHCP)
Link Layer 5-95
A day in the lifehellip HTTP requestreply
HTTP
TCP
IP
Eth
Phy
HTTP
HTTP request sent into TCP socket
IP datagram containing HTTP request routed to wwwgooglecom
IP datagram containing HTTP reply routed back to client
64233169105
web server
HTTP
TCP
IP
Eth
Phy web server responds with
HTTP reply (containing web page)
HTTP
HTTP
HTTP HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
web page finally () displayed
Link Layer 5-96
Chapter 5 Summary
principles behind data link layer services error detection correction
sharing a broadcast channel multiple access
link layer addressing
instantiation and implementation of various link layer technologies Ethernet
switched LANS VLANs
virtualized networks as a link layer MPLS
synthesis a day in the life of a web request
Link Layer 5-97
Chapter 5 letrsquos take a breath
journey down protocol stack complete (except PHY)
solid understanding of networking principles practice
hellip could stop here hellip but lots of interesting topics wireless
multimedia
security
network management
Link Layer 5-67
Interconnecting switches
switches can be connected together
Q sending from A to G - how does S1 know to forward frame destined to F via S4 and S3
A self learning (works exactly the same as in single-switch case)
A
B
S1
C D
E
F
S2
S4
S3
H
I
G
Link Layer 5-68
Self-learning multi-switch example
Suppose C sends frame to I I responds to C
Q show switch tables and packet forwarding in S1 S2 S3 S4
A
B
S1
C D
E
F
S2
S4
S3
H
I
G
Link Layer 5-69
Institutional network
to external
network
router
IP subnet
mail server
web server
Link Layer 5-70
Switches vs routers
both are store-and-forward
routers network-layer devices (examine network-layer headers)
switches link-layer devices (examine link-layer headers)
both have forwarding tables
routers compute tables using routing algorithms IP addresses
switches learn forwarding table using flooding learning MAC addresses
application
transport
network
link
physical
network
link
physical
link
physical
switch
datagram
application
transport
network
link
physical
frame
frame
frame
datagram
Link Layer 5-71
VLANs motivation
consider CS user moves office to
EE but wants connect to CS switch
single broadcast domain
all layer-2 broadcast traffic (ARP DHCP unknown location of destination MAC address) must cross entire LAN
securityprivacy efficiency issues
Computer
Science Electrical
Engineering
Computer
Engineering
Link Layer 5-72
VLANs port-based VLAN switch ports
grouped (by switch management software) so that single physical switch helliphellip
switch(es) supporting
VLAN capabilities can
be configured to
define multiple virtual
LANS over single
physical LAN
infrastructure
Virtual Local
Area Network 1
8
9
16 10 2
7
hellip
Electrical Engineering
(VLAN ports 1-8)
Computer Science
(VLAN ports 9-15)
15
hellip
Electrical Engineering
(VLAN ports 1-8)
hellip
1
8 2
7 9
16 10
15
hellip
Computer Science
(VLAN ports 9-16)
hellip operates as multiple virtual switches
Link Layer 5-73
Port-based VLAN
1
8
9
16 10 2
7
hellip
Electrical Engineering
(VLAN ports 1-8)
Computer Science
(VLAN ports 9-15)
15
hellip
traffic isolation frames tofrom ports 1-8 can only reach ports 1-8 can also define VLAN based on
MAC addresses of endpoints rather than switch port
dynamic membership ports can be dynamically assigned among VLANs
router
forwarding between VLANS done via routing (just as with separate switches) in practice vendors sell combined
switches plus routers
Link Layer 5-74
VLANS spanning multiple switches
trunk port carries frames between VLANS defined over multiple physical switches frames forwarded within VLAN between switches canrsquot be vanilla
8021 frames (must carry VLAN ID info)
8021q protocol addsremoved additional header fields for frames forwarded between trunk ports
1
8
9
10 2
7
hellip
Electrical Engineering
(VLAN ports 1-8)
Computer Science
(VLAN ports 9-15)
15
hellip
2
7 3
Ports 235 belong to EE VLAN
Ports 4678 belong to CS VLAN
5
4 6 8 16
1
Link Layer 5-75
type
2-byte Tag Protocol Identifier
(value 81-00)
Tag Control Information (12 bit VLAN ID field
3 bit priority field like IP TOS)
Recomputed CRC
8021Q VLAN frame format
8021 frame
8021Q frame
dest address
source address
data (payload) CRC preamble
dest address
source address
preamble data (payload) CRC
type
Link Layer 5-76
Link layer LANs outline
51 introduction services
52 error detection correction
53 multiple access protocols
54 LANs addressing ARP
Ethernet
switches
VLANS
55 link virtualization MPLS
56 data center networking
57 a day in the life of a web request
Link Layer 5-77
Multiprotocol label switching (MPLS)
initial goal high-speed IP forwarding using fixed length label (instead of IP address) fast lookup using fixed length identifier (rather than
shortest prefix matching)
borrowing ideas from Virtual Circuit (VC) approach
but IP datagram still keeps IP address
PPP or Ethernet
header IP header remainder of link-layer frame MPLS header
label Exp S TTL
20 3 1 5
Link Layer 5-78
MPLS capable routers
aka label-switched router
forward packets to outgoing interface based only on label value (donrsquot inspect IP address) MPLS forwarding table distinct from IP forwarding tables
flexibility MPLS forwarding decisions can differ from those of IP use destination and source addresses to route flows to
same destination differently (traffic engineering)
re-route flows quickly if link fails pre-computed backup paths (useful for VoIP)
Link Layer 5-79
R2
D
R3 R5
A
R6
MPLS versus IP paths
IP router IP routing path to destination determined
by destination address alone
R4
Link Layer 5-80
R2
D
R3 R4
R5
A
R6
MPLS versus IP paths
IP-only router
IP routing path to destination determined by destination address alone
MPLS and IP router
MPLS routing path to destination can be based on source and dest address fast reroute precompute backup routes in
case of link failure
entry router (R4) can use different MPLS
routes to A based eg on source address
Link Layer 5-81
MPLS signaling
modify OSPF IS-IS link-state flooding protocols to carry info used by MPLS routing eg link bandwidth amount of ldquoreservedrdquo link bandwidth
D
R4
R5
A
R6
entry MPLS router uses RSVP-TE signaling protocol to set up MPLS forwarding at downstream routers
modified link state flooding
RSVP-TE
Link Layer 5-82
R1 R2
D
R3 R4 R5
0
1
0 0
A
R6
in out out
label label dest interface
6 - A 0
in out out
label label dest interface
10 6 A 1
12 9 D 0
in out out
label label dest interface
10 A 0
12 D 0
1
in out out
label label dest interface
8 6 A 0
0
8 A 1
MPLS forwarding tables
Link Layer 5-83
Link layer LANs outline
51 introduction services
52 error detection correction
53 multiple access protocols
54 LANs addressing ARP
Ethernet
switches
VLANS
55 link virtualization MPLS
56 data center networking
57 a day in the life of a web request
Link Layer 5-84
Data center networks
10rsquos to 100rsquos of thousands of hosts often closely coupled in close proximity e-business (eg Amazon)
content-servers (eg YouTube Akamai Apple Microsoft)
search engines data mining (eg Google)
challenges
multiple applications each serving massive numbers of clients
managingbalancing load avoiding processing networking data bottlenecks
Inside a 40-ft Microsoft container
Chicago data center
Link Layer 5-85
Server racks
TOR switches
Tier-1 switches
Tier-2 switches
Load balancer
Load balancer
B
1 2 3 4 5 6 7 8
A C
Border router
Access router
Internet
Data center networks
load balancer application-layer routing receives external client requests
directs workload within data center
returns results to external client (hiding data
center internals from client)
Server racks
TOR switches
Tier-1 switches
Tier-2 switches
1 2 3 4 5 6 7 8
Data center networks
rich interconnection among switches racks
increased throughput between racks (multiple routing
paths possible)
increased reliability via redundancy
Link Layer 5-87
Link layer LANs outline
51 introduction services
52 error detection correction
53 multiple access protocols
54 LANs addressing ARP
Ethernet
switches
VLANS
55 link virtualization MPLS
56 data center networking
57 a day in the life of a web request
Link Layer 5-88
Synthesis a day in the life of a web request
journey down protocol stack complete application transport network link
putting-it-all-together synthesis goal identify review understand protocols (at all
layers) involved in seemingly simple scenario requesting www page
scenario student attaches laptop to campus network requestsreceives wwwgooglecom
Link Layer 5-89
A day in the life scenario
Comcast network
68800013
Googlersquos network
64233160019 64233169105
web server
DNS server
school network
68802024
web page
browser
router
(runs DHCP)
Link Layer 5-90
A day in the lifehellip connecting to the Internet
connecting laptop needs to get its own IP address addr of first-hop router addr of DNS server use DHCP
DHCP
UDP
IP
Eth
Phy
DHCP
DHCP
DHCP
DHCP
DHCP
DHCP
UDP
IP
Eth
Phy
DHCP
DHCP
DHCP
DHCP DHCP
DHCP request encapsulated in UDP encapsulated in IP encapsulated in 8023 Ethernet
Ethernet frame broadcast
(dest FFFFFFFFFFFF) on LAN received at router running DHCP server
Ethernet demuxed to IP demuxed UDP demuxed to DHCP
router
(runs DHCP)
Link Layer 5-91
DHCP server formulates DHCP ACK containing clientrsquos IP address IP address of first-hop router for client name amp IP address of DNS server
DHCP
UDP
IP
Eth
Phy
DHCP
DHCP
DHCP
DHCP
DHCP
UDP
IP
Eth
Phy
DHCP
DHCP
DHCP
DHCP
DHCP
encapsulation at DHCP server frame forwarded (switch learning) through LAN demultiplexing at client
Client now has IP address knows name amp addr of DNS
server IP address of its first-hop router
DHCP client receives DHCP ACK reply
A day in the lifehellip connecting to the Internet
router
(runs DHCP)
Link Layer 5-92
A day in the lifehellip ARP (before DNS before HTTP)
before sending HTTP request need IP address of wwwgooglecom DNS
DNS
UDP
IP
Eth
Phy
DNS
DNS
DNS
DNS query created encapsulated in UDP encapsulated in IP encapsulated in Eth To send frame to router need MAC address of router interface ARP
ARP query broadcast received by
router which replies with ARP reply giving MAC address of router interface
client now knows MAC address of first hop router so can now send frame containing DNS query
ARP query
Eth
Phy
ARP
ARP
ARP reply
router
(runs DHCP)
Link Layer 5-93
DNS
UDP
IP
Eth
Phy
DNS
DNS
DNS
DNS
DNS
IP datagram containing DNS query forwarded via LAN switch from client to 1st hop router
IP datagram forwarded from campus network into comcast network routed (tables created by RIP OSPF IS-IS andor BGP routing protocols) to DNS server
demuxrsquoed to DNS server
DNS server replies to client with IP address of wwwgooglecom
Comcast network
68800013
DNS server
DNS
UDP
IP
Eth
Phy
DNS
DNS
DNS
DNS
A day in the lifehellip using DNS
router
(runs DHCP)
Link Layer 5-94
A day in the lifehellipTCP connection carrying HTTP
HTTP
TCP
IP
Eth
Phy
HTTP
to send HTTP request client first opens TCP socket to web server
TCP SYN segment (step 1 in 3-way handshake) inter-domain routed to web server
TCP connection established 64233169105
web server
SYN
SYN
SYN
SYN
TCP
IP
Eth
Phy
SYN
SYN
SYN
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
web server responds with TCP SYNACK (step 2 in 3-way handshake)
router
(runs DHCP)
Link Layer 5-95
A day in the lifehellip HTTP requestreply
HTTP
TCP
IP
Eth
Phy
HTTP
HTTP request sent into TCP socket
IP datagram containing HTTP request routed to wwwgooglecom
IP datagram containing HTTP reply routed back to client
64233169105
web server
HTTP
TCP
IP
Eth
Phy web server responds with
HTTP reply (containing web page)
HTTP
HTTP
HTTP HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
web page finally () displayed
Link Layer 5-96
Chapter 5 Summary
principles behind data link layer services error detection correction
sharing a broadcast channel multiple access
link layer addressing
instantiation and implementation of various link layer technologies Ethernet
switched LANS VLANs
virtualized networks as a link layer MPLS
synthesis a day in the life of a web request
Link Layer 5-97
Chapter 5 letrsquos take a breath
journey down protocol stack complete (except PHY)
solid understanding of networking principles practice
hellip could stop here hellip but lots of interesting topics wireless
multimedia
security
network management
Link Layer 5-68
Self-learning multi-switch example
Suppose C sends frame to I I responds to C
Q show switch tables and packet forwarding in S1 S2 S3 S4
A
B
S1
C D
E
F
S2
S4
S3
H
I
G
Link Layer 5-69
Institutional network
to external
network
router
IP subnet
mail server
web server
Link Layer 5-70
Switches vs routers
both are store-and-forward
routers network-layer devices (examine network-layer headers)
switches link-layer devices (examine link-layer headers)
both have forwarding tables
routers compute tables using routing algorithms IP addresses
switches learn forwarding table using flooding learning MAC addresses
application
transport
network
link
physical
network
link
physical
link
physical
switch
datagram
application
transport
network
link
physical
frame
frame
frame
datagram
Link Layer 5-71
VLANs motivation
consider CS user moves office to
EE but wants connect to CS switch
single broadcast domain
all layer-2 broadcast traffic (ARP DHCP unknown location of destination MAC address) must cross entire LAN
securityprivacy efficiency issues
Computer
Science Electrical
Engineering
Computer
Engineering
Link Layer 5-72
VLANs port-based VLAN switch ports
grouped (by switch management software) so that single physical switch helliphellip
switch(es) supporting
VLAN capabilities can
be configured to
define multiple virtual
LANS over single
physical LAN
infrastructure
Virtual Local
Area Network 1
8
9
16 10 2
7
hellip
Electrical Engineering
(VLAN ports 1-8)
Computer Science
(VLAN ports 9-15)
15
hellip
Electrical Engineering
(VLAN ports 1-8)
hellip
1
8 2
7 9
16 10
15
hellip
Computer Science
(VLAN ports 9-16)
hellip operates as multiple virtual switches
Link Layer 5-73
Port-based VLAN
1
8
9
16 10 2
7
hellip
Electrical Engineering
(VLAN ports 1-8)
Computer Science
(VLAN ports 9-15)
15
hellip
traffic isolation frames tofrom ports 1-8 can only reach ports 1-8 can also define VLAN based on
MAC addresses of endpoints rather than switch port
dynamic membership ports can be dynamically assigned among VLANs
router
forwarding between VLANS done via routing (just as with separate switches) in practice vendors sell combined
switches plus routers
Link Layer 5-74
VLANS spanning multiple switches
trunk port carries frames between VLANS defined over multiple physical switches frames forwarded within VLAN between switches canrsquot be vanilla
8021 frames (must carry VLAN ID info)
8021q protocol addsremoved additional header fields for frames forwarded between trunk ports
1
8
9
10 2
7
hellip
Electrical Engineering
(VLAN ports 1-8)
Computer Science
(VLAN ports 9-15)
15
hellip
2
7 3
Ports 235 belong to EE VLAN
Ports 4678 belong to CS VLAN
5
4 6 8 16
1
Link Layer 5-75
type
2-byte Tag Protocol Identifier
(value 81-00)
Tag Control Information (12 bit VLAN ID field
3 bit priority field like IP TOS)
Recomputed CRC
8021Q VLAN frame format
8021 frame
8021Q frame
dest address
source address
data (payload) CRC preamble
dest address
source address
preamble data (payload) CRC
type
Link Layer 5-76
Link layer LANs outline
51 introduction services
52 error detection correction
53 multiple access protocols
54 LANs addressing ARP
Ethernet
switches
VLANS
55 link virtualization MPLS
56 data center networking
57 a day in the life of a web request
Link Layer 5-77
Multiprotocol label switching (MPLS)
initial goal high-speed IP forwarding using fixed length label (instead of IP address) fast lookup using fixed length identifier (rather than
shortest prefix matching)
borrowing ideas from Virtual Circuit (VC) approach
but IP datagram still keeps IP address
PPP or Ethernet
header IP header remainder of link-layer frame MPLS header
label Exp S TTL
20 3 1 5
Link Layer 5-78
MPLS capable routers
aka label-switched router
forward packets to outgoing interface based only on label value (donrsquot inspect IP address) MPLS forwarding table distinct from IP forwarding tables
flexibility MPLS forwarding decisions can differ from those of IP use destination and source addresses to route flows to
same destination differently (traffic engineering)
re-route flows quickly if link fails pre-computed backup paths (useful for VoIP)
Link Layer 5-79
R2
D
R3 R5
A
R6
MPLS versus IP paths
IP router IP routing path to destination determined
by destination address alone
R4
Link Layer 5-80
R2
D
R3 R4
R5
A
R6
MPLS versus IP paths
IP-only router
IP routing path to destination determined by destination address alone
MPLS and IP router
MPLS routing path to destination can be based on source and dest address fast reroute precompute backup routes in
case of link failure
entry router (R4) can use different MPLS
routes to A based eg on source address
Link Layer 5-81
MPLS signaling
modify OSPF IS-IS link-state flooding protocols to carry info used by MPLS routing eg link bandwidth amount of ldquoreservedrdquo link bandwidth
D
R4
R5
A
R6
entry MPLS router uses RSVP-TE signaling protocol to set up MPLS forwarding at downstream routers
modified link state flooding
RSVP-TE
Link Layer 5-82
R1 R2
D
R3 R4 R5
0
1
0 0
A
R6
in out out
label label dest interface
6 - A 0
in out out
label label dest interface
10 6 A 1
12 9 D 0
in out out
label label dest interface
10 A 0
12 D 0
1
in out out
label label dest interface
8 6 A 0
0
8 A 1
MPLS forwarding tables
Link Layer 5-83
Link layer LANs outline
51 introduction services
52 error detection correction
53 multiple access protocols
54 LANs addressing ARP
Ethernet
switches
VLANS
55 link virtualization MPLS
56 data center networking
57 a day in the life of a web request
Link Layer 5-84
Data center networks
10rsquos to 100rsquos of thousands of hosts often closely coupled in close proximity e-business (eg Amazon)
content-servers (eg YouTube Akamai Apple Microsoft)
search engines data mining (eg Google)
challenges
multiple applications each serving massive numbers of clients
managingbalancing load avoiding processing networking data bottlenecks
Inside a 40-ft Microsoft container
Chicago data center
Link Layer 5-85
Server racks
TOR switches
Tier-1 switches
Tier-2 switches
Load balancer
Load balancer
B
1 2 3 4 5 6 7 8
A C
Border router
Access router
Internet
Data center networks
load balancer application-layer routing receives external client requests
directs workload within data center
returns results to external client (hiding data
center internals from client)
Server racks
TOR switches
Tier-1 switches
Tier-2 switches
1 2 3 4 5 6 7 8
Data center networks
rich interconnection among switches racks
increased throughput between racks (multiple routing
paths possible)
increased reliability via redundancy
Link Layer 5-87
Link layer LANs outline
51 introduction services
52 error detection correction
53 multiple access protocols
54 LANs addressing ARP
Ethernet
switches
VLANS
55 link virtualization MPLS
56 data center networking
57 a day in the life of a web request
Link Layer 5-88
Synthesis a day in the life of a web request
journey down protocol stack complete application transport network link
putting-it-all-together synthesis goal identify review understand protocols (at all
layers) involved in seemingly simple scenario requesting www page
scenario student attaches laptop to campus network requestsreceives wwwgooglecom
Link Layer 5-89
A day in the life scenario
Comcast network
68800013
Googlersquos network
64233160019 64233169105
web server
DNS server
school network
68802024
web page
browser
router
(runs DHCP)
Link Layer 5-90
A day in the lifehellip connecting to the Internet
connecting laptop needs to get its own IP address addr of first-hop router addr of DNS server use DHCP
DHCP
UDP
IP
Eth
Phy
DHCP
DHCP
DHCP
DHCP
DHCP
DHCP
UDP
IP
Eth
Phy
DHCP
DHCP
DHCP
DHCP DHCP
DHCP request encapsulated in UDP encapsulated in IP encapsulated in 8023 Ethernet
Ethernet frame broadcast
(dest FFFFFFFFFFFF) on LAN received at router running DHCP server
Ethernet demuxed to IP demuxed UDP demuxed to DHCP
router
(runs DHCP)
Link Layer 5-91
DHCP server formulates DHCP ACK containing clientrsquos IP address IP address of first-hop router for client name amp IP address of DNS server
DHCP
UDP
IP
Eth
Phy
DHCP
DHCP
DHCP
DHCP
DHCP
UDP
IP
Eth
Phy
DHCP
DHCP
DHCP
DHCP
DHCP
encapsulation at DHCP server frame forwarded (switch learning) through LAN demultiplexing at client
Client now has IP address knows name amp addr of DNS
server IP address of its first-hop router
DHCP client receives DHCP ACK reply
A day in the lifehellip connecting to the Internet
router
(runs DHCP)
Link Layer 5-92
A day in the lifehellip ARP (before DNS before HTTP)
before sending HTTP request need IP address of wwwgooglecom DNS
DNS
UDP
IP
Eth
Phy
DNS
DNS
DNS
DNS query created encapsulated in UDP encapsulated in IP encapsulated in Eth To send frame to router need MAC address of router interface ARP
ARP query broadcast received by
router which replies with ARP reply giving MAC address of router interface
client now knows MAC address of first hop router so can now send frame containing DNS query
ARP query
Eth
Phy
ARP
ARP
ARP reply
router
(runs DHCP)
Link Layer 5-93
DNS
UDP
IP
Eth
Phy
DNS
DNS
DNS
DNS
DNS
IP datagram containing DNS query forwarded via LAN switch from client to 1st hop router
IP datagram forwarded from campus network into comcast network routed (tables created by RIP OSPF IS-IS andor BGP routing protocols) to DNS server
demuxrsquoed to DNS server
DNS server replies to client with IP address of wwwgooglecom
Comcast network
68800013
DNS server
DNS
UDP
IP
Eth
Phy
DNS
DNS
DNS
DNS
A day in the lifehellip using DNS
router
(runs DHCP)
Link Layer 5-94
A day in the lifehellipTCP connection carrying HTTP
HTTP
TCP
IP
Eth
Phy
HTTP
to send HTTP request client first opens TCP socket to web server
TCP SYN segment (step 1 in 3-way handshake) inter-domain routed to web server
TCP connection established 64233169105
web server
SYN
SYN
SYN
SYN
TCP
IP
Eth
Phy
SYN
SYN
SYN
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
web server responds with TCP SYNACK (step 2 in 3-way handshake)
router
(runs DHCP)
Link Layer 5-95
A day in the lifehellip HTTP requestreply
HTTP
TCP
IP
Eth
Phy
HTTP
HTTP request sent into TCP socket
IP datagram containing HTTP request routed to wwwgooglecom
IP datagram containing HTTP reply routed back to client
64233169105
web server
HTTP
TCP
IP
Eth
Phy web server responds with
HTTP reply (containing web page)
HTTP
HTTP
HTTP HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
web page finally () displayed
Link Layer 5-96
Chapter 5 Summary
principles behind data link layer services error detection correction
sharing a broadcast channel multiple access
link layer addressing
instantiation and implementation of various link layer technologies Ethernet
switched LANS VLANs
virtualized networks as a link layer MPLS
synthesis a day in the life of a web request
Link Layer 5-97
Chapter 5 letrsquos take a breath
journey down protocol stack complete (except PHY)
solid understanding of networking principles practice
hellip could stop here hellip but lots of interesting topics wireless
multimedia
security
network management
Link Layer 5-69
Institutional network
to external
network
router
IP subnet
mail server
web server
Link Layer 5-70
Switches vs routers
both are store-and-forward
routers network-layer devices (examine network-layer headers)
switches link-layer devices (examine link-layer headers)
both have forwarding tables
routers compute tables using routing algorithms IP addresses
switches learn forwarding table using flooding learning MAC addresses
application
transport
network
link
physical
network
link
physical
link
physical
switch
datagram
application
transport
network
link
physical
frame
frame
frame
datagram
Link Layer 5-71
VLANs motivation
consider CS user moves office to
EE but wants connect to CS switch
single broadcast domain
all layer-2 broadcast traffic (ARP DHCP unknown location of destination MAC address) must cross entire LAN
securityprivacy efficiency issues
Computer
Science Electrical
Engineering
Computer
Engineering
Link Layer 5-72
VLANs port-based VLAN switch ports
grouped (by switch management software) so that single physical switch helliphellip
switch(es) supporting
VLAN capabilities can
be configured to
define multiple virtual
LANS over single
physical LAN
infrastructure
Virtual Local
Area Network 1
8
9
16 10 2
7
hellip
Electrical Engineering
(VLAN ports 1-8)
Computer Science
(VLAN ports 9-15)
15
hellip
Electrical Engineering
(VLAN ports 1-8)
hellip
1
8 2
7 9
16 10
15
hellip
Computer Science
(VLAN ports 9-16)
hellip operates as multiple virtual switches
Link Layer 5-73
Port-based VLAN
1
8
9
16 10 2
7
hellip
Electrical Engineering
(VLAN ports 1-8)
Computer Science
(VLAN ports 9-15)
15
hellip
traffic isolation frames tofrom ports 1-8 can only reach ports 1-8 can also define VLAN based on
MAC addresses of endpoints rather than switch port
dynamic membership ports can be dynamically assigned among VLANs
router
forwarding between VLANS done via routing (just as with separate switches) in practice vendors sell combined
switches plus routers
Link Layer 5-74
VLANS spanning multiple switches
trunk port carries frames between VLANS defined over multiple physical switches frames forwarded within VLAN between switches canrsquot be vanilla
8021 frames (must carry VLAN ID info)
8021q protocol addsremoved additional header fields for frames forwarded between trunk ports
1
8
9
10 2
7
hellip
Electrical Engineering
(VLAN ports 1-8)
Computer Science
(VLAN ports 9-15)
15
hellip
2
7 3
Ports 235 belong to EE VLAN
Ports 4678 belong to CS VLAN
5
4 6 8 16
1
Link Layer 5-75
type
2-byte Tag Protocol Identifier
(value 81-00)
Tag Control Information (12 bit VLAN ID field
3 bit priority field like IP TOS)
Recomputed CRC
8021Q VLAN frame format
8021 frame
8021Q frame
dest address
source address
data (payload) CRC preamble
dest address
source address
preamble data (payload) CRC
type
Link Layer 5-76
Link layer LANs outline
51 introduction services
52 error detection correction
53 multiple access protocols
54 LANs addressing ARP
Ethernet
switches
VLANS
55 link virtualization MPLS
56 data center networking
57 a day in the life of a web request
Link Layer 5-77
Multiprotocol label switching (MPLS)
initial goal high-speed IP forwarding using fixed length label (instead of IP address) fast lookup using fixed length identifier (rather than
shortest prefix matching)
borrowing ideas from Virtual Circuit (VC) approach
but IP datagram still keeps IP address
PPP or Ethernet
header IP header remainder of link-layer frame MPLS header
label Exp S TTL
20 3 1 5
Link Layer 5-78
MPLS capable routers
aka label-switched router
forward packets to outgoing interface based only on label value (donrsquot inspect IP address) MPLS forwarding table distinct from IP forwarding tables
flexibility MPLS forwarding decisions can differ from those of IP use destination and source addresses to route flows to
same destination differently (traffic engineering)
re-route flows quickly if link fails pre-computed backup paths (useful for VoIP)
Link Layer 5-79
R2
D
R3 R5
A
R6
MPLS versus IP paths
IP router IP routing path to destination determined
by destination address alone
R4
Link Layer 5-80
R2
D
R3 R4
R5
A
R6
MPLS versus IP paths
IP-only router
IP routing path to destination determined by destination address alone
MPLS and IP router
MPLS routing path to destination can be based on source and dest address fast reroute precompute backup routes in
case of link failure
entry router (R4) can use different MPLS
routes to A based eg on source address
Link Layer 5-81
MPLS signaling
modify OSPF IS-IS link-state flooding protocols to carry info used by MPLS routing eg link bandwidth amount of ldquoreservedrdquo link bandwidth
D
R4
R5
A
R6
entry MPLS router uses RSVP-TE signaling protocol to set up MPLS forwarding at downstream routers
modified link state flooding
RSVP-TE
Link Layer 5-82
R1 R2
D
R3 R4 R5
0
1
0 0
A
R6
in out out
label label dest interface
6 - A 0
in out out
label label dest interface
10 6 A 1
12 9 D 0
in out out
label label dest interface
10 A 0
12 D 0
1
in out out
label label dest interface
8 6 A 0
0
8 A 1
MPLS forwarding tables
Link Layer 5-83
Link layer LANs outline
51 introduction services
52 error detection correction
53 multiple access protocols
54 LANs addressing ARP
Ethernet
switches
VLANS
55 link virtualization MPLS
56 data center networking
57 a day in the life of a web request
Link Layer 5-84
Data center networks
10rsquos to 100rsquos of thousands of hosts often closely coupled in close proximity e-business (eg Amazon)
content-servers (eg YouTube Akamai Apple Microsoft)
search engines data mining (eg Google)
challenges
multiple applications each serving massive numbers of clients
managingbalancing load avoiding processing networking data bottlenecks
Inside a 40-ft Microsoft container
Chicago data center
Link Layer 5-85
Server racks
TOR switches
Tier-1 switches
Tier-2 switches
Load balancer
Load balancer
B
1 2 3 4 5 6 7 8
A C
Border router
Access router
Internet
Data center networks
load balancer application-layer routing receives external client requests
directs workload within data center
returns results to external client (hiding data
center internals from client)
Server racks
TOR switches
Tier-1 switches
Tier-2 switches
1 2 3 4 5 6 7 8
Data center networks
rich interconnection among switches racks
increased throughput between racks (multiple routing
paths possible)
increased reliability via redundancy
Link Layer 5-87
Link layer LANs outline
51 introduction services
52 error detection correction
53 multiple access protocols
54 LANs addressing ARP
Ethernet
switches
VLANS
55 link virtualization MPLS
56 data center networking
57 a day in the life of a web request
Link Layer 5-88
Synthesis a day in the life of a web request
journey down protocol stack complete application transport network link
putting-it-all-together synthesis goal identify review understand protocols (at all
layers) involved in seemingly simple scenario requesting www page
scenario student attaches laptop to campus network requestsreceives wwwgooglecom
Link Layer 5-89
A day in the life scenario
Comcast network
68800013
Googlersquos network
64233160019 64233169105
web server
DNS server
school network
68802024
web page
browser
router
(runs DHCP)
Link Layer 5-90
A day in the lifehellip connecting to the Internet
connecting laptop needs to get its own IP address addr of first-hop router addr of DNS server use DHCP
DHCP
UDP
IP
Eth
Phy
DHCP
DHCP
DHCP
DHCP
DHCP
DHCP
UDP
IP
Eth
Phy
DHCP
DHCP
DHCP
DHCP DHCP
DHCP request encapsulated in UDP encapsulated in IP encapsulated in 8023 Ethernet
Ethernet frame broadcast
(dest FFFFFFFFFFFF) on LAN received at router running DHCP server
Ethernet demuxed to IP demuxed UDP demuxed to DHCP
router
(runs DHCP)
Link Layer 5-91
DHCP server formulates DHCP ACK containing clientrsquos IP address IP address of first-hop router for client name amp IP address of DNS server
DHCP
UDP
IP
Eth
Phy
DHCP
DHCP
DHCP
DHCP
DHCP
UDP
IP
Eth
Phy
DHCP
DHCP
DHCP
DHCP
DHCP
encapsulation at DHCP server frame forwarded (switch learning) through LAN demultiplexing at client
Client now has IP address knows name amp addr of DNS
server IP address of its first-hop router
DHCP client receives DHCP ACK reply
A day in the lifehellip connecting to the Internet
router
(runs DHCP)
Link Layer 5-92
A day in the lifehellip ARP (before DNS before HTTP)
before sending HTTP request need IP address of wwwgooglecom DNS
DNS
UDP
IP
Eth
Phy
DNS
DNS
DNS
DNS query created encapsulated in UDP encapsulated in IP encapsulated in Eth To send frame to router need MAC address of router interface ARP
ARP query broadcast received by
router which replies with ARP reply giving MAC address of router interface
client now knows MAC address of first hop router so can now send frame containing DNS query
ARP query
Eth
Phy
ARP
ARP
ARP reply
router
(runs DHCP)
Link Layer 5-93
DNS
UDP
IP
Eth
Phy
DNS
DNS
DNS
DNS
DNS
IP datagram containing DNS query forwarded via LAN switch from client to 1st hop router
IP datagram forwarded from campus network into comcast network routed (tables created by RIP OSPF IS-IS andor BGP routing protocols) to DNS server
demuxrsquoed to DNS server
DNS server replies to client with IP address of wwwgooglecom
Comcast network
68800013
DNS server
DNS
UDP
IP
Eth
Phy
DNS
DNS
DNS
DNS
A day in the lifehellip using DNS
router
(runs DHCP)
Link Layer 5-94
A day in the lifehellipTCP connection carrying HTTP
HTTP
TCP
IP
Eth
Phy
HTTP
to send HTTP request client first opens TCP socket to web server
TCP SYN segment (step 1 in 3-way handshake) inter-domain routed to web server
TCP connection established 64233169105
web server
SYN
SYN
SYN
SYN
TCP
IP
Eth
Phy
SYN
SYN
SYN
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
web server responds with TCP SYNACK (step 2 in 3-way handshake)
router
(runs DHCP)
Link Layer 5-95
A day in the lifehellip HTTP requestreply
HTTP
TCP
IP
Eth
Phy
HTTP
HTTP request sent into TCP socket
IP datagram containing HTTP request routed to wwwgooglecom
IP datagram containing HTTP reply routed back to client
64233169105
web server
HTTP
TCP
IP
Eth
Phy web server responds with
HTTP reply (containing web page)
HTTP
HTTP
HTTP HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
web page finally () displayed
Link Layer 5-96
Chapter 5 Summary
principles behind data link layer services error detection correction
sharing a broadcast channel multiple access
link layer addressing
instantiation and implementation of various link layer technologies Ethernet
switched LANS VLANs
virtualized networks as a link layer MPLS
synthesis a day in the life of a web request
Link Layer 5-97
Chapter 5 letrsquos take a breath
journey down protocol stack complete (except PHY)
solid understanding of networking principles practice
hellip could stop here hellip but lots of interesting topics wireless
multimedia
security
network management
Link Layer 5-70
Switches vs routers
both are store-and-forward
routers network-layer devices (examine network-layer headers)
switches link-layer devices (examine link-layer headers)
both have forwarding tables
routers compute tables using routing algorithms IP addresses
switches learn forwarding table using flooding learning MAC addresses
application
transport
network
link
physical
network
link
physical
link
physical
switch
datagram
application
transport
network
link
physical
frame
frame
frame
datagram
Link Layer 5-71
VLANs motivation
consider CS user moves office to
EE but wants connect to CS switch
single broadcast domain
all layer-2 broadcast traffic (ARP DHCP unknown location of destination MAC address) must cross entire LAN
securityprivacy efficiency issues
Computer
Science Electrical
Engineering
Computer
Engineering
Link Layer 5-72
VLANs port-based VLAN switch ports
grouped (by switch management software) so that single physical switch helliphellip
switch(es) supporting
VLAN capabilities can
be configured to
define multiple virtual
LANS over single
physical LAN
infrastructure
Virtual Local
Area Network 1
8
9
16 10 2
7
hellip
Electrical Engineering
(VLAN ports 1-8)
Computer Science
(VLAN ports 9-15)
15
hellip
Electrical Engineering
(VLAN ports 1-8)
hellip
1
8 2
7 9
16 10
15
hellip
Computer Science
(VLAN ports 9-16)
hellip operates as multiple virtual switches
Link Layer 5-73
Port-based VLAN
1
8
9
16 10 2
7
hellip
Electrical Engineering
(VLAN ports 1-8)
Computer Science
(VLAN ports 9-15)
15
hellip
traffic isolation frames tofrom ports 1-8 can only reach ports 1-8 can also define VLAN based on
MAC addresses of endpoints rather than switch port
dynamic membership ports can be dynamically assigned among VLANs
router
forwarding between VLANS done via routing (just as with separate switches) in practice vendors sell combined
switches plus routers
Link Layer 5-74
VLANS spanning multiple switches
trunk port carries frames between VLANS defined over multiple physical switches frames forwarded within VLAN between switches canrsquot be vanilla
8021 frames (must carry VLAN ID info)
8021q protocol addsremoved additional header fields for frames forwarded between trunk ports
1
8
9
10 2
7
hellip
Electrical Engineering
(VLAN ports 1-8)
Computer Science
(VLAN ports 9-15)
15
hellip
2
7 3
Ports 235 belong to EE VLAN
Ports 4678 belong to CS VLAN
5
4 6 8 16
1
Link Layer 5-75
type
2-byte Tag Protocol Identifier
(value 81-00)
Tag Control Information (12 bit VLAN ID field
3 bit priority field like IP TOS)
Recomputed CRC
8021Q VLAN frame format
8021 frame
8021Q frame
dest address
source address
data (payload) CRC preamble
dest address
source address
preamble data (payload) CRC
type
Link Layer 5-76
Link layer LANs outline
51 introduction services
52 error detection correction
53 multiple access protocols
54 LANs addressing ARP
Ethernet
switches
VLANS
55 link virtualization MPLS
56 data center networking
57 a day in the life of a web request
Link Layer 5-77
Multiprotocol label switching (MPLS)
initial goal high-speed IP forwarding using fixed length label (instead of IP address) fast lookup using fixed length identifier (rather than
shortest prefix matching)
borrowing ideas from Virtual Circuit (VC) approach
but IP datagram still keeps IP address
PPP or Ethernet
header IP header remainder of link-layer frame MPLS header
label Exp S TTL
20 3 1 5
Link Layer 5-78
MPLS capable routers
aka label-switched router
forward packets to outgoing interface based only on label value (donrsquot inspect IP address) MPLS forwarding table distinct from IP forwarding tables
flexibility MPLS forwarding decisions can differ from those of IP use destination and source addresses to route flows to
same destination differently (traffic engineering)
re-route flows quickly if link fails pre-computed backup paths (useful for VoIP)
Link Layer 5-79
R2
D
R3 R5
A
R6
MPLS versus IP paths
IP router IP routing path to destination determined
by destination address alone
R4
Link Layer 5-80
R2
D
R3 R4
R5
A
R6
MPLS versus IP paths
IP-only router
IP routing path to destination determined by destination address alone
MPLS and IP router
MPLS routing path to destination can be based on source and dest address fast reroute precompute backup routes in
case of link failure
entry router (R4) can use different MPLS
routes to A based eg on source address
Link Layer 5-81
MPLS signaling
modify OSPF IS-IS link-state flooding protocols to carry info used by MPLS routing eg link bandwidth amount of ldquoreservedrdquo link bandwidth
D
R4
R5
A
R6
entry MPLS router uses RSVP-TE signaling protocol to set up MPLS forwarding at downstream routers
modified link state flooding
RSVP-TE
Link Layer 5-82
R1 R2
D
R3 R4 R5
0
1
0 0
A
R6
in out out
label label dest interface
6 - A 0
in out out
label label dest interface
10 6 A 1
12 9 D 0
in out out
label label dest interface
10 A 0
12 D 0
1
in out out
label label dest interface
8 6 A 0
0
8 A 1
MPLS forwarding tables
Link Layer 5-83
Link layer LANs outline
51 introduction services
52 error detection correction
53 multiple access protocols
54 LANs addressing ARP
Ethernet
switches
VLANS
55 link virtualization MPLS
56 data center networking
57 a day in the life of a web request
Link Layer 5-84
Data center networks
10rsquos to 100rsquos of thousands of hosts often closely coupled in close proximity e-business (eg Amazon)
content-servers (eg YouTube Akamai Apple Microsoft)
search engines data mining (eg Google)
challenges
multiple applications each serving massive numbers of clients
managingbalancing load avoiding processing networking data bottlenecks
Inside a 40-ft Microsoft container
Chicago data center
Link Layer 5-85
Server racks
TOR switches
Tier-1 switches
Tier-2 switches
Load balancer
Load balancer
B
1 2 3 4 5 6 7 8
A C
Border router
Access router
Internet
Data center networks
load balancer application-layer routing receives external client requests
directs workload within data center
returns results to external client (hiding data
center internals from client)
Server racks
TOR switches
Tier-1 switches
Tier-2 switches
1 2 3 4 5 6 7 8
Data center networks
rich interconnection among switches racks
increased throughput between racks (multiple routing
paths possible)
increased reliability via redundancy
Link Layer 5-87
Link layer LANs outline
51 introduction services
52 error detection correction
53 multiple access protocols
54 LANs addressing ARP
Ethernet
switches
VLANS
55 link virtualization MPLS
56 data center networking
57 a day in the life of a web request
Link Layer 5-88
Synthesis a day in the life of a web request
journey down protocol stack complete application transport network link
putting-it-all-together synthesis goal identify review understand protocols (at all
layers) involved in seemingly simple scenario requesting www page
scenario student attaches laptop to campus network requestsreceives wwwgooglecom
Link Layer 5-89
A day in the life scenario
Comcast network
68800013
Googlersquos network
64233160019 64233169105
web server
DNS server
school network
68802024
web page
browser
router
(runs DHCP)
Link Layer 5-90
A day in the lifehellip connecting to the Internet
connecting laptop needs to get its own IP address addr of first-hop router addr of DNS server use DHCP
DHCP
UDP
IP
Eth
Phy
DHCP
DHCP
DHCP
DHCP
DHCP
DHCP
UDP
IP
Eth
Phy
DHCP
DHCP
DHCP
DHCP DHCP
DHCP request encapsulated in UDP encapsulated in IP encapsulated in 8023 Ethernet
Ethernet frame broadcast
(dest FFFFFFFFFFFF) on LAN received at router running DHCP server
Ethernet demuxed to IP demuxed UDP demuxed to DHCP
router
(runs DHCP)
Link Layer 5-91
DHCP server formulates DHCP ACK containing clientrsquos IP address IP address of first-hop router for client name amp IP address of DNS server
DHCP
UDP
IP
Eth
Phy
DHCP
DHCP
DHCP
DHCP
DHCP
UDP
IP
Eth
Phy
DHCP
DHCP
DHCP
DHCP
DHCP
encapsulation at DHCP server frame forwarded (switch learning) through LAN demultiplexing at client
Client now has IP address knows name amp addr of DNS
server IP address of its first-hop router
DHCP client receives DHCP ACK reply
A day in the lifehellip connecting to the Internet
router
(runs DHCP)
Link Layer 5-92
A day in the lifehellip ARP (before DNS before HTTP)
before sending HTTP request need IP address of wwwgooglecom DNS
DNS
UDP
IP
Eth
Phy
DNS
DNS
DNS
DNS query created encapsulated in UDP encapsulated in IP encapsulated in Eth To send frame to router need MAC address of router interface ARP
ARP query broadcast received by
router which replies with ARP reply giving MAC address of router interface
client now knows MAC address of first hop router so can now send frame containing DNS query
ARP query
Eth
Phy
ARP
ARP
ARP reply
router
(runs DHCP)
Link Layer 5-93
DNS
UDP
IP
Eth
Phy
DNS
DNS
DNS
DNS
DNS
IP datagram containing DNS query forwarded via LAN switch from client to 1st hop router
IP datagram forwarded from campus network into comcast network routed (tables created by RIP OSPF IS-IS andor BGP routing protocols) to DNS server
demuxrsquoed to DNS server
DNS server replies to client with IP address of wwwgooglecom
Comcast network
68800013
DNS server
DNS
UDP
IP
Eth
Phy
DNS
DNS
DNS
DNS
A day in the lifehellip using DNS
router
(runs DHCP)
Link Layer 5-94
A day in the lifehellipTCP connection carrying HTTP
HTTP
TCP
IP
Eth
Phy
HTTP
to send HTTP request client first opens TCP socket to web server
TCP SYN segment (step 1 in 3-way handshake) inter-domain routed to web server
TCP connection established 64233169105
web server
SYN
SYN
SYN
SYN
TCP
IP
Eth
Phy
SYN
SYN
SYN
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
web server responds with TCP SYNACK (step 2 in 3-way handshake)
router
(runs DHCP)
Link Layer 5-95
A day in the lifehellip HTTP requestreply
HTTP
TCP
IP
Eth
Phy
HTTP
HTTP request sent into TCP socket
IP datagram containing HTTP request routed to wwwgooglecom
IP datagram containing HTTP reply routed back to client
64233169105
web server
HTTP
TCP
IP
Eth
Phy web server responds with
HTTP reply (containing web page)
HTTP
HTTP
HTTP HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
web page finally () displayed
Link Layer 5-96
Chapter 5 Summary
principles behind data link layer services error detection correction
sharing a broadcast channel multiple access
link layer addressing
instantiation and implementation of various link layer technologies Ethernet
switched LANS VLANs
virtualized networks as a link layer MPLS
synthesis a day in the life of a web request
Link Layer 5-97
Chapter 5 letrsquos take a breath
journey down protocol stack complete (except PHY)
solid understanding of networking principles practice
hellip could stop here hellip but lots of interesting topics wireless
multimedia
security
network management
Link Layer 5-71
VLANs motivation
consider CS user moves office to
EE but wants connect to CS switch
single broadcast domain
all layer-2 broadcast traffic (ARP DHCP unknown location of destination MAC address) must cross entire LAN
securityprivacy efficiency issues
Computer
Science Electrical
Engineering
Computer
Engineering
Link Layer 5-72
VLANs port-based VLAN switch ports
grouped (by switch management software) so that single physical switch helliphellip
switch(es) supporting
VLAN capabilities can
be configured to
define multiple virtual
LANS over single
physical LAN
infrastructure
Virtual Local
Area Network 1
8
9
16 10 2
7
hellip
Electrical Engineering
(VLAN ports 1-8)
Computer Science
(VLAN ports 9-15)
15
hellip
Electrical Engineering
(VLAN ports 1-8)
hellip
1
8 2
7 9
16 10
15
hellip
Computer Science
(VLAN ports 9-16)
hellip operates as multiple virtual switches
Link Layer 5-73
Port-based VLAN
1
8
9
16 10 2
7
hellip
Electrical Engineering
(VLAN ports 1-8)
Computer Science
(VLAN ports 9-15)
15
hellip
traffic isolation frames tofrom ports 1-8 can only reach ports 1-8 can also define VLAN based on
MAC addresses of endpoints rather than switch port
dynamic membership ports can be dynamically assigned among VLANs
router
forwarding between VLANS done via routing (just as with separate switches) in practice vendors sell combined
switches plus routers
Link Layer 5-74
VLANS spanning multiple switches
trunk port carries frames between VLANS defined over multiple physical switches frames forwarded within VLAN between switches canrsquot be vanilla
8021 frames (must carry VLAN ID info)
8021q protocol addsremoved additional header fields for frames forwarded between trunk ports
1
8
9
10 2
7
hellip
Electrical Engineering
(VLAN ports 1-8)
Computer Science
(VLAN ports 9-15)
15
hellip
2
7 3
Ports 235 belong to EE VLAN
Ports 4678 belong to CS VLAN
5
4 6 8 16
1
Link Layer 5-75
type
2-byte Tag Protocol Identifier
(value 81-00)
Tag Control Information (12 bit VLAN ID field
3 bit priority field like IP TOS)
Recomputed CRC
8021Q VLAN frame format
8021 frame
8021Q frame
dest address
source address
data (payload) CRC preamble
dest address
source address
preamble data (payload) CRC
type
Link Layer 5-76
Link layer LANs outline
51 introduction services
52 error detection correction
53 multiple access protocols
54 LANs addressing ARP
Ethernet
switches
VLANS
55 link virtualization MPLS
56 data center networking
57 a day in the life of a web request
Link Layer 5-77
Multiprotocol label switching (MPLS)
initial goal high-speed IP forwarding using fixed length label (instead of IP address) fast lookup using fixed length identifier (rather than
shortest prefix matching)
borrowing ideas from Virtual Circuit (VC) approach
but IP datagram still keeps IP address
PPP or Ethernet
header IP header remainder of link-layer frame MPLS header
label Exp S TTL
20 3 1 5
Link Layer 5-78
MPLS capable routers
aka label-switched router
forward packets to outgoing interface based only on label value (donrsquot inspect IP address) MPLS forwarding table distinct from IP forwarding tables
flexibility MPLS forwarding decisions can differ from those of IP use destination and source addresses to route flows to
same destination differently (traffic engineering)
re-route flows quickly if link fails pre-computed backup paths (useful for VoIP)
Link Layer 5-79
R2
D
R3 R5
A
R6
MPLS versus IP paths
IP router IP routing path to destination determined
by destination address alone
R4
Link Layer 5-80
R2
D
R3 R4
R5
A
R6
MPLS versus IP paths
IP-only router
IP routing path to destination determined by destination address alone
MPLS and IP router
MPLS routing path to destination can be based on source and dest address fast reroute precompute backup routes in
case of link failure
entry router (R4) can use different MPLS
routes to A based eg on source address
Link Layer 5-81
MPLS signaling
modify OSPF IS-IS link-state flooding protocols to carry info used by MPLS routing eg link bandwidth amount of ldquoreservedrdquo link bandwidth
D
R4
R5
A
R6
entry MPLS router uses RSVP-TE signaling protocol to set up MPLS forwarding at downstream routers
modified link state flooding
RSVP-TE
Link Layer 5-82
R1 R2
D
R3 R4 R5
0
1
0 0
A
R6
in out out
label label dest interface
6 - A 0
in out out
label label dest interface
10 6 A 1
12 9 D 0
in out out
label label dest interface
10 A 0
12 D 0
1
in out out
label label dest interface
8 6 A 0
0
8 A 1
MPLS forwarding tables
Link Layer 5-83
Link layer LANs outline
51 introduction services
52 error detection correction
53 multiple access protocols
54 LANs addressing ARP
Ethernet
switches
VLANS
55 link virtualization MPLS
56 data center networking
57 a day in the life of a web request
Link Layer 5-84
Data center networks
10rsquos to 100rsquos of thousands of hosts often closely coupled in close proximity e-business (eg Amazon)
content-servers (eg YouTube Akamai Apple Microsoft)
search engines data mining (eg Google)
challenges
multiple applications each serving massive numbers of clients
managingbalancing load avoiding processing networking data bottlenecks
Inside a 40-ft Microsoft container
Chicago data center
Link Layer 5-85
Server racks
TOR switches
Tier-1 switches
Tier-2 switches
Load balancer
Load balancer
B
1 2 3 4 5 6 7 8
A C
Border router
Access router
Internet
Data center networks
load balancer application-layer routing receives external client requests
directs workload within data center
returns results to external client (hiding data
center internals from client)
Server racks
TOR switches
Tier-1 switches
Tier-2 switches
1 2 3 4 5 6 7 8
Data center networks
rich interconnection among switches racks
increased throughput between racks (multiple routing
paths possible)
increased reliability via redundancy
Link Layer 5-87
Link layer LANs outline
51 introduction services
52 error detection correction
53 multiple access protocols
54 LANs addressing ARP
Ethernet
switches
VLANS
55 link virtualization MPLS
56 data center networking
57 a day in the life of a web request
Link Layer 5-88
Synthesis a day in the life of a web request
journey down protocol stack complete application transport network link
putting-it-all-together synthesis goal identify review understand protocols (at all
layers) involved in seemingly simple scenario requesting www page
scenario student attaches laptop to campus network requestsreceives wwwgooglecom
Link Layer 5-89
A day in the life scenario
Comcast network
68800013
Googlersquos network
64233160019 64233169105
web server
DNS server
school network
68802024
web page
browser
router
(runs DHCP)
Link Layer 5-90
A day in the lifehellip connecting to the Internet
connecting laptop needs to get its own IP address addr of first-hop router addr of DNS server use DHCP
DHCP
UDP
IP
Eth
Phy
DHCP
DHCP
DHCP
DHCP
DHCP
DHCP
UDP
IP
Eth
Phy
DHCP
DHCP
DHCP
DHCP DHCP
DHCP request encapsulated in UDP encapsulated in IP encapsulated in 8023 Ethernet
Ethernet frame broadcast
(dest FFFFFFFFFFFF) on LAN received at router running DHCP server
Ethernet demuxed to IP demuxed UDP demuxed to DHCP
router
(runs DHCP)
Link Layer 5-91
DHCP server formulates DHCP ACK containing clientrsquos IP address IP address of first-hop router for client name amp IP address of DNS server
DHCP
UDP
IP
Eth
Phy
DHCP
DHCP
DHCP
DHCP
DHCP
UDP
IP
Eth
Phy
DHCP
DHCP
DHCP
DHCP
DHCP
encapsulation at DHCP server frame forwarded (switch learning) through LAN demultiplexing at client
Client now has IP address knows name amp addr of DNS
server IP address of its first-hop router
DHCP client receives DHCP ACK reply
A day in the lifehellip connecting to the Internet
router
(runs DHCP)
Link Layer 5-92
A day in the lifehellip ARP (before DNS before HTTP)
before sending HTTP request need IP address of wwwgooglecom DNS
DNS
UDP
IP
Eth
Phy
DNS
DNS
DNS
DNS query created encapsulated in UDP encapsulated in IP encapsulated in Eth To send frame to router need MAC address of router interface ARP
ARP query broadcast received by
router which replies with ARP reply giving MAC address of router interface
client now knows MAC address of first hop router so can now send frame containing DNS query
ARP query
Eth
Phy
ARP
ARP
ARP reply
router
(runs DHCP)
Link Layer 5-93
DNS
UDP
IP
Eth
Phy
DNS
DNS
DNS
DNS
DNS
IP datagram containing DNS query forwarded via LAN switch from client to 1st hop router
IP datagram forwarded from campus network into comcast network routed (tables created by RIP OSPF IS-IS andor BGP routing protocols) to DNS server
demuxrsquoed to DNS server
DNS server replies to client with IP address of wwwgooglecom
Comcast network
68800013
DNS server
DNS
UDP
IP
Eth
Phy
DNS
DNS
DNS
DNS
A day in the lifehellip using DNS
router
(runs DHCP)
Link Layer 5-94
A day in the lifehellipTCP connection carrying HTTP
HTTP
TCP
IP
Eth
Phy
HTTP
to send HTTP request client first opens TCP socket to web server
TCP SYN segment (step 1 in 3-way handshake) inter-domain routed to web server
TCP connection established 64233169105
web server
SYN
SYN
SYN
SYN
TCP
IP
Eth
Phy
SYN
SYN
SYN
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
web server responds with TCP SYNACK (step 2 in 3-way handshake)
router
(runs DHCP)
Link Layer 5-95
A day in the lifehellip HTTP requestreply
HTTP
TCP
IP
Eth
Phy
HTTP
HTTP request sent into TCP socket
IP datagram containing HTTP request routed to wwwgooglecom
IP datagram containing HTTP reply routed back to client
64233169105
web server
HTTP
TCP
IP
Eth
Phy web server responds with
HTTP reply (containing web page)
HTTP
HTTP
HTTP HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
web page finally () displayed
Link Layer 5-96
Chapter 5 Summary
principles behind data link layer services error detection correction
sharing a broadcast channel multiple access
link layer addressing
instantiation and implementation of various link layer technologies Ethernet
switched LANS VLANs
virtualized networks as a link layer MPLS
synthesis a day in the life of a web request
Link Layer 5-97
Chapter 5 letrsquos take a breath
journey down protocol stack complete (except PHY)
solid understanding of networking principles practice
hellip could stop here hellip but lots of interesting topics wireless
multimedia
security
network management
Link Layer 5-72
VLANs port-based VLAN switch ports
grouped (by switch management software) so that single physical switch helliphellip
switch(es) supporting
VLAN capabilities can
be configured to
define multiple virtual
LANS over single
physical LAN
infrastructure
Virtual Local
Area Network 1
8
9
16 10 2
7
hellip
Electrical Engineering
(VLAN ports 1-8)
Computer Science
(VLAN ports 9-15)
15
hellip
Electrical Engineering
(VLAN ports 1-8)
hellip
1
8 2
7 9
16 10
15
hellip
Computer Science
(VLAN ports 9-16)
hellip operates as multiple virtual switches
Link Layer 5-73
Port-based VLAN
1
8
9
16 10 2
7
hellip
Electrical Engineering
(VLAN ports 1-8)
Computer Science
(VLAN ports 9-15)
15
hellip
traffic isolation frames tofrom ports 1-8 can only reach ports 1-8 can also define VLAN based on
MAC addresses of endpoints rather than switch port
dynamic membership ports can be dynamically assigned among VLANs
router
forwarding between VLANS done via routing (just as with separate switches) in practice vendors sell combined
switches plus routers
Link Layer 5-74
VLANS spanning multiple switches
trunk port carries frames between VLANS defined over multiple physical switches frames forwarded within VLAN between switches canrsquot be vanilla
8021 frames (must carry VLAN ID info)
8021q protocol addsremoved additional header fields for frames forwarded between trunk ports
1
8
9
10 2
7
hellip
Electrical Engineering
(VLAN ports 1-8)
Computer Science
(VLAN ports 9-15)
15
hellip
2
7 3
Ports 235 belong to EE VLAN
Ports 4678 belong to CS VLAN
5
4 6 8 16
1
Link Layer 5-75
type
2-byte Tag Protocol Identifier
(value 81-00)
Tag Control Information (12 bit VLAN ID field
3 bit priority field like IP TOS)
Recomputed CRC
8021Q VLAN frame format
8021 frame
8021Q frame
dest address
source address
data (payload) CRC preamble
dest address
source address
preamble data (payload) CRC
type
Link Layer 5-76
Link layer LANs outline
51 introduction services
52 error detection correction
53 multiple access protocols
54 LANs addressing ARP
Ethernet
switches
VLANS
55 link virtualization MPLS
56 data center networking
57 a day in the life of a web request
Link Layer 5-77
Multiprotocol label switching (MPLS)
initial goal high-speed IP forwarding using fixed length label (instead of IP address) fast lookup using fixed length identifier (rather than
shortest prefix matching)
borrowing ideas from Virtual Circuit (VC) approach
but IP datagram still keeps IP address
PPP or Ethernet
header IP header remainder of link-layer frame MPLS header
label Exp S TTL
20 3 1 5
Link Layer 5-78
MPLS capable routers
aka label-switched router
forward packets to outgoing interface based only on label value (donrsquot inspect IP address) MPLS forwarding table distinct from IP forwarding tables
flexibility MPLS forwarding decisions can differ from those of IP use destination and source addresses to route flows to
same destination differently (traffic engineering)
re-route flows quickly if link fails pre-computed backup paths (useful for VoIP)
Link Layer 5-79
R2
D
R3 R5
A
R6
MPLS versus IP paths
IP router IP routing path to destination determined
by destination address alone
R4
Link Layer 5-80
R2
D
R3 R4
R5
A
R6
MPLS versus IP paths
IP-only router
IP routing path to destination determined by destination address alone
MPLS and IP router
MPLS routing path to destination can be based on source and dest address fast reroute precompute backup routes in
case of link failure
entry router (R4) can use different MPLS
routes to A based eg on source address
Link Layer 5-81
MPLS signaling
modify OSPF IS-IS link-state flooding protocols to carry info used by MPLS routing eg link bandwidth amount of ldquoreservedrdquo link bandwidth
D
R4
R5
A
R6
entry MPLS router uses RSVP-TE signaling protocol to set up MPLS forwarding at downstream routers
modified link state flooding
RSVP-TE
Link Layer 5-82
R1 R2
D
R3 R4 R5
0
1
0 0
A
R6
in out out
label label dest interface
6 - A 0
in out out
label label dest interface
10 6 A 1
12 9 D 0
in out out
label label dest interface
10 A 0
12 D 0
1
in out out
label label dest interface
8 6 A 0
0
8 A 1
MPLS forwarding tables
Link Layer 5-83
Link layer LANs outline
51 introduction services
52 error detection correction
53 multiple access protocols
54 LANs addressing ARP
Ethernet
switches
VLANS
55 link virtualization MPLS
56 data center networking
57 a day in the life of a web request
Link Layer 5-84
Data center networks
10rsquos to 100rsquos of thousands of hosts often closely coupled in close proximity e-business (eg Amazon)
content-servers (eg YouTube Akamai Apple Microsoft)
search engines data mining (eg Google)
challenges
multiple applications each serving massive numbers of clients
managingbalancing load avoiding processing networking data bottlenecks
Inside a 40-ft Microsoft container
Chicago data center
Link Layer 5-85
Server racks
TOR switches
Tier-1 switches
Tier-2 switches
Load balancer
Load balancer
B
1 2 3 4 5 6 7 8
A C
Border router
Access router
Internet
Data center networks
load balancer application-layer routing receives external client requests
directs workload within data center
returns results to external client (hiding data
center internals from client)
Server racks
TOR switches
Tier-1 switches
Tier-2 switches
1 2 3 4 5 6 7 8
Data center networks
rich interconnection among switches racks
increased throughput between racks (multiple routing
paths possible)
increased reliability via redundancy
Link Layer 5-87
Link layer LANs outline
51 introduction services
52 error detection correction
53 multiple access protocols
54 LANs addressing ARP
Ethernet
switches
VLANS
55 link virtualization MPLS
56 data center networking
57 a day in the life of a web request
Link Layer 5-88
Synthesis a day in the life of a web request
journey down protocol stack complete application transport network link
putting-it-all-together synthesis goal identify review understand protocols (at all
layers) involved in seemingly simple scenario requesting www page
scenario student attaches laptop to campus network requestsreceives wwwgooglecom
Link Layer 5-89
A day in the life scenario
Comcast network
68800013
Googlersquos network
64233160019 64233169105
web server
DNS server
school network
68802024
web page
browser
router
(runs DHCP)
Link Layer 5-90
A day in the lifehellip connecting to the Internet
connecting laptop needs to get its own IP address addr of first-hop router addr of DNS server use DHCP
DHCP
UDP
IP
Eth
Phy
DHCP
DHCP
DHCP
DHCP
DHCP
DHCP
UDP
IP
Eth
Phy
DHCP
DHCP
DHCP
DHCP DHCP
DHCP request encapsulated in UDP encapsulated in IP encapsulated in 8023 Ethernet
Ethernet frame broadcast
(dest FFFFFFFFFFFF) on LAN received at router running DHCP server
Ethernet demuxed to IP demuxed UDP demuxed to DHCP
router
(runs DHCP)
Link Layer 5-91
DHCP server formulates DHCP ACK containing clientrsquos IP address IP address of first-hop router for client name amp IP address of DNS server
DHCP
UDP
IP
Eth
Phy
DHCP
DHCP
DHCP
DHCP
DHCP
UDP
IP
Eth
Phy
DHCP
DHCP
DHCP
DHCP
DHCP
encapsulation at DHCP server frame forwarded (switch learning) through LAN demultiplexing at client
Client now has IP address knows name amp addr of DNS
server IP address of its first-hop router
DHCP client receives DHCP ACK reply
A day in the lifehellip connecting to the Internet
router
(runs DHCP)
Link Layer 5-92
A day in the lifehellip ARP (before DNS before HTTP)
before sending HTTP request need IP address of wwwgooglecom DNS
DNS
UDP
IP
Eth
Phy
DNS
DNS
DNS
DNS query created encapsulated in UDP encapsulated in IP encapsulated in Eth To send frame to router need MAC address of router interface ARP
ARP query broadcast received by
router which replies with ARP reply giving MAC address of router interface
client now knows MAC address of first hop router so can now send frame containing DNS query
ARP query
Eth
Phy
ARP
ARP
ARP reply
router
(runs DHCP)
Link Layer 5-93
DNS
UDP
IP
Eth
Phy
DNS
DNS
DNS
DNS
DNS
IP datagram containing DNS query forwarded via LAN switch from client to 1st hop router
IP datagram forwarded from campus network into comcast network routed (tables created by RIP OSPF IS-IS andor BGP routing protocols) to DNS server
demuxrsquoed to DNS server
DNS server replies to client with IP address of wwwgooglecom
Comcast network
68800013
DNS server
DNS
UDP
IP
Eth
Phy
DNS
DNS
DNS
DNS
A day in the lifehellip using DNS
router
(runs DHCP)
Link Layer 5-94
A day in the lifehellipTCP connection carrying HTTP
HTTP
TCP
IP
Eth
Phy
HTTP
to send HTTP request client first opens TCP socket to web server
TCP SYN segment (step 1 in 3-way handshake) inter-domain routed to web server
TCP connection established 64233169105
web server
SYN
SYN
SYN
SYN
TCP
IP
Eth
Phy
SYN
SYN
SYN
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
web server responds with TCP SYNACK (step 2 in 3-way handshake)
router
(runs DHCP)
Link Layer 5-95
A day in the lifehellip HTTP requestreply
HTTP
TCP
IP
Eth
Phy
HTTP
HTTP request sent into TCP socket
IP datagram containing HTTP request routed to wwwgooglecom
IP datagram containing HTTP reply routed back to client
64233169105
web server
HTTP
TCP
IP
Eth
Phy web server responds with
HTTP reply (containing web page)
HTTP
HTTP
HTTP HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
web page finally () displayed
Link Layer 5-96
Chapter 5 Summary
principles behind data link layer services error detection correction
sharing a broadcast channel multiple access
link layer addressing
instantiation and implementation of various link layer technologies Ethernet
switched LANS VLANs
virtualized networks as a link layer MPLS
synthesis a day in the life of a web request
Link Layer 5-97
Chapter 5 letrsquos take a breath
journey down protocol stack complete (except PHY)
solid understanding of networking principles practice
hellip could stop here hellip but lots of interesting topics wireless
multimedia
security
network management
Link Layer 5-73
Port-based VLAN
1
8
9
16 10 2
7
hellip
Electrical Engineering
(VLAN ports 1-8)
Computer Science
(VLAN ports 9-15)
15
hellip
traffic isolation frames tofrom ports 1-8 can only reach ports 1-8 can also define VLAN based on
MAC addresses of endpoints rather than switch port
dynamic membership ports can be dynamically assigned among VLANs
router
forwarding between VLANS done via routing (just as with separate switches) in practice vendors sell combined
switches plus routers
Link Layer 5-74
VLANS spanning multiple switches
trunk port carries frames between VLANS defined over multiple physical switches frames forwarded within VLAN between switches canrsquot be vanilla
8021 frames (must carry VLAN ID info)
8021q protocol addsremoved additional header fields for frames forwarded between trunk ports
1
8
9
10 2
7
hellip
Electrical Engineering
(VLAN ports 1-8)
Computer Science
(VLAN ports 9-15)
15
hellip
2
7 3
Ports 235 belong to EE VLAN
Ports 4678 belong to CS VLAN
5
4 6 8 16
1
Link Layer 5-75
type
2-byte Tag Protocol Identifier
(value 81-00)
Tag Control Information (12 bit VLAN ID field
3 bit priority field like IP TOS)
Recomputed CRC
8021Q VLAN frame format
8021 frame
8021Q frame
dest address
source address
data (payload) CRC preamble
dest address
source address
preamble data (payload) CRC
type
Link Layer 5-76
Link layer LANs outline
51 introduction services
52 error detection correction
53 multiple access protocols
54 LANs addressing ARP
Ethernet
switches
VLANS
55 link virtualization MPLS
56 data center networking
57 a day in the life of a web request
Link Layer 5-77
Multiprotocol label switching (MPLS)
initial goal high-speed IP forwarding using fixed length label (instead of IP address) fast lookup using fixed length identifier (rather than
shortest prefix matching)
borrowing ideas from Virtual Circuit (VC) approach
but IP datagram still keeps IP address
PPP or Ethernet
header IP header remainder of link-layer frame MPLS header
label Exp S TTL
20 3 1 5
Link Layer 5-78
MPLS capable routers
aka label-switched router
forward packets to outgoing interface based only on label value (donrsquot inspect IP address) MPLS forwarding table distinct from IP forwarding tables
flexibility MPLS forwarding decisions can differ from those of IP use destination and source addresses to route flows to
same destination differently (traffic engineering)
re-route flows quickly if link fails pre-computed backup paths (useful for VoIP)
Link Layer 5-79
R2
D
R3 R5
A
R6
MPLS versus IP paths
IP router IP routing path to destination determined
by destination address alone
R4
Link Layer 5-80
R2
D
R3 R4
R5
A
R6
MPLS versus IP paths
IP-only router
IP routing path to destination determined by destination address alone
MPLS and IP router
MPLS routing path to destination can be based on source and dest address fast reroute precompute backup routes in
case of link failure
entry router (R4) can use different MPLS
routes to A based eg on source address
Link Layer 5-81
MPLS signaling
modify OSPF IS-IS link-state flooding protocols to carry info used by MPLS routing eg link bandwidth amount of ldquoreservedrdquo link bandwidth
D
R4
R5
A
R6
entry MPLS router uses RSVP-TE signaling protocol to set up MPLS forwarding at downstream routers
modified link state flooding
RSVP-TE
Link Layer 5-82
R1 R2
D
R3 R4 R5
0
1
0 0
A
R6
in out out
label label dest interface
6 - A 0
in out out
label label dest interface
10 6 A 1
12 9 D 0
in out out
label label dest interface
10 A 0
12 D 0
1
in out out
label label dest interface
8 6 A 0
0
8 A 1
MPLS forwarding tables
Link Layer 5-83
Link layer LANs outline
51 introduction services
52 error detection correction
53 multiple access protocols
54 LANs addressing ARP
Ethernet
switches
VLANS
55 link virtualization MPLS
56 data center networking
57 a day in the life of a web request
Link Layer 5-84
Data center networks
10rsquos to 100rsquos of thousands of hosts often closely coupled in close proximity e-business (eg Amazon)
content-servers (eg YouTube Akamai Apple Microsoft)
search engines data mining (eg Google)
challenges
multiple applications each serving massive numbers of clients
managingbalancing load avoiding processing networking data bottlenecks
Inside a 40-ft Microsoft container
Chicago data center
Link Layer 5-85
Server racks
TOR switches
Tier-1 switches
Tier-2 switches
Load balancer
Load balancer
B
1 2 3 4 5 6 7 8
A C
Border router
Access router
Internet
Data center networks
load balancer application-layer routing receives external client requests
directs workload within data center
returns results to external client (hiding data
center internals from client)
Server racks
TOR switches
Tier-1 switches
Tier-2 switches
1 2 3 4 5 6 7 8
Data center networks
rich interconnection among switches racks
increased throughput between racks (multiple routing
paths possible)
increased reliability via redundancy
Link Layer 5-87
Link layer LANs outline
51 introduction services
52 error detection correction
53 multiple access protocols
54 LANs addressing ARP
Ethernet
switches
VLANS
55 link virtualization MPLS
56 data center networking
57 a day in the life of a web request
Link Layer 5-88
Synthesis a day in the life of a web request
journey down protocol stack complete application transport network link
putting-it-all-together synthesis goal identify review understand protocols (at all
layers) involved in seemingly simple scenario requesting www page
scenario student attaches laptop to campus network requestsreceives wwwgooglecom
Link Layer 5-89
A day in the life scenario
Comcast network
68800013
Googlersquos network
64233160019 64233169105
web server
DNS server
school network
68802024
web page
browser
router
(runs DHCP)
Link Layer 5-90
A day in the lifehellip connecting to the Internet
connecting laptop needs to get its own IP address addr of first-hop router addr of DNS server use DHCP
DHCP
UDP
IP
Eth
Phy
DHCP
DHCP
DHCP
DHCP
DHCP
DHCP
UDP
IP
Eth
Phy
DHCP
DHCP
DHCP
DHCP DHCP
DHCP request encapsulated in UDP encapsulated in IP encapsulated in 8023 Ethernet
Ethernet frame broadcast
(dest FFFFFFFFFFFF) on LAN received at router running DHCP server
Ethernet demuxed to IP demuxed UDP demuxed to DHCP
router
(runs DHCP)
Link Layer 5-91
DHCP server formulates DHCP ACK containing clientrsquos IP address IP address of first-hop router for client name amp IP address of DNS server
DHCP
UDP
IP
Eth
Phy
DHCP
DHCP
DHCP
DHCP
DHCP
UDP
IP
Eth
Phy
DHCP
DHCP
DHCP
DHCP
DHCP
encapsulation at DHCP server frame forwarded (switch learning) through LAN demultiplexing at client
Client now has IP address knows name amp addr of DNS
server IP address of its first-hop router
DHCP client receives DHCP ACK reply
A day in the lifehellip connecting to the Internet
router
(runs DHCP)
Link Layer 5-92
A day in the lifehellip ARP (before DNS before HTTP)
before sending HTTP request need IP address of wwwgooglecom DNS
DNS
UDP
IP
Eth
Phy
DNS
DNS
DNS
DNS query created encapsulated in UDP encapsulated in IP encapsulated in Eth To send frame to router need MAC address of router interface ARP
ARP query broadcast received by
router which replies with ARP reply giving MAC address of router interface
client now knows MAC address of first hop router so can now send frame containing DNS query
ARP query
Eth
Phy
ARP
ARP
ARP reply
router
(runs DHCP)
Link Layer 5-93
DNS
UDP
IP
Eth
Phy
DNS
DNS
DNS
DNS
DNS
IP datagram containing DNS query forwarded via LAN switch from client to 1st hop router
IP datagram forwarded from campus network into comcast network routed (tables created by RIP OSPF IS-IS andor BGP routing protocols) to DNS server
demuxrsquoed to DNS server
DNS server replies to client with IP address of wwwgooglecom
Comcast network
68800013
DNS server
DNS
UDP
IP
Eth
Phy
DNS
DNS
DNS
DNS
A day in the lifehellip using DNS
router
(runs DHCP)
Link Layer 5-94
A day in the lifehellipTCP connection carrying HTTP
HTTP
TCP
IP
Eth
Phy
HTTP
to send HTTP request client first opens TCP socket to web server
TCP SYN segment (step 1 in 3-way handshake) inter-domain routed to web server
TCP connection established 64233169105
web server
SYN
SYN
SYN
SYN
TCP
IP
Eth
Phy
SYN
SYN
SYN
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
web server responds with TCP SYNACK (step 2 in 3-way handshake)
router
(runs DHCP)
Link Layer 5-95
A day in the lifehellip HTTP requestreply
HTTP
TCP
IP
Eth
Phy
HTTP
HTTP request sent into TCP socket
IP datagram containing HTTP request routed to wwwgooglecom
IP datagram containing HTTP reply routed back to client
64233169105
web server
HTTP
TCP
IP
Eth
Phy web server responds with
HTTP reply (containing web page)
HTTP
HTTP
HTTP HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
web page finally () displayed
Link Layer 5-96
Chapter 5 Summary
principles behind data link layer services error detection correction
sharing a broadcast channel multiple access
link layer addressing
instantiation and implementation of various link layer technologies Ethernet
switched LANS VLANs
virtualized networks as a link layer MPLS
synthesis a day in the life of a web request
Link Layer 5-97
Chapter 5 letrsquos take a breath
journey down protocol stack complete (except PHY)
solid understanding of networking principles practice
hellip could stop here hellip but lots of interesting topics wireless
multimedia
security
network management
Link Layer 5-74
VLANS spanning multiple switches
trunk port carries frames between VLANS defined over multiple physical switches frames forwarded within VLAN between switches canrsquot be vanilla
8021 frames (must carry VLAN ID info)
8021q protocol addsremoved additional header fields for frames forwarded between trunk ports
1
8
9
10 2
7
hellip
Electrical Engineering
(VLAN ports 1-8)
Computer Science
(VLAN ports 9-15)
15
hellip
2
7 3
Ports 235 belong to EE VLAN
Ports 4678 belong to CS VLAN
5
4 6 8 16
1
Link Layer 5-75
type
2-byte Tag Protocol Identifier
(value 81-00)
Tag Control Information (12 bit VLAN ID field
3 bit priority field like IP TOS)
Recomputed CRC
8021Q VLAN frame format
8021 frame
8021Q frame
dest address
source address
data (payload) CRC preamble
dest address
source address
preamble data (payload) CRC
type
Link Layer 5-76
Link layer LANs outline
51 introduction services
52 error detection correction
53 multiple access protocols
54 LANs addressing ARP
Ethernet
switches
VLANS
55 link virtualization MPLS
56 data center networking
57 a day in the life of a web request
Link Layer 5-77
Multiprotocol label switching (MPLS)
initial goal high-speed IP forwarding using fixed length label (instead of IP address) fast lookup using fixed length identifier (rather than
shortest prefix matching)
borrowing ideas from Virtual Circuit (VC) approach
but IP datagram still keeps IP address
PPP or Ethernet
header IP header remainder of link-layer frame MPLS header
label Exp S TTL
20 3 1 5
Link Layer 5-78
MPLS capable routers
aka label-switched router
forward packets to outgoing interface based only on label value (donrsquot inspect IP address) MPLS forwarding table distinct from IP forwarding tables
flexibility MPLS forwarding decisions can differ from those of IP use destination and source addresses to route flows to
same destination differently (traffic engineering)
re-route flows quickly if link fails pre-computed backup paths (useful for VoIP)
Link Layer 5-79
R2
D
R3 R5
A
R6
MPLS versus IP paths
IP router IP routing path to destination determined
by destination address alone
R4
Link Layer 5-80
R2
D
R3 R4
R5
A
R6
MPLS versus IP paths
IP-only router
IP routing path to destination determined by destination address alone
MPLS and IP router
MPLS routing path to destination can be based on source and dest address fast reroute precompute backup routes in
case of link failure
entry router (R4) can use different MPLS
routes to A based eg on source address
Link Layer 5-81
MPLS signaling
modify OSPF IS-IS link-state flooding protocols to carry info used by MPLS routing eg link bandwidth amount of ldquoreservedrdquo link bandwidth
D
R4
R5
A
R6
entry MPLS router uses RSVP-TE signaling protocol to set up MPLS forwarding at downstream routers
modified link state flooding
RSVP-TE
Link Layer 5-82
R1 R2
D
R3 R4 R5
0
1
0 0
A
R6
in out out
label label dest interface
6 - A 0
in out out
label label dest interface
10 6 A 1
12 9 D 0
in out out
label label dest interface
10 A 0
12 D 0
1
in out out
label label dest interface
8 6 A 0
0
8 A 1
MPLS forwarding tables
Link Layer 5-83
Link layer LANs outline
51 introduction services
52 error detection correction
53 multiple access protocols
54 LANs addressing ARP
Ethernet
switches
VLANS
55 link virtualization MPLS
56 data center networking
57 a day in the life of a web request
Link Layer 5-84
Data center networks
10rsquos to 100rsquos of thousands of hosts often closely coupled in close proximity e-business (eg Amazon)
content-servers (eg YouTube Akamai Apple Microsoft)
search engines data mining (eg Google)
challenges
multiple applications each serving massive numbers of clients
managingbalancing load avoiding processing networking data bottlenecks
Inside a 40-ft Microsoft container
Chicago data center
Link Layer 5-85
Server racks
TOR switches
Tier-1 switches
Tier-2 switches
Load balancer
Load balancer
B
1 2 3 4 5 6 7 8
A C
Border router
Access router
Internet
Data center networks
load balancer application-layer routing receives external client requests
directs workload within data center
returns results to external client (hiding data
center internals from client)
Server racks
TOR switches
Tier-1 switches
Tier-2 switches
1 2 3 4 5 6 7 8
Data center networks
rich interconnection among switches racks
increased throughput between racks (multiple routing
paths possible)
increased reliability via redundancy
Link Layer 5-87
Link layer LANs outline
51 introduction services
52 error detection correction
53 multiple access protocols
54 LANs addressing ARP
Ethernet
switches
VLANS
55 link virtualization MPLS
56 data center networking
57 a day in the life of a web request
Link Layer 5-88
Synthesis a day in the life of a web request
journey down protocol stack complete application transport network link
putting-it-all-together synthesis goal identify review understand protocols (at all
layers) involved in seemingly simple scenario requesting www page
scenario student attaches laptop to campus network requestsreceives wwwgooglecom
Link Layer 5-89
A day in the life scenario
Comcast network
68800013
Googlersquos network
64233160019 64233169105
web server
DNS server
school network
68802024
web page
browser
router
(runs DHCP)
Link Layer 5-90
A day in the lifehellip connecting to the Internet
connecting laptop needs to get its own IP address addr of first-hop router addr of DNS server use DHCP
DHCP
UDP
IP
Eth
Phy
DHCP
DHCP
DHCP
DHCP
DHCP
DHCP
UDP
IP
Eth
Phy
DHCP
DHCP
DHCP
DHCP DHCP
DHCP request encapsulated in UDP encapsulated in IP encapsulated in 8023 Ethernet
Ethernet frame broadcast
(dest FFFFFFFFFFFF) on LAN received at router running DHCP server
Ethernet demuxed to IP demuxed UDP demuxed to DHCP
router
(runs DHCP)
Link Layer 5-91
DHCP server formulates DHCP ACK containing clientrsquos IP address IP address of first-hop router for client name amp IP address of DNS server
DHCP
UDP
IP
Eth
Phy
DHCP
DHCP
DHCP
DHCP
DHCP
UDP
IP
Eth
Phy
DHCP
DHCP
DHCP
DHCP
DHCP
encapsulation at DHCP server frame forwarded (switch learning) through LAN demultiplexing at client
Client now has IP address knows name amp addr of DNS
server IP address of its first-hop router
DHCP client receives DHCP ACK reply
A day in the lifehellip connecting to the Internet
router
(runs DHCP)
Link Layer 5-92
A day in the lifehellip ARP (before DNS before HTTP)
before sending HTTP request need IP address of wwwgooglecom DNS
DNS
UDP
IP
Eth
Phy
DNS
DNS
DNS
DNS query created encapsulated in UDP encapsulated in IP encapsulated in Eth To send frame to router need MAC address of router interface ARP
ARP query broadcast received by
router which replies with ARP reply giving MAC address of router interface
client now knows MAC address of first hop router so can now send frame containing DNS query
ARP query
Eth
Phy
ARP
ARP
ARP reply
router
(runs DHCP)
Link Layer 5-93
DNS
UDP
IP
Eth
Phy
DNS
DNS
DNS
DNS
DNS
IP datagram containing DNS query forwarded via LAN switch from client to 1st hop router
IP datagram forwarded from campus network into comcast network routed (tables created by RIP OSPF IS-IS andor BGP routing protocols) to DNS server
demuxrsquoed to DNS server
DNS server replies to client with IP address of wwwgooglecom
Comcast network
68800013
DNS server
DNS
UDP
IP
Eth
Phy
DNS
DNS
DNS
DNS
A day in the lifehellip using DNS
router
(runs DHCP)
Link Layer 5-94
A day in the lifehellipTCP connection carrying HTTP
HTTP
TCP
IP
Eth
Phy
HTTP
to send HTTP request client first opens TCP socket to web server
TCP SYN segment (step 1 in 3-way handshake) inter-domain routed to web server
TCP connection established 64233169105
web server
SYN
SYN
SYN
SYN
TCP
IP
Eth
Phy
SYN
SYN
SYN
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
web server responds with TCP SYNACK (step 2 in 3-way handshake)
router
(runs DHCP)
Link Layer 5-95
A day in the lifehellip HTTP requestreply
HTTP
TCP
IP
Eth
Phy
HTTP
HTTP request sent into TCP socket
IP datagram containing HTTP request routed to wwwgooglecom
IP datagram containing HTTP reply routed back to client
64233169105
web server
HTTP
TCP
IP
Eth
Phy web server responds with
HTTP reply (containing web page)
HTTP
HTTP
HTTP HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
web page finally () displayed
Link Layer 5-96
Chapter 5 Summary
principles behind data link layer services error detection correction
sharing a broadcast channel multiple access
link layer addressing
instantiation and implementation of various link layer technologies Ethernet
switched LANS VLANs
virtualized networks as a link layer MPLS
synthesis a day in the life of a web request
Link Layer 5-97
Chapter 5 letrsquos take a breath
journey down protocol stack complete (except PHY)
solid understanding of networking principles practice
hellip could stop here hellip but lots of interesting topics wireless
multimedia
security
network management
Link Layer 5-75
type
2-byte Tag Protocol Identifier
(value 81-00)
Tag Control Information (12 bit VLAN ID field
3 bit priority field like IP TOS)
Recomputed CRC
8021Q VLAN frame format
8021 frame
8021Q frame
dest address
source address
data (payload) CRC preamble
dest address
source address
preamble data (payload) CRC
type
Link Layer 5-76
Link layer LANs outline
51 introduction services
52 error detection correction
53 multiple access protocols
54 LANs addressing ARP
Ethernet
switches
VLANS
55 link virtualization MPLS
56 data center networking
57 a day in the life of a web request
Link Layer 5-77
Multiprotocol label switching (MPLS)
initial goal high-speed IP forwarding using fixed length label (instead of IP address) fast lookup using fixed length identifier (rather than
shortest prefix matching)
borrowing ideas from Virtual Circuit (VC) approach
but IP datagram still keeps IP address
PPP or Ethernet
header IP header remainder of link-layer frame MPLS header
label Exp S TTL
20 3 1 5
Link Layer 5-78
MPLS capable routers
aka label-switched router
forward packets to outgoing interface based only on label value (donrsquot inspect IP address) MPLS forwarding table distinct from IP forwarding tables
flexibility MPLS forwarding decisions can differ from those of IP use destination and source addresses to route flows to
same destination differently (traffic engineering)
re-route flows quickly if link fails pre-computed backup paths (useful for VoIP)
Link Layer 5-79
R2
D
R3 R5
A
R6
MPLS versus IP paths
IP router IP routing path to destination determined
by destination address alone
R4
Link Layer 5-80
R2
D
R3 R4
R5
A
R6
MPLS versus IP paths
IP-only router
IP routing path to destination determined by destination address alone
MPLS and IP router
MPLS routing path to destination can be based on source and dest address fast reroute precompute backup routes in
case of link failure
entry router (R4) can use different MPLS
routes to A based eg on source address
Link Layer 5-81
MPLS signaling
modify OSPF IS-IS link-state flooding protocols to carry info used by MPLS routing eg link bandwidth amount of ldquoreservedrdquo link bandwidth
D
R4
R5
A
R6
entry MPLS router uses RSVP-TE signaling protocol to set up MPLS forwarding at downstream routers
modified link state flooding
RSVP-TE
Link Layer 5-82
R1 R2
D
R3 R4 R5
0
1
0 0
A
R6
in out out
label label dest interface
6 - A 0
in out out
label label dest interface
10 6 A 1
12 9 D 0
in out out
label label dest interface
10 A 0
12 D 0
1
in out out
label label dest interface
8 6 A 0
0
8 A 1
MPLS forwarding tables
Link Layer 5-83
Link layer LANs outline
51 introduction services
52 error detection correction
53 multiple access protocols
54 LANs addressing ARP
Ethernet
switches
VLANS
55 link virtualization MPLS
56 data center networking
57 a day in the life of a web request
Link Layer 5-84
Data center networks
10rsquos to 100rsquos of thousands of hosts often closely coupled in close proximity e-business (eg Amazon)
content-servers (eg YouTube Akamai Apple Microsoft)
search engines data mining (eg Google)
challenges
multiple applications each serving massive numbers of clients
managingbalancing load avoiding processing networking data bottlenecks
Inside a 40-ft Microsoft container
Chicago data center
Link Layer 5-85
Server racks
TOR switches
Tier-1 switches
Tier-2 switches
Load balancer
Load balancer
B
1 2 3 4 5 6 7 8
A C
Border router
Access router
Internet
Data center networks
load balancer application-layer routing receives external client requests
directs workload within data center
returns results to external client (hiding data
center internals from client)
Server racks
TOR switches
Tier-1 switches
Tier-2 switches
1 2 3 4 5 6 7 8
Data center networks
rich interconnection among switches racks
increased throughput between racks (multiple routing
paths possible)
increased reliability via redundancy
Link Layer 5-87
Link layer LANs outline
51 introduction services
52 error detection correction
53 multiple access protocols
54 LANs addressing ARP
Ethernet
switches
VLANS
55 link virtualization MPLS
56 data center networking
57 a day in the life of a web request
Link Layer 5-88
Synthesis a day in the life of a web request
journey down protocol stack complete application transport network link
putting-it-all-together synthesis goal identify review understand protocols (at all
layers) involved in seemingly simple scenario requesting www page
scenario student attaches laptop to campus network requestsreceives wwwgooglecom
Link Layer 5-89
A day in the life scenario
Comcast network
68800013
Googlersquos network
64233160019 64233169105
web server
DNS server
school network
68802024
web page
browser
router
(runs DHCP)
Link Layer 5-90
A day in the lifehellip connecting to the Internet
connecting laptop needs to get its own IP address addr of first-hop router addr of DNS server use DHCP
DHCP
UDP
IP
Eth
Phy
DHCP
DHCP
DHCP
DHCP
DHCP
DHCP
UDP
IP
Eth
Phy
DHCP
DHCP
DHCP
DHCP DHCP
DHCP request encapsulated in UDP encapsulated in IP encapsulated in 8023 Ethernet
Ethernet frame broadcast
(dest FFFFFFFFFFFF) on LAN received at router running DHCP server
Ethernet demuxed to IP demuxed UDP demuxed to DHCP
router
(runs DHCP)
Link Layer 5-91
DHCP server formulates DHCP ACK containing clientrsquos IP address IP address of first-hop router for client name amp IP address of DNS server
DHCP
UDP
IP
Eth
Phy
DHCP
DHCP
DHCP
DHCP
DHCP
UDP
IP
Eth
Phy
DHCP
DHCP
DHCP
DHCP
DHCP
encapsulation at DHCP server frame forwarded (switch learning) through LAN demultiplexing at client
Client now has IP address knows name amp addr of DNS
server IP address of its first-hop router
DHCP client receives DHCP ACK reply
A day in the lifehellip connecting to the Internet
router
(runs DHCP)
Link Layer 5-92
A day in the lifehellip ARP (before DNS before HTTP)
before sending HTTP request need IP address of wwwgooglecom DNS
DNS
UDP
IP
Eth
Phy
DNS
DNS
DNS
DNS query created encapsulated in UDP encapsulated in IP encapsulated in Eth To send frame to router need MAC address of router interface ARP
ARP query broadcast received by
router which replies with ARP reply giving MAC address of router interface
client now knows MAC address of first hop router so can now send frame containing DNS query
ARP query
Eth
Phy
ARP
ARP
ARP reply
router
(runs DHCP)
Link Layer 5-93
DNS
UDP
IP
Eth
Phy
DNS
DNS
DNS
DNS
DNS
IP datagram containing DNS query forwarded via LAN switch from client to 1st hop router
IP datagram forwarded from campus network into comcast network routed (tables created by RIP OSPF IS-IS andor BGP routing protocols) to DNS server
demuxrsquoed to DNS server
DNS server replies to client with IP address of wwwgooglecom
Comcast network
68800013
DNS server
DNS
UDP
IP
Eth
Phy
DNS
DNS
DNS
DNS
A day in the lifehellip using DNS
router
(runs DHCP)
Link Layer 5-94
A day in the lifehellipTCP connection carrying HTTP
HTTP
TCP
IP
Eth
Phy
HTTP
to send HTTP request client first opens TCP socket to web server
TCP SYN segment (step 1 in 3-way handshake) inter-domain routed to web server
TCP connection established 64233169105
web server
SYN
SYN
SYN
SYN
TCP
IP
Eth
Phy
SYN
SYN
SYN
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
web server responds with TCP SYNACK (step 2 in 3-way handshake)
router
(runs DHCP)
Link Layer 5-95
A day in the lifehellip HTTP requestreply
HTTP
TCP
IP
Eth
Phy
HTTP
HTTP request sent into TCP socket
IP datagram containing HTTP request routed to wwwgooglecom
IP datagram containing HTTP reply routed back to client
64233169105
web server
HTTP
TCP
IP
Eth
Phy web server responds with
HTTP reply (containing web page)
HTTP
HTTP
HTTP HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
web page finally () displayed
Link Layer 5-96
Chapter 5 Summary
principles behind data link layer services error detection correction
sharing a broadcast channel multiple access
link layer addressing
instantiation and implementation of various link layer technologies Ethernet
switched LANS VLANs
virtualized networks as a link layer MPLS
synthesis a day in the life of a web request
Link Layer 5-97
Chapter 5 letrsquos take a breath
journey down protocol stack complete (except PHY)
solid understanding of networking principles practice
hellip could stop here hellip but lots of interesting topics wireless
multimedia
security
network management
Link Layer 5-76
Link layer LANs outline
51 introduction services
52 error detection correction
53 multiple access protocols
54 LANs addressing ARP
Ethernet
switches
VLANS
55 link virtualization MPLS
56 data center networking
57 a day in the life of a web request
Link Layer 5-77
Multiprotocol label switching (MPLS)
initial goal high-speed IP forwarding using fixed length label (instead of IP address) fast lookup using fixed length identifier (rather than
shortest prefix matching)
borrowing ideas from Virtual Circuit (VC) approach
but IP datagram still keeps IP address
PPP or Ethernet
header IP header remainder of link-layer frame MPLS header
label Exp S TTL
20 3 1 5
Link Layer 5-78
MPLS capable routers
aka label-switched router
forward packets to outgoing interface based only on label value (donrsquot inspect IP address) MPLS forwarding table distinct from IP forwarding tables
flexibility MPLS forwarding decisions can differ from those of IP use destination and source addresses to route flows to
same destination differently (traffic engineering)
re-route flows quickly if link fails pre-computed backup paths (useful for VoIP)
Link Layer 5-79
R2
D
R3 R5
A
R6
MPLS versus IP paths
IP router IP routing path to destination determined
by destination address alone
R4
Link Layer 5-80
R2
D
R3 R4
R5
A
R6
MPLS versus IP paths
IP-only router
IP routing path to destination determined by destination address alone
MPLS and IP router
MPLS routing path to destination can be based on source and dest address fast reroute precompute backup routes in
case of link failure
entry router (R4) can use different MPLS
routes to A based eg on source address
Link Layer 5-81
MPLS signaling
modify OSPF IS-IS link-state flooding protocols to carry info used by MPLS routing eg link bandwidth amount of ldquoreservedrdquo link bandwidth
D
R4
R5
A
R6
entry MPLS router uses RSVP-TE signaling protocol to set up MPLS forwarding at downstream routers
modified link state flooding
RSVP-TE
Link Layer 5-82
R1 R2
D
R3 R4 R5
0
1
0 0
A
R6
in out out
label label dest interface
6 - A 0
in out out
label label dest interface
10 6 A 1
12 9 D 0
in out out
label label dest interface
10 A 0
12 D 0
1
in out out
label label dest interface
8 6 A 0
0
8 A 1
MPLS forwarding tables
Link Layer 5-83
Link layer LANs outline
51 introduction services
52 error detection correction
53 multiple access protocols
54 LANs addressing ARP
Ethernet
switches
VLANS
55 link virtualization MPLS
56 data center networking
57 a day in the life of a web request
Link Layer 5-84
Data center networks
10rsquos to 100rsquos of thousands of hosts often closely coupled in close proximity e-business (eg Amazon)
content-servers (eg YouTube Akamai Apple Microsoft)
search engines data mining (eg Google)
challenges
multiple applications each serving massive numbers of clients
managingbalancing load avoiding processing networking data bottlenecks
Inside a 40-ft Microsoft container
Chicago data center
Link Layer 5-85
Server racks
TOR switches
Tier-1 switches
Tier-2 switches
Load balancer
Load balancer
B
1 2 3 4 5 6 7 8
A C
Border router
Access router
Internet
Data center networks
load balancer application-layer routing receives external client requests
directs workload within data center
returns results to external client (hiding data
center internals from client)
Server racks
TOR switches
Tier-1 switches
Tier-2 switches
1 2 3 4 5 6 7 8
Data center networks
rich interconnection among switches racks
increased throughput between racks (multiple routing
paths possible)
increased reliability via redundancy
Link Layer 5-87
Link layer LANs outline
51 introduction services
52 error detection correction
53 multiple access protocols
54 LANs addressing ARP
Ethernet
switches
VLANS
55 link virtualization MPLS
56 data center networking
57 a day in the life of a web request
Link Layer 5-88
Synthesis a day in the life of a web request
journey down protocol stack complete application transport network link
putting-it-all-together synthesis goal identify review understand protocols (at all
layers) involved in seemingly simple scenario requesting www page
scenario student attaches laptop to campus network requestsreceives wwwgooglecom
Link Layer 5-89
A day in the life scenario
Comcast network
68800013
Googlersquos network
64233160019 64233169105
web server
DNS server
school network
68802024
web page
browser
router
(runs DHCP)
Link Layer 5-90
A day in the lifehellip connecting to the Internet
connecting laptop needs to get its own IP address addr of first-hop router addr of DNS server use DHCP
DHCP
UDP
IP
Eth
Phy
DHCP
DHCP
DHCP
DHCP
DHCP
DHCP
UDP
IP
Eth
Phy
DHCP
DHCP
DHCP
DHCP DHCP
DHCP request encapsulated in UDP encapsulated in IP encapsulated in 8023 Ethernet
Ethernet frame broadcast
(dest FFFFFFFFFFFF) on LAN received at router running DHCP server
Ethernet demuxed to IP demuxed UDP demuxed to DHCP
router
(runs DHCP)
Link Layer 5-91
DHCP server formulates DHCP ACK containing clientrsquos IP address IP address of first-hop router for client name amp IP address of DNS server
DHCP
UDP
IP
Eth
Phy
DHCP
DHCP
DHCP
DHCP
DHCP
UDP
IP
Eth
Phy
DHCP
DHCP
DHCP
DHCP
DHCP
encapsulation at DHCP server frame forwarded (switch learning) through LAN demultiplexing at client
Client now has IP address knows name amp addr of DNS
server IP address of its first-hop router
DHCP client receives DHCP ACK reply
A day in the lifehellip connecting to the Internet
router
(runs DHCP)
Link Layer 5-92
A day in the lifehellip ARP (before DNS before HTTP)
before sending HTTP request need IP address of wwwgooglecom DNS
DNS
UDP
IP
Eth
Phy
DNS
DNS
DNS
DNS query created encapsulated in UDP encapsulated in IP encapsulated in Eth To send frame to router need MAC address of router interface ARP
ARP query broadcast received by
router which replies with ARP reply giving MAC address of router interface
client now knows MAC address of first hop router so can now send frame containing DNS query
ARP query
Eth
Phy
ARP
ARP
ARP reply
router
(runs DHCP)
Link Layer 5-93
DNS
UDP
IP
Eth
Phy
DNS
DNS
DNS
DNS
DNS
IP datagram containing DNS query forwarded via LAN switch from client to 1st hop router
IP datagram forwarded from campus network into comcast network routed (tables created by RIP OSPF IS-IS andor BGP routing protocols) to DNS server
demuxrsquoed to DNS server
DNS server replies to client with IP address of wwwgooglecom
Comcast network
68800013
DNS server
DNS
UDP
IP
Eth
Phy
DNS
DNS
DNS
DNS
A day in the lifehellip using DNS
router
(runs DHCP)
Link Layer 5-94
A day in the lifehellipTCP connection carrying HTTP
HTTP
TCP
IP
Eth
Phy
HTTP
to send HTTP request client first opens TCP socket to web server
TCP SYN segment (step 1 in 3-way handshake) inter-domain routed to web server
TCP connection established 64233169105
web server
SYN
SYN
SYN
SYN
TCP
IP
Eth
Phy
SYN
SYN
SYN
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
web server responds with TCP SYNACK (step 2 in 3-way handshake)
router
(runs DHCP)
Link Layer 5-95
A day in the lifehellip HTTP requestreply
HTTP
TCP
IP
Eth
Phy
HTTP
HTTP request sent into TCP socket
IP datagram containing HTTP request routed to wwwgooglecom
IP datagram containing HTTP reply routed back to client
64233169105
web server
HTTP
TCP
IP
Eth
Phy web server responds with
HTTP reply (containing web page)
HTTP
HTTP
HTTP HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
web page finally () displayed
Link Layer 5-96
Chapter 5 Summary
principles behind data link layer services error detection correction
sharing a broadcast channel multiple access
link layer addressing
instantiation and implementation of various link layer technologies Ethernet
switched LANS VLANs
virtualized networks as a link layer MPLS
synthesis a day in the life of a web request
Link Layer 5-97
Chapter 5 letrsquos take a breath
journey down protocol stack complete (except PHY)
solid understanding of networking principles practice
hellip could stop here hellip but lots of interesting topics wireless
multimedia
security
network management
Link Layer 5-77
Multiprotocol label switching (MPLS)
initial goal high-speed IP forwarding using fixed length label (instead of IP address) fast lookup using fixed length identifier (rather than
shortest prefix matching)
borrowing ideas from Virtual Circuit (VC) approach
but IP datagram still keeps IP address
PPP or Ethernet
header IP header remainder of link-layer frame MPLS header
label Exp S TTL
20 3 1 5
Link Layer 5-78
MPLS capable routers
aka label-switched router
forward packets to outgoing interface based only on label value (donrsquot inspect IP address) MPLS forwarding table distinct from IP forwarding tables
flexibility MPLS forwarding decisions can differ from those of IP use destination and source addresses to route flows to
same destination differently (traffic engineering)
re-route flows quickly if link fails pre-computed backup paths (useful for VoIP)
Link Layer 5-79
R2
D
R3 R5
A
R6
MPLS versus IP paths
IP router IP routing path to destination determined
by destination address alone
R4
Link Layer 5-80
R2
D
R3 R4
R5
A
R6
MPLS versus IP paths
IP-only router
IP routing path to destination determined by destination address alone
MPLS and IP router
MPLS routing path to destination can be based on source and dest address fast reroute precompute backup routes in
case of link failure
entry router (R4) can use different MPLS
routes to A based eg on source address
Link Layer 5-81
MPLS signaling
modify OSPF IS-IS link-state flooding protocols to carry info used by MPLS routing eg link bandwidth amount of ldquoreservedrdquo link bandwidth
D
R4
R5
A
R6
entry MPLS router uses RSVP-TE signaling protocol to set up MPLS forwarding at downstream routers
modified link state flooding
RSVP-TE
Link Layer 5-82
R1 R2
D
R3 R4 R5
0
1
0 0
A
R6
in out out
label label dest interface
6 - A 0
in out out
label label dest interface
10 6 A 1
12 9 D 0
in out out
label label dest interface
10 A 0
12 D 0
1
in out out
label label dest interface
8 6 A 0
0
8 A 1
MPLS forwarding tables
Link Layer 5-83
Link layer LANs outline
51 introduction services
52 error detection correction
53 multiple access protocols
54 LANs addressing ARP
Ethernet
switches
VLANS
55 link virtualization MPLS
56 data center networking
57 a day in the life of a web request
Link Layer 5-84
Data center networks
10rsquos to 100rsquos of thousands of hosts often closely coupled in close proximity e-business (eg Amazon)
content-servers (eg YouTube Akamai Apple Microsoft)
search engines data mining (eg Google)
challenges
multiple applications each serving massive numbers of clients
managingbalancing load avoiding processing networking data bottlenecks
Inside a 40-ft Microsoft container
Chicago data center
Link Layer 5-85
Server racks
TOR switches
Tier-1 switches
Tier-2 switches
Load balancer
Load balancer
B
1 2 3 4 5 6 7 8
A C
Border router
Access router
Internet
Data center networks
load balancer application-layer routing receives external client requests
directs workload within data center
returns results to external client (hiding data
center internals from client)
Server racks
TOR switches
Tier-1 switches
Tier-2 switches
1 2 3 4 5 6 7 8
Data center networks
rich interconnection among switches racks
increased throughput between racks (multiple routing
paths possible)
increased reliability via redundancy
Link Layer 5-87
Link layer LANs outline
51 introduction services
52 error detection correction
53 multiple access protocols
54 LANs addressing ARP
Ethernet
switches
VLANS
55 link virtualization MPLS
56 data center networking
57 a day in the life of a web request
Link Layer 5-88
Synthesis a day in the life of a web request
journey down protocol stack complete application transport network link
putting-it-all-together synthesis goal identify review understand protocols (at all
layers) involved in seemingly simple scenario requesting www page
scenario student attaches laptop to campus network requestsreceives wwwgooglecom
Link Layer 5-89
A day in the life scenario
Comcast network
68800013
Googlersquos network
64233160019 64233169105
web server
DNS server
school network
68802024
web page
browser
router
(runs DHCP)
Link Layer 5-90
A day in the lifehellip connecting to the Internet
connecting laptop needs to get its own IP address addr of first-hop router addr of DNS server use DHCP
DHCP
UDP
IP
Eth
Phy
DHCP
DHCP
DHCP
DHCP
DHCP
DHCP
UDP
IP
Eth
Phy
DHCP
DHCP
DHCP
DHCP DHCP
DHCP request encapsulated in UDP encapsulated in IP encapsulated in 8023 Ethernet
Ethernet frame broadcast
(dest FFFFFFFFFFFF) on LAN received at router running DHCP server
Ethernet demuxed to IP demuxed UDP demuxed to DHCP
router
(runs DHCP)
Link Layer 5-91
DHCP server formulates DHCP ACK containing clientrsquos IP address IP address of first-hop router for client name amp IP address of DNS server
DHCP
UDP
IP
Eth
Phy
DHCP
DHCP
DHCP
DHCP
DHCP
UDP
IP
Eth
Phy
DHCP
DHCP
DHCP
DHCP
DHCP
encapsulation at DHCP server frame forwarded (switch learning) through LAN demultiplexing at client
Client now has IP address knows name amp addr of DNS
server IP address of its first-hop router
DHCP client receives DHCP ACK reply
A day in the lifehellip connecting to the Internet
router
(runs DHCP)
Link Layer 5-92
A day in the lifehellip ARP (before DNS before HTTP)
before sending HTTP request need IP address of wwwgooglecom DNS
DNS
UDP
IP
Eth
Phy
DNS
DNS
DNS
DNS query created encapsulated in UDP encapsulated in IP encapsulated in Eth To send frame to router need MAC address of router interface ARP
ARP query broadcast received by
router which replies with ARP reply giving MAC address of router interface
client now knows MAC address of first hop router so can now send frame containing DNS query
ARP query
Eth
Phy
ARP
ARP
ARP reply
router
(runs DHCP)
Link Layer 5-93
DNS
UDP
IP
Eth
Phy
DNS
DNS
DNS
DNS
DNS
IP datagram containing DNS query forwarded via LAN switch from client to 1st hop router
IP datagram forwarded from campus network into comcast network routed (tables created by RIP OSPF IS-IS andor BGP routing protocols) to DNS server
demuxrsquoed to DNS server
DNS server replies to client with IP address of wwwgooglecom
Comcast network
68800013
DNS server
DNS
UDP
IP
Eth
Phy
DNS
DNS
DNS
DNS
A day in the lifehellip using DNS
router
(runs DHCP)
Link Layer 5-94
A day in the lifehellipTCP connection carrying HTTP
HTTP
TCP
IP
Eth
Phy
HTTP
to send HTTP request client first opens TCP socket to web server
TCP SYN segment (step 1 in 3-way handshake) inter-domain routed to web server
TCP connection established 64233169105
web server
SYN
SYN
SYN
SYN
TCP
IP
Eth
Phy
SYN
SYN
SYN
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
web server responds with TCP SYNACK (step 2 in 3-way handshake)
router
(runs DHCP)
Link Layer 5-95
A day in the lifehellip HTTP requestreply
HTTP
TCP
IP
Eth
Phy
HTTP
HTTP request sent into TCP socket
IP datagram containing HTTP request routed to wwwgooglecom
IP datagram containing HTTP reply routed back to client
64233169105
web server
HTTP
TCP
IP
Eth
Phy web server responds with
HTTP reply (containing web page)
HTTP
HTTP
HTTP HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
web page finally () displayed
Link Layer 5-96
Chapter 5 Summary
principles behind data link layer services error detection correction
sharing a broadcast channel multiple access
link layer addressing
instantiation and implementation of various link layer technologies Ethernet
switched LANS VLANs
virtualized networks as a link layer MPLS
synthesis a day in the life of a web request
Link Layer 5-97
Chapter 5 letrsquos take a breath
journey down protocol stack complete (except PHY)
solid understanding of networking principles practice
hellip could stop here hellip but lots of interesting topics wireless
multimedia
security
network management
Link Layer 5-78
MPLS capable routers
aka label-switched router
forward packets to outgoing interface based only on label value (donrsquot inspect IP address) MPLS forwarding table distinct from IP forwarding tables
flexibility MPLS forwarding decisions can differ from those of IP use destination and source addresses to route flows to
same destination differently (traffic engineering)
re-route flows quickly if link fails pre-computed backup paths (useful for VoIP)
Link Layer 5-79
R2
D
R3 R5
A
R6
MPLS versus IP paths
IP router IP routing path to destination determined
by destination address alone
R4
Link Layer 5-80
R2
D
R3 R4
R5
A
R6
MPLS versus IP paths
IP-only router
IP routing path to destination determined by destination address alone
MPLS and IP router
MPLS routing path to destination can be based on source and dest address fast reroute precompute backup routes in
case of link failure
entry router (R4) can use different MPLS
routes to A based eg on source address
Link Layer 5-81
MPLS signaling
modify OSPF IS-IS link-state flooding protocols to carry info used by MPLS routing eg link bandwidth amount of ldquoreservedrdquo link bandwidth
D
R4
R5
A
R6
entry MPLS router uses RSVP-TE signaling protocol to set up MPLS forwarding at downstream routers
modified link state flooding
RSVP-TE
Link Layer 5-82
R1 R2
D
R3 R4 R5
0
1
0 0
A
R6
in out out
label label dest interface
6 - A 0
in out out
label label dest interface
10 6 A 1
12 9 D 0
in out out
label label dest interface
10 A 0
12 D 0
1
in out out
label label dest interface
8 6 A 0
0
8 A 1
MPLS forwarding tables
Link Layer 5-83
Link layer LANs outline
51 introduction services
52 error detection correction
53 multiple access protocols
54 LANs addressing ARP
Ethernet
switches
VLANS
55 link virtualization MPLS
56 data center networking
57 a day in the life of a web request
Link Layer 5-84
Data center networks
10rsquos to 100rsquos of thousands of hosts often closely coupled in close proximity e-business (eg Amazon)
content-servers (eg YouTube Akamai Apple Microsoft)
search engines data mining (eg Google)
challenges
multiple applications each serving massive numbers of clients
managingbalancing load avoiding processing networking data bottlenecks
Inside a 40-ft Microsoft container
Chicago data center
Link Layer 5-85
Server racks
TOR switches
Tier-1 switches
Tier-2 switches
Load balancer
Load balancer
B
1 2 3 4 5 6 7 8
A C
Border router
Access router
Internet
Data center networks
load balancer application-layer routing receives external client requests
directs workload within data center
returns results to external client (hiding data
center internals from client)
Server racks
TOR switches
Tier-1 switches
Tier-2 switches
1 2 3 4 5 6 7 8
Data center networks
rich interconnection among switches racks
increased throughput between racks (multiple routing
paths possible)
increased reliability via redundancy
Link Layer 5-87
Link layer LANs outline
51 introduction services
52 error detection correction
53 multiple access protocols
54 LANs addressing ARP
Ethernet
switches
VLANS
55 link virtualization MPLS
56 data center networking
57 a day in the life of a web request
Link Layer 5-88
Synthesis a day in the life of a web request
journey down protocol stack complete application transport network link
putting-it-all-together synthesis goal identify review understand protocols (at all
layers) involved in seemingly simple scenario requesting www page
scenario student attaches laptop to campus network requestsreceives wwwgooglecom
Link Layer 5-89
A day in the life scenario
Comcast network
68800013
Googlersquos network
64233160019 64233169105
web server
DNS server
school network
68802024
web page
browser
router
(runs DHCP)
Link Layer 5-90
A day in the lifehellip connecting to the Internet
connecting laptop needs to get its own IP address addr of first-hop router addr of DNS server use DHCP
DHCP
UDP
IP
Eth
Phy
DHCP
DHCP
DHCP
DHCP
DHCP
DHCP
UDP
IP
Eth
Phy
DHCP
DHCP
DHCP
DHCP DHCP
DHCP request encapsulated in UDP encapsulated in IP encapsulated in 8023 Ethernet
Ethernet frame broadcast
(dest FFFFFFFFFFFF) on LAN received at router running DHCP server
Ethernet demuxed to IP demuxed UDP demuxed to DHCP
router
(runs DHCP)
Link Layer 5-91
DHCP server formulates DHCP ACK containing clientrsquos IP address IP address of first-hop router for client name amp IP address of DNS server
DHCP
UDP
IP
Eth
Phy
DHCP
DHCP
DHCP
DHCP
DHCP
UDP
IP
Eth
Phy
DHCP
DHCP
DHCP
DHCP
DHCP
encapsulation at DHCP server frame forwarded (switch learning) through LAN demultiplexing at client
Client now has IP address knows name amp addr of DNS
server IP address of its first-hop router
DHCP client receives DHCP ACK reply
A day in the lifehellip connecting to the Internet
router
(runs DHCP)
Link Layer 5-92
A day in the lifehellip ARP (before DNS before HTTP)
before sending HTTP request need IP address of wwwgooglecom DNS
DNS
UDP
IP
Eth
Phy
DNS
DNS
DNS
DNS query created encapsulated in UDP encapsulated in IP encapsulated in Eth To send frame to router need MAC address of router interface ARP
ARP query broadcast received by
router which replies with ARP reply giving MAC address of router interface
client now knows MAC address of first hop router so can now send frame containing DNS query
ARP query
Eth
Phy
ARP
ARP
ARP reply
router
(runs DHCP)
Link Layer 5-93
DNS
UDP
IP
Eth
Phy
DNS
DNS
DNS
DNS
DNS
IP datagram containing DNS query forwarded via LAN switch from client to 1st hop router
IP datagram forwarded from campus network into comcast network routed (tables created by RIP OSPF IS-IS andor BGP routing protocols) to DNS server
demuxrsquoed to DNS server
DNS server replies to client with IP address of wwwgooglecom
Comcast network
68800013
DNS server
DNS
UDP
IP
Eth
Phy
DNS
DNS
DNS
DNS
A day in the lifehellip using DNS
router
(runs DHCP)
Link Layer 5-94
A day in the lifehellipTCP connection carrying HTTP
HTTP
TCP
IP
Eth
Phy
HTTP
to send HTTP request client first opens TCP socket to web server
TCP SYN segment (step 1 in 3-way handshake) inter-domain routed to web server
TCP connection established 64233169105
web server
SYN
SYN
SYN
SYN
TCP
IP
Eth
Phy
SYN
SYN
SYN
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
web server responds with TCP SYNACK (step 2 in 3-way handshake)
router
(runs DHCP)
Link Layer 5-95
A day in the lifehellip HTTP requestreply
HTTP
TCP
IP
Eth
Phy
HTTP
HTTP request sent into TCP socket
IP datagram containing HTTP request routed to wwwgooglecom
IP datagram containing HTTP reply routed back to client
64233169105
web server
HTTP
TCP
IP
Eth
Phy web server responds with
HTTP reply (containing web page)
HTTP
HTTP
HTTP HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
web page finally () displayed
Link Layer 5-96
Chapter 5 Summary
principles behind data link layer services error detection correction
sharing a broadcast channel multiple access
link layer addressing
instantiation and implementation of various link layer technologies Ethernet
switched LANS VLANs
virtualized networks as a link layer MPLS
synthesis a day in the life of a web request
Link Layer 5-97
Chapter 5 letrsquos take a breath
journey down protocol stack complete (except PHY)
solid understanding of networking principles practice
hellip could stop here hellip but lots of interesting topics wireless
multimedia
security
network management
Link Layer 5-79
R2
D
R3 R5
A
R6
MPLS versus IP paths
IP router IP routing path to destination determined
by destination address alone
R4
Link Layer 5-80
R2
D
R3 R4
R5
A
R6
MPLS versus IP paths
IP-only router
IP routing path to destination determined by destination address alone
MPLS and IP router
MPLS routing path to destination can be based on source and dest address fast reroute precompute backup routes in
case of link failure
entry router (R4) can use different MPLS
routes to A based eg on source address
Link Layer 5-81
MPLS signaling
modify OSPF IS-IS link-state flooding protocols to carry info used by MPLS routing eg link bandwidth amount of ldquoreservedrdquo link bandwidth
D
R4
R5
A
R6
entry MPLS router uses RSVP-TE signaling protocol to set up MPLS forwarding at downstream routers
modified link state flooding
RSVP-TE
Link Layer 5-82
R1 R2
D
R3 R4 R5
0
1
0 0
A
R6
in out out
label label dest interface
6 - A 0
in out out
label label dest interface
10 6 A 1
12 9 D 0
in out out
label label dest interface
10 A 0
12 D 0
1
in out out
label label dest interface
8 6 A 0
0
8 A 1
MPLS forwarding tables
Link Layer 5-83
Link layer LANs outline
51 introduction services
52 error detection correction
53 multiple access protocols
54 LANs addressing ARP
Ethernet
switches
VLANS
55 link virtualization MPLS
56 data center networking
57 a day in the life of a web request
Link Layer 5-84
Data center networks
10rsquos to 100rsquos of thousands of hosts often closely coupled in close proximity e-business (eg Amazon)
content-servers (eg YouTube Akamai Apple Microsoft)
search engines data mining (eg Google)
challenges
multiple applications each serving massive numbers of clients
managingbalancing load avoiding processing networking data bottlenecks
Inside a 40-ft Microsoft container
Chicago data center
Link Layer 5-85
Server racks
TOR switches
Tier-1 switches
Tier-2 switches
Load balancer
Load balancer
B
1 2 3 4 5 6 7 8
A C
Border router
Access router
Internet
Data center networks
load balancer application-layer routing receives external client requests
directs workload within data center
returns results to external client (hiding data
center internals from client)
Server racks
TOR switches
Tier-1 switches
Tier-2 switches
1 2 3 4 5 6 7 8
Data center networks
rich interconnection among switches racks
increased throughput between racks (multiple routing
paths possible)
increased reliability via redundancy
Link Layer 5-87
Link layer LANs outline
51 introduction services
52 error detection correction
53 multiple access protocols
54 LANs addressing ARP
Ethernet
switches
VLANS
55 link virtualization MPLS
56 data center networking
57 a day in the life of a web request
Link Layer 5-88
Synthesis a day in the life of a web request
journey down protocol stack complete application transport network link
putting-it-all-together synthesis goal identify review understand protocols (at all
layers) involved in seemingly simple scenario requesting www page
scenario student attaches laptop to campus network requestsreceives wwwgooglecom
Link Layer 5-89
A day in the life scenario
Comcast network
68800013
Googlersquos network
64233160019 64233169105
web server
DNS server
school network
68802024
web page
browser
router
(runs DHCP)
Link Layer 5-90
A day in the lifehellip connecting to the Internet
connecting laptop needs to get its own IP address addr of first-hop router addr of DNS server use DHCP
DHCP
UDP
IP
Eth
Phy
DHCP
DHCP
DHCP
DHCP
DHCP
DHCP
UDP
IP
Eth
Phy
DHCP
DHCP
DHCP
DHCP DHCP
DHCP request encapsulated in UDP encapsulated in IP encapsulated in 8023 Ethernet
Ethernet frame broadcast
(dest FFFFFFFFFFFF) on LAN received at router running DHCP server
Ethernet demuxed to IP demuxed UDP demuxed to DHCP
router
(runs DHCP)
Link Layer 5-91
DHCP server formulates DHCP ACK containing clientrsquos IP address IP address of first-hop router for client name amp IP address of DNS server
DHCP
UDP
IP
Eth
Phy
DHCP
DHCP
DHCP
DHCP
DHCP
UDP
IP
Eth
Phy
DHCP
DHCP
DHCP
DHCP
DHCP
encapsulation at DHCP server frame forwarded (switch learning) through LAN demultiplexing at client
Client now has IP address knows name amp addr of DNS
server IP address of its first-hop router
DHCP client receives DHCP ACK reply
A day in the lifehellip connecting to the Internet
router
(runs DHCP)
Link Layer 5-92
A day in the lifehellip ARP (before DNS before HTTP)
before sending HTTP request need IP address of wwwgooglecom DNS
DNS
UDP
IP
Eth
Phy
DNS
DNS
DNS
DNS query created encapsulated in UDP encapsulated in IP encapsulated in Eth To send frame to router need MAC address of router interface ARP
ARP query broadcast received by
router which replies with ARP reply giving MAC address of router interface
client now knows MAC address of first hop router so can now send frame containing DNS query
ARP query
Eth
Phy
ARP
ARP
ARP reply
router
(runs DHCP)
Link Layer 5-93
DNS
UDP
IP
Eth
Phy
DNS
DNS
DNS
DNS
DNS
IP datagram containing DNS query forwarded via LAN switch from client to 1st hop router
IP datagram forwarded from campus network into comcast network routed (tables created by RIP OSPF IS-IS andor BGP routing protocols) to DNS server
demuxrsquoed to DNS server
DNS server replies to client with IP address of wwwgooglecom
Comcast network
68800013
DNS server
DNS
UDP
IP
Eth
Phy
DNS
DNS
DNS
DNS
A day in the lifehellip using DNS
router
(runs DHCP)
Link Layer 5-94
A day in the lifehellipTCP connection carrying HTTP
HTTP
TCP
IP
Eth
Phy
HTTP
to send HTTP request client first opens TCP socket to web server
TCP SYN segment (step 1 in 3-way handshake) inter-domain routed to web server
TCP connection established 64233169105
web server
SYN
SYN
SYN
SYN
TCP
IP
Eth
Phy
SYN
SYN
SYN
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
web server responds with TCP SYNACK (step 2 in 3-way handshake)
router
(runs DHCP)
Link Layer 5-95
A day in the lifehellip HTTP requestreply
HTTP
TCP
IP
Eth
Phy
HTTP
HTTP request sent into TCP socket
IP datagram containing HTTP request routed to wwwgooglecom
IP datagram containing HTTP reply routed back to client
64233169105
web server
HTTP
TCP
IP
Eth
Phy web server responds with
HTTP reply (containing web page)
HTTP
HTTP
HTTP HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
web page finally () displayed
Link Layer 5-96
Chapter 5 Summary
principles behind data link layer services error detection correction
sharing a broadcast channel multiple access
link layer addressing
instantiation and implementation of various link layer technologies Ethernet
switched LANS VLANs
virtualized networks as a link layer MPLS
synthesis a day in the life of a web request
Link Layer 5-97
Chapter 5 letrsquos take a breath
journey down protocol stack complete (except PHY)
solid understanding of networking principles practice
hellip could stop here hellip but lots of interesting topics wireless
multimedia
security
network management
Link Layer 5-80
R2
D
R3 R4
R5
A
R6
MPLS versus IP paths
IP-only router
IP routing path to destination determined by destination address alone
MPLS and IP router
MPLS routing path to destination can be based on source and dest address fast reroute precompute backup routes in
case of link failure
entry router (R4) can use different MPLS
routes to A based eg on source address
Link Layer 5-81
MPLS signaling
modify OSPF IS-IS link-state flooding protocols to carry info used by MPLS routing eg link bandwidth amount of ldquoreservedrdquo link bandwidth
D
R4
R5
A
R6
entry MPLS router uses RSVP-TE signaling protocol to set up MPLS forwarding at downstream routers
modified link state flooding
RSVP-TE
Link Layer 5-82
R1 R2
D
R3 R4 R5
0
1
0 0
A
R6
in out out
label label dest interface
6 - A 0
in out out
label label dest interface
10 6 A 1
12 9 D 0
in out out
label label dest interface
10 A 0
12 D 0
1
in out out
label label dest interface
8 6 A 0
0
8 A 1
MPLS forwarding tables
Link Layer 5-83
Link layer LANs outline
51 introduction services
52 error detection correction
53 multiple access protocols
54 LANs addressing ARP
Ethernet
switches
VLANS
55 link virtualization MPLS
56 data center networking
57 a day in the life of a web request
Link Layer 5-84
Data center networks
10rsquos to 100rsquos of thousands of hosts often closely coupled in close proximity e-business (eg Amazon)
content-servers (eg YouTube Akamai Apple Microsoft)
search engines data mining (eg Google)
challenges
multiple applications each serving massive numbers of clients
managingbalancing load avoiding processing networking data bottlenecks
Inside a 40-ft Microsoft container
Chicago data center
Link Layer 5-85
Server racks
TOR switches
Tier-1 switches
Tier-2 switches
Load balancer
Load balancer
B
1 2 3 4 5 6 7 8
A C
Border router
Access router
Internet
Data center networks
load balancer application-layer routing receives external client requests
directs workload within data center
returns results to external client (hiding data
center internals from client)
Server racks
TOR switches
Tier-1 switches
Tier-2 switches
1 2 3 4 5 6 7 8
Data center networks
rich interconnection among switches racks
increased throughput between racks (multiple routing
paths possible)
increased reliability via redundancy
Link Layer 5-87
Link layer LANs outline
51 introduction services
52 error detection correction
53 multiple access protocols
54 LANs addressing ARP
Ethernet
switches
VLANS
55 link virtualization MPLS
56 data center networking
57 a day in the life of a web request
Link Layer 5-88
Synthesis a day in the life of a web request
journey down protocol stack complete application transport network link
putting-it-all-together synthesis goal identify review understand protocols (at all
layers) involved in seemingly simple scenario requesting www page
scenario student attaches laptop to campus network requestsreceives wwwgooglecom
Link Layer 5-89
A day in the life scenario
Comcast network
68800013
Googlersquos network
64233160019 64233169105
web server
DNS server
school network
68802024
web page
browser
router
(runs DHCP)
Link Layer 5-90
A day in the lifehellip connecting to the Internet
connecting laptop needs to get its own IP address addr of first-hop router addr of DNS server use DHCP
DHCP
UDP
IP
Eth
Phy
DHCP
DHCP
DHCP
DHCP
DHCP
DHCP
UDP
IP
Eth
Phy
DHCP
DHCP
DHCP
DHCP DHCP
DHCP request encapsulated in UDP encapsulated in IP encapsulated in 8023 Ethernet
Ethernet frame broadcast
(dest FFFFFFFFFFFF) on LAN received at router running DHCP server
Ethernet demuxed to IP demuxed UDP demuxed to DHCP
router
(runs DHCP)
Link Layer 5-91
DHCP server formulates DHCP ACK containing clientrsquos IP address IP address of first-hop router for client name amp IP address of DNS server
DHCP
UDP
IP
Eth
Phy
DHCP
DHCP
DHCP
DHCP
DHCP
UDP
IP
Eth
Phy
DHCP
DHCP
DHCP
DHCP
DHCP
encapsulation at DHCP server frame forwarded (switch learning) through LAN demultiplexing at client
Client now has IP address knows name amp addr of DNS
server IP address of its first-hop router
DHCP client receives DHCP ACK reply
A day in the lifehellip connecting to the Internet
router
(runs DHCP)
Link Layer 5-92
A day in the lifehellip ARP (before DNS before HTTP)
before sending HTTP request need IP address of wwwgooglecom DNS
DNS
UDP
IP
Eth
Phy
DNS
DNS
DNS
DNS query created encapsulated in UDP encapsulated in IP encapsulated in Eth To send frame to router need MAC address of router interface ARP
ARP query broadcast received by
router which replies with ARP reply giving MAC address of router interface
client now knows MAC address of first hop router so can now send frame containing DNS query
ARP query
Eth
Phy
ARP
ARP
ARP reply
router
(runs DHCP)
Link Layer 5-93
DNS
UDP
IP
Eth
Phy
DNS
DNS
DNS
DNS
DNS
IP datagram containing DNS query forwarded via LAN switch from client to 1st hop router
IP datagram forwarded from campus network into comcast network routed (tables created by RIP OSPF IS-IS andor BGP routing protocols) to DNS server
demuxrsquoed to DNS server
DNS server replies to client with IP address of wwwgooglecom
Comcast network
68800013
DNS server
DNS
UDP
IP
Eth
Phy
DNS
DNS
DNS
DNS
A day in the lifehellip using DNS
router
(runs DHCP)
Link Layer 5-94
A day in the lifehellipTCP connection carrying HTTP
HTTP
TCP
IP
Eth
Phy
HTTP
to send HTTP request client first opens TCP socket to web server
TCP SYN segment (step 1 in 3-way handshake) inter-domain routed to web server
TCP connection established 64233169105
web server
SYN
SYN
SYN
SYN
TCP
IP
Eth
Phy
SYN
SYN
SYN
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
web server responds with TCP SYNACK (step 2 in 3-way handshake)
router
(runs DHCP)
Link Layer 5-95
A day in the lifehellip HTTP requestreply
HTTP
TCP
IP
Eth
Phy
HTTP
HTTP request sent into TCP socket
IP datagram containing HTTP request routed to wwwgooglecom
IP datagram containing HTTP reply routed back to client
64233169105
web server
HTTP
TCP
IP
Eth
Phy web server responds with
HTTP reply (containing web page)
HTTP
HTTP
HTTP HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
web page finally () displayed
Link Layer 5-96
Chapter 5 Summary
principles behind data link layer services error detection correction
sharing a broadcast channel multiple access
link layer addressing
instantiation and implementation of various link layer technologies Ethernet
switched LANS VLANs
virtualized networks as a link layer MPLS
synthesis a day in the life of a web request
Link Layer 5-97
Chapter 5 letrsquos take a breath
journey down protocol stack complete (except PHY)
solid understanding of networking principles practice
hellip could stop here hellip but lots of interesting topics wireless
multimedia
security
network management
Link Layer 5-81
MPLS signaling
modify OSPF IS-IS link-state flooding protocols to carry info used by MPLS routing eg link bandwidth amount of ldquoreservedrdquo link bandwidth
D
R4
R5
A
R6
entry MPLS router uses RSVP-TE signaling protocol to set up MPLS forwarding at downstream routers
modified link state flooding
RSVP-TE
Link Layer 5-82
R1 R2
D
R3 R4 R5
0
1
0 0
A
R6
in out out
label label dest interface
6 - A 0
in out out
label label dest interface
10 6 A 1
12 9 D 0
in out out
label label dest interface
10 A 0
12 D 0
1
in out out
label label dest interface
8 6 A 0
0
8 A 1
MPLS forwarding tables
Link Layer 5-83
Link layer LANs outline
51 introduction services
52 error detection correction
53 multiple access protocols
54 LANs addressing ARP
Ethernet
switches
VLANS
55 link virtualization MPLS
56 data center networking
57 a day in the life of a web request
Link Layer 5-84
Data center networks
10rsquos to 100rsquos of thousands of hosts often closely coupled in close proximity e-business (eg Amazon)
content-servers (eg YouTube Akamai Apple Microsoft)
search engines data mining (eg Google)
challenges
multiple applications each serving massive numbers of clients
managingbalancing load avoiding processing networking data bottlenecks
Inside a 40-ft Microsoft container
Chicago data center
Link Layer 5-85
Server racks
TOR switches
Tier-1 switches
Tier-2 switches
Load balancer
Load balancer
B
1 2 3 4 5 6 7 8
A C
Border router
Access router
Internet
Data center networks
load balancer application-layer routing receives external client requests
directs workload within data center
returns results to external client (hiding data
center internals from client)
Server racks
TOR switches
Tier-1 switches
Tier-2 switches
1 2 3 4 5 6 7 8
Data center networks
rich interconnection among switches racks
increased throughput between racks (multiple routing
paths possible)
increased reliability via redundancy
Link Layer 5-87
Link layer LANs outline
51 introduction services
52 error detection correction
53 multiple access protocols
54 LANs addressing ARP
Ethernet
switches
VLANS
55 link virtualization MPLS
56 data center networking
57 a day in the life of a web request
Link Layer 5-88
Synthesis a day in the life of a web request
journey down protocol stack complete application transport network link
putting-it-all-together synthesis goal identify review understand protocols (at all
layers) involved in seemingly simple scenario requesting www page
scenario student attaches laptop to campus network requestsreceives wwwgooglecom
Link Layer 5-89
A day in the life scenario
Comcast network
68800013
Googlersquos network
64233160019 64233169105
web server
DNS server
school network
68802024
web page
browser
router
(runs DHCP)
Link Layer 5-90
A day in the lifehellip connecting to the Internet
connecting laptop needs to get its own IP address addr of first-hop router addr of DNS server use DHCP
DHCP
UDP
IP
Eth
Phy
DHCP
DHCP
DHCP
DHCP
DHCP
DHCP
UDP
IP
Eth
Phy
DHCP
DHCP
DHCP
DHCP DHCP
DHCP request encapsulated in UDP encapsulated in IP encapsulated in 8023 Ethernet
Ethernet frame broadcast
(dest FFFFFFFFFFFF) on LAN received at router running DHCP server
Ethernet demuxed to IP demuxed UDP demuxed to DHCP
router
(runs DHCP)
Link Layer 5-91
DHCP server formulates DHCP ACK containing clientrsquos IP address IP address of first-hop router for client name amp IP address of DNS server
DHCP
UDP
IP
Eth
Phy
DHCP
DHCP
DHCP
DHCP
DHCP
UDP
IP
Eth
Phy
DHCP
DHCP
DHCP
DHCP
DHCP
encapsulation at DHCP server frame forwarded (switch learning) through LAN demultiplexing at client
Client now has IP address knows name amp addr of DNS
server IP address of its first-hop router
DHCP client receives DHCP ACK reply
A day in the lifehellip connecting to the Internet
router
(runs DHCP)
Link Layer 5-92
A day in the lifehellip ARP (before DNS before HTTP)
before sending HTTP request need IP address of wwwgooglecom DNS
DNS
UDP
IP
Eth
Phy
DNS
DNS
DNS
DNS query created encapsulated in UDP encapsulated in IP encapsulated in Eth To send frame to router need MAC address of router interface ARP
ARP query broadcast received by
router which replies with ARP reply giving MAC address of router interface
client now knows MAC address of first hop router so can now send frame containing DNS query
ARP query
Eth
Phy
ARP
ARP
ARP reply
router
(runs DHCP)
Link Layer 5-93
DNS
UDP
IP
Eth
Phy
DNS
DNS
DNS
DNS
DNS
IP datagram containing DNS query forwarded via LAN switch from client to 1st hop router
IP datagram forwarded from campus network into comcast network routed (tables created by RIP OSPF IS-IS andor BGP routing protocols) to DNS server
demuxrsquoed to DNS server
DNS server replies to client with IP address of wwwgooglecom
Comcast network
68800013
DNS server
DNS
UDP
IP
Eth
Phy
DNS
DNS
DNS
DNS
A day in the lifehellip using DNS
router
(runs DHCP)
Link Layer 5-94
A day in the lifehellipTCP connection carrying HTTP
HTTP
TCP
IP
Eth
Phy
HTTP
to send HTTP request client first opens TCP socket to web server
TCP SYN segment (step 1 in 3-way handshake) inter-domain routed to web server
TCP connection established 64233169105
web server
SYN
SYN
SYN
SYN
TCP
IP
Eth
Phy
SYN
SYN
SYN
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
web server responds with TCP SYNACK (step 2 in 3-way handshake)
router
(runs DHCP)
Link Layer 5-95
A day in the lifehellip HTTP requestreply
HTTP
TCP
IP
Eth
Phy
HTTP
HTTP request sent into TCP socket
IP datagram containing HTTP request routed to wwwgooglecom
IP datagram containing HTTP reply routed back to client
64233169105
web server
HTTP
TCP
IP
Eth
Phy web server responds with
HTTP reply (containing web page)
HTTP
HTTP
HTTP HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
web page finally () displayed
Link Layer 5-96
Chapter 5 Summary
principles behind data link layer services error detection correction
sharing a broadcast channel multiple access
link layer addressing
instantiation and implementation of various link layer technologies Ethernet
switched LANS VLANs
virtualized networks as a link layer MPLS
synthesis a day in the life of a web request
Link Layer 5-97
Chapter 5 letrsquos take a breath
journey down protocol stack complete (except PHY)
solid understanding of networking principles practice
hellip could stop here hellip but lots of interesting topics wireless
multimedia
security
network management
Link Layer 5-82
R1 R2
D
R3 R4 R5
0
1
0 0
A
R6
in out out
label label dest interface
6 - A 0
in out out
label label dest interface
10 6 A 1
12 9 D 0
in out out
label label dest interface
10 A 0
12 D 0
1
in out out
label label dest interface
8 6 A 0
0
8 A 1
MPLS forwarding tables
Link Layer 5-83
Link layer LANs outline
51 introduction services
52 error detection correction
53 multiple access protocols
54 LANs addressing ARP
Ethernet
switches
VLANS
55 link virtualization MPLS
56 data center networking
57 a day in the life of a web request
Link Layer 5-84
Data center networks
10rsquos to 100rsquos of thousands of hosts often closely coupled in close proximity e-business (eg Amazon)
content-servers (eg YouTube Akamai Apple Microsoft)
search engines data mining (eg Google)
challenges
multiple applications each serving massive numbers of clients
managingbalancing load avoiding processing networking data bottlenecks
Inside a 40-ft Microsoft container
Chicago data center
Link Layer 5-85
Server racks
TOR switches
Tier-1 switches
Tier-2 switches
Load balancer
Load balancer
B
1 2 3 4 5 6 7 8
A C
Border router
Access router
Internet
Data center networks
load balancer application-layer routing receives external client requests
directs workload within data center
returns results to external client (hiding data
center internals from client)
Server racks
TOR switches
Tier-1 switches
Tier-2 switches
1 2 3 4 5 6 7 8
Data center networks
rich interconnection among switches racks
increased throughput between racks (multiple routing
paths possible)
increased reliability via redundancy
Link Layer 5-87
Link layer LANs outline
51 introduction services
52 error detection correction
53 multiple access protocols
54 LANs addressing ARP
Ethernet
switches
VLANS
55 link virtualization MPLS
56 data center networking
57 a day in the life of a web request
Link Layer 5-88
Synthesis a day in the life of a web request
journey down protocol stack complete application transport network link
putting-it-all-together synthesis goal identify review understand protocols (at all
layers) involved in seemingly simple scenario requesting www page
scenario student attaches laptop to campus network requestsreceives wwwgooglecom
Link Layer 5-89
A day in the life scenario
Comcast network
68800013
Googlersquos network
64233160019 64233169105
web server
DNS server
school network
68802024
web page
browser
router
(runs DHCP)
Link Layer 5-90
A day in the lifehellip connecting to the Internet
connecting laptop needs to get its own IP address addr of first-hop router addr of DNS server use DHCP
DHCP
UDP
IP
Eth
Phy
DHCP
DHCP
DHCP
DHCP
DHCP
DHCP
UDP
IP
Eth
Phy
DHCP
DHCP
DHCP
DHCP DHCP
DHCP request encapsulated in UDP encapsulated in IP encapsulated in 8023 Ethernet
Ethernet frame broadcast
(dest FFFFFFFFFFFF) on LAN received at router running DHCP server
Ethernet demuxed to IP demuxed UDP demuxed to DHCP
router
(runs DHCP)
Link Layer 5-91
DHCP server formulates DHCP ACK containing clientrsquos IP address IP address of first-hop router for client name amp IP address of DNS server
DHCP
UDP
IP
Eth
Phy
DHCP
DHCP
DHCP
DHCP
DHCP
UDP
IP
Eth
Phy
DHCP
DHCP
DHCP
DHCP
DHCP
encapsulation at DHCP server frame forwarded (switch learning) through LAN demultiplexing at client
Client now has IP address knows name amp addr of DNS
server IP address of its first-hop router
DHCP client receives DHCP ACK reply
A day in the lifehellip connecting to the Internet
router
(runs DHCP)
Link Layer 5-92
A day in the lifehellip ARP (before DNS before HTTP)
before sending HTTP request need IP address of wwwgooglecom DNS
DNS
UDP
IP
Eth
Phy
DNS
DNS
DNS
DNS query created encapsulated in UDP encapsulated in IP encapsulated in Eth To send frame to router need MAC address of router interface ARP
ARP query broadcast received by
router which replies with ARP reply giving MAC address of router interface
client now knows MAC address of first hop router so can now send frame containing DNS query
ARP query
Eth
Phy
ARP
ARP
ARP reply
router
(runs DHCP)
Link Layer 5-93
DNS
UDP
IP
Eth
Phy
DNS
DNS
DNS
DNS
DNS
IP datagram containing DNS query forwarded via LAN switch from client to 1st hop router
IP datagram forwarded from campus network into comcast network routed (tables created by RIP OSPF IS-IS andor BGP routing protocols) to DNS server
demuxrsquoed to DNS server
DNS server replies to client with IP address of wwwgooglecom
Comcast network
68800013
DNS server
DNS
UDP
IP
Eth
Phy
DNS
DNS
DNS
DNS
A day in the lifehellip using DNS
router
(runs DHCP)
Link Layer 5-94
A day in the lifehellipTCP connection carrying HTTP
HTTP
TCP
IP
Eth
Phy
HTTP
to send HTTP request client first opens TCP socket to web server
TCP SYN segment (step 1 in 3-way handshake) inter-domain routed to web server
TCP connection established 64233169105
web server
SYN
SYN
SYN
SYN
TCP
IP
Eth
Phy
SYN
SYN
SYN
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
web server responds with TCP SYNACK (step 2 in 3-way handshake)
router
(runs DHCP)
Link Layer 5-95
A day in the lifehellip HTTP requestreply
HTTP
TCP
IP
Eth
Phy
HTTP
HTTP request sent into TCP socket
IP datagram containing HTTP request routed to wwwgooglecom
IP datagram containing HTTP reply routed back to client
64233169105
web server
HTTP
TCP
IP
Eth
Phy web server responds with
HTTP reply (containing web page)
HTTP
HTTP
HTTP HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
web page finally () displayed
Link Layer 5-96
Chapter 5 Summary
principles behind data link layer services error detection correction
sharing a broadcast channel multiple access
link layer addressing
instantiation and implementation of various link layer technologies Ethernet
switched LANS VLANs
virtualized networks as a link layer MPLS
synthesis a day in the life of a web request
Link Layer 5-97
Chapter 5 letrsquos take a breath
journey down protocol stack complete (except PHY)
solid understanding of networking principles practice
hellip could stop here hellip but lots of interesting topics wireless
multimedia
security
network management
Link Layer 5-83
Link layer LANs outline
51 introduction services
52 error detection correction
53 multiple access protocols
54 LANs addressing ARP
Ethernet
switches
VLANS
55 link virtualization MPLS
56 data center networking
57 a day in the life of a web request
Link Layer 5-84
Data center networks
10rsquos to 100rsquos of thousands of hosts often closely coupled in close proximity e-business (eg Amazon)
content-servers (eg YouTube Akamai Apple Microsoft)
search engines data mining (eg Google)
challenges
multiple applications each serving massive numbers of clients
managingbalancing load avoiding processing networking data bottlenecks
Inside a 40-ft Microsoft container
Chicago data center
Link Layer 5-85
Server racks
TOR switches
Tier-1 switches
Tier-2 switches
Load balancer
Load balancer
B
1 2 3 4 5 6 7 8
A C
Border router
Access router
Internet
Data center networks
load balancer application-layer routing receives external client requests
directs workload within data center
returns results to external client (hiding data
center internals from client)
Server racks
TOR switches
Tier-1 switches
Tier-2 switches
1 2 3 4 5 6 7 8
Data center networks
rich interconnection among switches racks
increased throughput between racks (multiple routing
paths possible)
increased reliability via redundancy
Link Layer 5-87
Link layer LANs outline
51 introduction services
52 error detection correction
53 multiple access protocols
54 LANs addressing ARP
Ethernet
switches
VLANS
55 link virtualization MPLS
56 data center networking
57 a day in the life of a web request
Link Layer 5-88
Synthesis a day in the life of a web request
journey down protocol stack complete application transport network link
putting-it-all-together synthesis goal identify review understand protocols (at all
layers) involved in seemingly simple scenario requesting www page
scenario student attaches laptop to campus network requestsreceives wwwgooglecom
Link Layer 5-89
A day in the life scenario
Comcast network
68800013
Googlersquos network
64233160019 64233169105
web server
DNS server
school network
68802024
web page
browser
router
(runs DHCP)
Link Layer 5-90
A day in the lifehellip connecting to the Internet
connecting laptop needs to get its own IP address addr of first-hop router addr of DNS server use DHCP
DHCP
UDP
IP
Eth
Phy
DHCP
DHCP
DHCP
DHCP
DHCP
DHCP
UDP
IP
Eth
Phy
DHCP
DHCP
DHCP
DHCP DHCP
DHCP request encapsulated in UDP encapsulated in IP encapsulated in 8023 Ethernet
Ethernet frame broadcast
(dest FFFFFFFFFFFF) on LAN received at router running DHCP server
Ethernet demuxed to IP demuxed UDP demuxed to DHCP
router
(runs DHCP)
Link Layer 5-91
DHCP server formulates DHCP ACK containing clientrsquos IP address IP address of first-hop router for client name amp IP address of DNS server
DHCP
UDP
IP
Eth
Phy
DHCP
DHCP
DHCP
DHCP
DHCP
UDP
IP
Eth
Phy
DHCP
DHCP
DHCP
DHCP
DHCP
encapsulation at DHCP server frame forwarded (switch learning) through LAN demultiplexing at client
Client now has IP address knows name amp addr of DNS
server IP address of its first-hop router
DHCP client receives DHCP ACK reply
A day in the lifehellip connecting to the Internet
router
(runs DHCP)
Link Layer 5-92
A day in the lifehellip ARP (before DNS before HTTP)
before sending HTTP request need IP address of wwwgooglecom DNS
DNS
UDP
IP
Eth
Phy
DNS
DNS
DNS
DNS query created encapsulated in UDP encapsulated in IP encapsulated in Eth To send frame to router need MAC address of router interface ARP
ARP query broadcast received by
router which replies with ARP reply giving MAC address of router interface
client now knows MAC address of first hop router so can now send frame containing DNS query
ARP query
Eth
Phy
ARP
ARP
ARP reply
router
(runs DHCP)
Link Layer 5-93
DNS
UDP
IP
Eth
Phy
DNS
DNS
DNS
DNS
DNS
IP datagram containing DNS query forwarded via LAN switch from client to 1st hop router
IP datagram forwarded from campus network into comcast network routed (tables created by RIP OSPF IS-IS andor BGP routing protocols) to DNS server
demuxrsquoed to DNS server
DNS server replies to client with IP address of wwwgooglecom
Comcast network
68800013
DNS server
DNS
UDP
IP
Eth
Phy
DNS
DNS
DNS
DNS
A day in the lifehellip using DNS
router
(runs DHCP)
Link Layer 5-94
A day in the lifehellipTCP connection carrying HTTP
HTTP
TCP
IP
Eth
Phy
HTTP
to send HTTP request client first opens TCP socket to web server
TCP SYN segment (step 1 in 3-way handshake) inter-domain routed to web server
TCP connection established 64233169105
web server
SYN
SYN
SYN
SYN
TCP
IP
Eth
Phy
SYN
SYN
SYN
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
web server responds with TCP SYNACK (step 2 in 3-way handshake)
router
(runs DHCP)
Link Layer 5-95
A day in the lifehellip HTTP requestreply
HTTP
TCP
IP
Eth
Phy
HTTP
HTTP request sent into TCP socket
IP datagram containing HTTP request routed to wwwgooglecom
IP datagram containing HTTP reply routed back to client
64233169105
web server
HTTP
TCP
IP
Eth
Phy web server responds with
HTTP reply (containing web page)
HTTP
HTTP
HTTP HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
web page finally () displayed
Link Layer 5-96
Chapter 5 Summary
principles behind data link layer services error detection correction
sharing a broadcast channel multiple access
link layer addressing
instantiation and implementation of various link layer technologies Ethernet
switched LANS VLANs
virtualized networks as a link layer MPLS
synthesis a day in the life of a web request
Link Layer 5-97
Chapter 5 letrsquos take a breath
journey down protocol stack complete (except PHY)
solid understanding of networking principles practice
hellip could stop here hellip but lots of interesting topics wireless
multimedia
security
network management
Link Layer 5-84
Data center networks
10rsquos to 100rsquos of thousands of hosts often closely coupled in close proximity e-business (eg Amazon)
content-servers (eg YouTube Akamai Apple Microsoft)
search engines data mining (eg Google)
challenges
multiple applications each serving massive numbers of clients
managingbalancing load avoiding processing networking data bottlenecks
Inside a 40-ft Microsoft container
Chicago data center
Link Layer 5-85
Server racks
TOR switches
Tier-1 switches
Tier-2 switches
Load balancer
Load balancer
B
1 2 3 4 5 6 7 8
A C
Border router
Access router
Internet
Data center networks
load balancer application-layer routing receives external client requests
directs workload within data center
returns results to external client (hiding data
center internals from client)
Server racks
TOR switches
Tier-1 switches
Tier-2 switches
1 2 3 4 5 6 7 8
Data center networks
rich interconnection among switches racks
increased throughput between racks (multiple routing
paths possible)
increased reliability via redundancy
Link Layer 5-87
Link layer LANs outline
51 introduction services
52 error detection correction
53 multiple access protocols
54 LANs addressing ARP
Ethernet
switches
VLANS
55 link virtualization MPLS
56 data center networking
57 a day in the life of a web request
Link Layer 5-88
Synthesis a day in the life of a web request
journey down protocol stack complete application transport network link
putting-it-all-together synthesis goal identify review understand protocols (at all
layers) involved in seemingly simple scenario requesting www page
scenario student attaches laptop to campus network requestsreceives wwwgooglecom
Link Layer 5-89
A day in the life scenario
Comcast network
68800013
Googlersquos network
64233160019 64233169105
web server
DNS server
school network
68802024
web page
browser
router
(runs DHCP)
Link Layer 5-90
A day in the lifehellip connecting to the Internet
connecting laptop needs to get its own IP address addr of first-hop router addr of DNS server use DHCP
DHCP
UDP
IP
Eth
Phy
DHCP
DHCP
DHCP
DHCP
DHCP
DHCP
UDP
IP
Eth
Phy
DHCP
DHCP
DHCP
DHCP DHCP
DHCP request encapsulated in UDP encapsulated in IP encapsulated in 8023 Ethernet
Ethernet frame broadcast
(dest FFFFFFFFFFFF) on LAN received at router running DHCP server
Ethernet demuxed to IP demuxed UDP demuxed to DHCP
router
(runs DHCP)
Link Layer 5-91
DHCP server formulates DHCP ACK containing clientrsquos IP address IP address of first-hop router for client name amp IP address of DNS server
DHCP
UDP
IP
Eth
Phy
DHCP
DHCP
DHCP
DHCP
DHCP
UDP
IP
Eth
Phy
DHCP
DHCP
DHCP
DHCP
DHCP
encapsulation at DHCP server frame forwarded (switch learning) through LAN demultiplexing at client
Client now has IP address knows name amp addr of DNS
server IP address of its first-hop router
DHCP client receives DHCP ACK reply
A day in the lifehellip connecting to the Internet
router
(runs DHCP)
Link Layer 5-92
A day in the lifehellip ARP (before DNS before HTTP)
before sending HTTP request need IP address of wwwgooglecom DNS
DNS
UDP
IP
Eth
Phy
DNS
DNS
DNS
DNS query created encapsulated in UDP encapsulated in IP encapsulated in Eth To send frame to router need MAC address of router interface ARP
ARP query broadcast received by
router which replies with ARP reply giving MAC address of router interface
client now knows MAC address of first hop router so can now send frame containing DNS query
ARP query
Eth
Phy
ARP
ARP
ARP reply
router
(runs DHCP)
Link Layer 5-93
DNS
UDP
IP
Eth
Phy
DNS
DNS
DNS
DNS
DNS
IP datagram containing DNS query forwarded via LAN switch from client to 1st hop router
IP datagram forwarded from campus network into comcast network routed (tables created by RIP OSPF IS-IS andor BGP routing protocols) to DNS server
demuxrsquoed to DNS server
DNS server replies to client with IP address of wwwgooglecom
Comcast network
68800013
DNS server
DNS
UDP
IP
Eth
Phy
DNS
DNS
DNS
DNS
A day in the lifehellip using DNS
router
(runs DHCP)
Link Layer 5-94
A day in the lifehellipTCP connection carrying HTTP
HTTP
TCP
IP
Eth
Phy
HTTP
to send HTTP request client first opens TCP socket to web server
TCP SYN segment (step 1 in 3-way handshake) inter-domain routed to web server
TCP connection established 64233169105
web server
SYN
SYN
SYN
SYN
TCP
IP
Eth
Phy
SYN
SYN
SYN
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
web server responds with TCP SYNACK (step 2 in 3-way handshake)
router
(runs DHCP)
Link Layer 5-95
A day in the lifehellip HTTP requestreply
HTTP
TCP
IP
Eth
Phy
HTTP
HTTP request sent into TCP socket
IP datagram containing HTTP request routed to wwwgooglecom
IP datagram containing HTTP reply routed back to client
64233169105
web server
HTTP
TCP
IP
Eth
Phy web server responds with
HTTP reply (containing web page)
HTTP
HTTP
HTTP HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
web page finally () displayed
Link Layer 5-96
Chapter 5 Summary
principles behind data link layer services error detection correction
sharing a broadcast channel multiple access
link layer addressing
instantiation and implementation of various link layer technologies Ethernet
switched LANS VLANs
virtualized networks as a link layer MPLS
synthesis a day in the life of a web request
Link Layer 5-97
Chapter 5 letrsquos take a breath
journey down protocol stack complete (except PHY)
solid understanding of networking principles practice
hellip could stop here hellip but lots of interesting topics wireless
multimedia
security
network management
Link Layer 5-85
Server racks
TOR switches
Tier-1 switches
Tier-2 switches
Load balancer
Load balancer
B
1 2 3 4 5 6 7 8
A C
Border router
Access router
Internet
Data center networks
load balancer application-layer routing receives external client requests
directs workload within data center
returns results to external client (hiding data
center internals from client)
Server racks
TOR switches
Tier-1 switches
Tier-2 switches
1 2 3 4 5 6 7 8
Data center networks
rich interconnection among switches racks
increased throughput between racks (multiple routing
paths possible)
increased reliability via redundancy
Link Layer 5-87
Link layer LANs outline
51 introduction services
52 error detection correction
53 multiple access protocols
54 LANs addressing ARP
Ethernet
switches
VLANS
55 link virtualization MPLS
56 data center networking
57 a day in the life of a web request
Link Layer 5-88
Synthesis a day in the life of a web request
journey down protocol stack complete application transport network link
putting-it-all-together synthesis goal identify review understand protocols (at all
layers) involved in seemingly simple scenario requesting www page
scenario student attaches laptop to campus network requestsreceives wwwgooglecom
Link Layer 5-89
A day in the life scenario
Comcast network
68800013
Googlersquos network
64233160019 64233169105
web server
DNS server
school network
68802024
web page
browser
router
(runs DHCP)
Link Layer 5-90
A day in the lifehellip connecting to the Internet
connecting laptop needs to get its own IP address addr of first-hop router addr of DNS server use DHCP
DHCP
UDP
IP
Eth
Phy
DHCP
DHCP
DHCP
DHCP
DHCP
DHCP
UDP
IP
Eth
Phy
DHCP
DHCP
DHCP
DHCP DHCP
DHCP request encapsulated in UDP encapsulated in IP encapsulated in 8023 Ethernet
Ethernet frame broadcast
(dest FFFFFFFFFFFF) on LAN received at router running DHCP server
Ethernet demuxed to IP demuxed UDP demuxed to DHCP
router
(runs DHCP)
Link Layer 5-91
DHCP server formulates DHCP ACK containing clientrsquos IP address IP address of first-hop router for client name amp IP address of DNS server
DHCP
UDP
IP
Eth
Phy
DHCP
DHCP
DHCP
DHCP
DHCP
UDP
IP
Eth
Phy
DHCP
DHCP
DHCP
DHCP
DHCP
encapsulation at DHCP server frame forwarded (switch learning) through LAN demultiplexing at client
Client now has IP address knows name amp addr of DNS
server IP address of its first-hop router
DHCP client receives DHCP ACK reply
A day in the lifehellip connecting to the Internet
router
(runs DHCP)
Link Layer 5-92
A day in the lifehellip ARP (before DNS before HTTP)
before sending HTTP request need IP address of wwwgooglecom DNS
DNS
UDP
IP
Eth
Phy
DNS
DNS
DNS
DNS query created encapsulated in UDP encapsulated in IP encapsulated in Eth To send frame to router need MAC address of router interface ARP
ARP query broadcast received by
router which replies with ARP reply giving MAC address of router interface
client now knows MAC address of first hop router so can now send frame containing DNS query
ARP query
Eth
Phy
ARP
ARP
ARP reply
router
(runs DHCP)
Link Layer 5-93
DNS
UDP
IP
Eth
Phy
DNS
DNS
DNS
DNS
DNS
IP datagram containing DNS query forwarded via LAN switch from client to 1st hop router
IP datagram forwarded from campus network into comcast network routed (tables created by RIP OSPF IS-IS andor BGP routing protocols) to DNS server
demuxrsquoed to DNS server
DNS server replies to client with IP address of wwwgooglecom
Comcast network
68800013
DNS server
DNS
UDP
IP
Eth
Phy
DNS
DNS
DNS
DNS
A day in the lifehellip using DNS
router
(runs DHCP)
Link Layer 5-94
A day in the lifehellipTCP connection carrying HTTP
HTTP
TCP
IP
Eth
Phy
HTTP
to send HTTP request client first opens TCP socket to web server
TCP SYN segment (step 1 in 3-way handshake) inter-domain routed to web server
TCP connection established 64233169105
web server
SYN
SYN
SYN
SYN
TCP
IP
Eth
Phy
SYN
SYN
SYN
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
web server responds with TCP SYNACK (step 2 in 3-way handshake)
router
(runs DHCP)
Link Layer 5-95
A day in the lifehellip HTTP requestreply
HTTP
TCP
IP
Eth
Phy
HTTP
HTTP request sent into TCP socket
IP datagram containing HTTP request routed to wwwgooglecom
IP datagram containing HTTP reply routed back to client
64233169105
web server
HTTP
TCP
IP
Eth
Phy web server responds with
HTTP reply (containing web page)
HTTP
HTTP
HTTP HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
web page finally () displayed
Link Layer 5-96
Chapter 5 Summary
principles behind data link layer services error detection correction
sharing a broadcast channel multiple access
link layer addressing
instantiation and implementation of various link layer technologies Ethernet
switched LANS VLANs
virtualized networks as a link layer MPLS
synthesis a day in the life of a web request
Link Layer 5-97
Chapter 5 letrsquos take a breath
journey down protocol stack complete (except PHY)
solid understanding of networking principles practice
hellip could stop here hellip but lots of interesting topics wireless
multimedia
security
network management
Server racks
TOR switches
Tier-1 switches
Tier-2 switches
1 2 3 4 5 6 7 8
Data center networks
rich interconnection among switches racks
increased throughput between racks (multiple routing
paths possible)
increased reliability via redundancy
Link Layer 5-87
Link layer LANs outline
51 introduction services
52 error detection correction
53 multiple access protocols
54 LANs addressing ARP
Ethernet
switches
VLANS
55 link virtualization MPLS
56 data center networking
57 a day in the life of a web request
Link Layer 5-88
Synthesis a day in the life of a web request
journey down protocol stack complete application transport network link
putting-it-all-together synthesis goal identify review understand protocols (at all
layers) involved in seemingly simple scenario requesting www page
scenario student attaches laptop to campus network requestsreceives wwwgooglecom
Link Layer 5-89
A day in the life scenario
Comcast network
68800013
Googlersquos network
64233160019 64233169105
web server
DNS server
school network
68802024
web page
browser
router
(runs DHCP)
Link Layer 5-90
A day in the lifehellip connecting to the Internet
connecting laptop needs to get its own IP address addr of first-hop router addr of DNS server use DHCP
DHCP
UDP
IP
Eth
Phy
DHCP
DHCP
DHCP
DHCP
DHCP
DHCP
UDP
IP
Eth
Phy
DHCP
DHCP
DHCP
DHCP DHCP
DHCP request encapsulated in UDP encapsulated in IP encapsulated in 8023 Ethernet
Ethernet frame broadcast
(dest FFFFFFFFFFFF) on LAN received at router running DHCP server
Ethernet demuxed to IP demuxed UDP demuxed to DHCP
router
(runs DHCP)
Link Layer 5-91
DHCP server formulates DHCP ACK containing clientrsquos IP address IP address of first-hop router for client name amp IP address of DNS server
DHCP
UDP
IP
Eth
Phy
DHCP
DHCP
DHCP
DHCP
DHCP
UDP
IP
Eth
Phy
DHCP
DHCP
DHCP
DHCP
DHCP
encapsulation at DHCP server frame forwarded (switch learning) through LAN demultiplexing at client
Client now has IP address knows name amp addr of DNS
server IP address of its first-hop router
DHCP client receives DHCP ACK reply
A day in the lifehellip connecting to the Internet
router
(runs DHCP)
Link Layer 5-92
A day in the lifehellip ARP (before DNS before HTTP)
before sending HTTP request need IP address of wwwgooglecom DNS
DNS
UDP
IP
Eth
Phy
DNS
DNS
DNS
DNS query created encapsulated in UDP encapsulated in IP encapsulated in Eth To send frame to router need MAC address of router interface ARP
ARP query broadcast received by
router which replies with ARP reply giving MAC address of router interface
client now knows MAC address of first hop router so can now send frame containing DNS query
ARP query
Eth
Phy
ARP
ARP
ARP reply
router
(runs DHCP)
Link Layer 5-93
DNS
UDP
IP
Eth
Phy
DNS
DNS
DNS
DNS
DNS
IP datagram containing DNS query forwarded via LAN switch from client to 1st hop router
IP datagram forwarded from campus network into comcast network routed (tables created by RIP OSPF IS-IS andor BGP routing protocols) to DNS server
demuxrsquoed to DNS server
DNS server replies to client with IP address of wwwgooglecom
Comcast network
68800013
DNS server
DNS
UDP
IP
Eth
Phy
DNS
DNS
DNS
DNS
A day in the lifehellip using DNS
router
(runs DHCP)
Link Layer 5-94
A day in the lifehellipTCP connection carrying HTTP
HTTP
TCP
IP
Eth
Phy
HTTP
to send HTTP request client first opens TCP socket to web server
TCP SYN segment (step 1 in 3-way handshake) inter-domain routed to web server
TCP connection established 64233169105
web server
SYN
SYN
SYN
SYN
TCP
IP
Eth
Phy
SYN
SYN
SYN
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
web server responds with TCP SYNACK (step 2 in 3-way handshake)
router
(runs DHCP)
Link Layer 5-95
A day in the lifehellip HTTP requestreply
HTTP
TCP
IP
Eth
Phy
HTTP
HTTP request sent into TCP socket
IP datagram containing HTTP request routed to wwwgooglecom
IP datagram containing HTTP reply routed back to client
64233169105
web server
HTTP
TCP
IP
Eth
Phy web server responds with
HTTP reply (containing web page)
HTTP
HTTP
HTTP HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
web page finally () displayed
Link Layer 5-96
Chapter 5 Summary
principles behind data link layer services error detection correction
sharing a broadcast channel multiple access
link layer addressing
instantiation and implementation of various link layer technologies Ethernet
switched LANS VLANs
virtualized networks as a link layer MPLS
synthesis a day in the life of a web request
Link Layer 5-97
Chapter 5 letrsquos take a breath
journey down protocol stack complete (except PHY)
solid understanding of networking principles practice
hellip could stop here hellip but lots of interesting topics wireless
multimedia
security
network management
Link Layer 5-87
Link layer LANs outline
51 introduction services
52 error detection correction
53 multiple access protocols
54 LANs addressing ARP
Ethernet
switches
VLANS
55 link virtualization MPLS
56 data center networking
57 a day in the life of a web request
Link Layer 5-88
Synthesis a day in the life of a web request
journey down protocol stack complete application transport network link
putting-it-all-together synthesis goal identify review understand protocols (at all
layers) involved in seemingly simple scenario requesting www page
scenario student attaches laptop to campus network requestsreceives wwwgooglecom
Link Layer 5-89
A day in the life scenario
Comcast network
68800013
Googlersquos network
64233160019 64233169105
web server
DNS server
school network
68802024
web page
browser
router
(runs DHCP)
Link Layer 5-90
A day in the lifehellip connecting to the Internet
connecting laptop needs to get its own IP address addr of first-hop router addr of DNS server use DHCP
DHCP
UDP
IP
Eth
Phy
DHCP
DHCP
DHCP
DHCP
DHCP
DHCP
UDP
IP
Eth
Phy
DHCP
DHCP
DHCP
DHCP DHCP
DHCP request encapsulated in UDP encapsulated in IP encapsulated in 8023 Ethernet
Ethernet frame broadcast
(dest FFFFFFFFFFFF) on LAN received at router running DHCP server
Ethernet demuxed to IP demuxed UDP demuxed to DHCP
router
(runs DHCP)
Link Layer 5-91
DHCP server formulates DHCP ACK containing clientrsquos IP address IP address of first-hop router for client name amp IP address of DNS server
DHCP
UDP
IP
Eth
Phy
DHCP
DHCP
DHCP
DHCP
DHCP
UDP
IP
Eth
Phy
DHCP
DHCP
DHCP
DHCP
DHCP
encapsulation at DHCP server frame forwarded (switch learning) through LAN demultiplexing at client
Client now has IP address knows name amp addr of DNS
server IP address of its first-hop router
DHCP client receives DHCP ACK reply
A day in the lifehellip connecting to the Internet
router
(runs DHCP)
Link Layer 5-92
A day in the lifehellip ARP (before DNS before HTTP)
before sending HTTP request need IP address of wwwgooglecom DNS
DNS
UDP
IP
Eth
Phy
DNS
DNS
DNS
DNS query created encapsulated in UDP encapsulated in IP encapsulated in Eth To send frame to router need MAC address of router interface ARP
ARP query broadcast received by
router which replies with ARP reply giving MAC address of router interface
client now knows MAC address of first hop router so can now send frame containing DNS query
ARP query
Eth
Phy
ARP
ARP
ARP reply
router
(runs DHCP)
Link Layer 5-93
DNS
UDP
IP
Eth
Phy
DNS
DNS
DNS
DNS
DNS
IP datagram containing DNS query forwarded via LAN switch from client to 1st hop router
IP datagram forwarded from campus network into comcast network routed (tables created by RIP OSPF IS-IS andor BGP routing protocols) to DNS server
demuxrsquoed to DNS server
DNS server replies to client with IP address of wwwgooglecom
Comcast network
68800013
DNS server
DNS
UDP
IP
Eth
Phy
DNS
DNS
DNS
DNS
A day in the lifehellip using DNS
router
(runs DHCP)
Link Layer 5-94
A day in the lifehellipTCP connection carrying HTTP
HTTP
TCP
IP
Eth
Phy
HTTP
to send HTTP request client first opens TCP socket to web server
TCP SYN segment (step 1 in 3-way handshake) inter-domain routed to web server
TCP connection established 64233169105
web server
SYN
SYN
SYN
SYN
TCP
IP
Eth
Phy
SYN
SYN
SYN
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
web server responds with TCP SYNACK (step 2 in 3-way handshake)
router
(runs DHCP)
Link Layer 5-95
A day in the lifehellip HTTP requestreply
HTTP
TCP
IP
Eth
Phy
HTTP
HTTP request sent into TCP socket
IP datagram containing HTTP request routed to wwwgooglecom
IP datagram containing HTTP reply routed back to client
64233169105
web server
HTTP
TCP
IP
Eth
Phy web server responds with
HTTP reply (containing web page)
HTTP
HTTP
HTTP HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
web page finally () displayed
Link Layer 5-96
Chapter 5 Summary
principles behind data link layer services error detection correction
sharing a broadcast channel multiple access
link layer addressing
instantiation and implementation of various link layer technologies Ethernet
switched LANS VLANs
virtualized networks as a link layer MPLS
synthesis a day in the life of a web request
Link Layer 5-97
Chapter 5 letrsquos take a breath
journey down protocol stack complete (except PHY)
solid understanding of networking principles practice
hellip could stop here hellip but lots of interesting topics wireless
multimedia
security
network management
Link Layer 5-88
Synthesis a day in the life of a web request
journey down protocol stack complete application transport network link
putting-it-all-together synthesis goal identify review understand protocols (at all
layers) involved in seemingly simple scenario requesting www page
scenario student attaches laptop to campus network requestsreceives wwwgooglecom
Link Layer 5-89
A day in the life scenario
Comcast network
68800013
Googlersquos network
64233160019 64233169105
web server
DNS server
school network
68802024
web page
browser
router
(runs DHCP)
Link Layer 5-90
A day in the lifehellip connecting to the Internet
connecting laptop needs to get its own IP address addr of first-hop router addr of DNS server use DHCP
DHCP
UDP
IP
Eth
Phy
DHCP
DHCP
DHCP
DHCP
DHCP
DHCP
UDP
IP
Eth
Phy
DHCP
DHCP
DHCP
DHCP DHCP
DHCP request encapsulated in UDP encapsulated in IP encapsulated in 8023 Ethernet
Ethernet frame broadcast
(dest FFFFFFFFFFFF) on LAN received at router running DHCP server
Ethernet demuxed to IP demuxed UDP demuxed to DHCP
router
(runs DHCP)
Link Layer 5-91
DHCP server formulates DHCP ACK containing clientrsquos IP address IP address of first-hop router for client name amp IP address of DNS server
DHCP
UDP
IP
Eth
Phy
DHCP
DHCP
DHCP
DHCP
DHCP
UDP
IP
Eth
Phy
DHCP
DHCP
DHCP
DHCP
DHCP
encapsulation at DHCP server frame forwarded (switch learning) through LAN demultiplexing at client
Client now has IP address knows name amp addr of DNS
server IP address of its first-hop router
DHCP client receives DHCP ACK reply
A day in the lifehellip connecting to the Internet
router
(runs DHCP)
Link Layer 5-92
A day in the lifehellip ARP (before DNS before HTTP)
before sending HTTP request need IP address of wwwgooglecom DNS
DNS
UDP
IP
Eth
Phy
DNS
DNS
DNS
DNS query created encapsulated in UDP encapsulated in IP encapsulated in Eth To send frame to router need MAC address of router interface ARP
ARP query broadcast received by
router which replies with ARP reply giving MAC address of router interface
client now knows MAC address of first hop router so can now send frame containing DNS query
ARP query
Eth
Phy
ARP
ARP
ARP reply
router
(runs DHCP)
Link Layer 5-93
DNS
UDP
IP
Eth
Phy
DNS
DNS
DNS
DNS
DNS
IP datagram containing DNS query forwarded via LAN switch from client to 1st hop router
IP datagram forwarded from campus network into comcast network routed (tables created by RIP OSPF IS-IS andor BGP routing protocols) to DNS server
demuxrsquoed to DNS server
DNS server replies to client with IP address of wwwgooglecom
Comcast network
68800013
DNS server
DNS
UDP
IP
Eth
Phy
DNS
DNS
DNS
DNS
A day in the lifehellip using DNS
router
(runs DHCP)
Link Layer 5-94
A day in the lifehellipTCP connection carrying HTTP
HTTP
TCP
IP
Eth
Phy
HTTP
to send HTTP request client first opens TCP socket to web server
TCP SYN segment (step 1 in 3-way handshake) inter-domain routed to web server
TCP connection established 64233169105
web server
SYN
SYN
SYN
SYN
TCP
IP
Eth
Phy
SYN
SYN
SYN
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
web server responds with TCP SYNACK (step 2 in 3-way handshake)
router
(runs DHCP)
Link Layer 5-95
A day in the lifehellip HTTP requestreply
HTTP
TCP
IP
Eth
Phy
HTTP
HTTP request sent into TCP socket
IP datagram containing HTTP request routed to wwwgooglecom
IP datagram containing HTTP reply routed back to client
64233169105
web server
HTTP
TCP
IP
Eth
Phy web server responds with
HTTP reply (containing web page)
HTTP
HTTP
HTTP HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
web page finally () displayed
Link Layer 5-96
Chapter 5 Summary
principles behind data link layer services error detection correction
sharing a broadcast channel multiple access
link layer addressing
instantiation and implementation of various link layer technologies Ethernet
switched LANS VLANs
virtualized networks as a link layer MPLS
synthesis a day in the life of a web request
Link Layer 5-97
Chapter 5 letrsquos take a breath
journey down protocol stack complete (except PHY)
solid understanding of networking principles practice
hellip could stop here hellip but lots of interesting topics wireless
multimedia
security
network management
Link Layer 5-89
A day in the life scenario
Comcast network
68800013
Googlersquos network
64233160019 64233169105
web server
DNS server
school network
68802024
web page
browser
router
(runs DHCP)
Link Layer 5-90
A day in the lifehellip connecting to the Internet
connecting laptop needs to get its own IP address addr of first-hop router addr of DNS server use DHCP
DHCP
UDP
IP
Eth
Phy
DHCP
DHCP
DHCP
DHCP
DHCP
DHCP
UDP
IP
Eth
Phy
DHCP
DHCP
DHCP
DHCP DHCP
DHCP request encapsulated in UDP encapsulated in IP encapsulated in 8023 Ethernet
Ethernet frame broadcast
(dest FFFFFFFFFFFF) on LAN received at router running DHCP server
Ethernet demuxed to IP demuxed UDP demuxed to DHCP
router
(runs DHCP)
Link Layer 5-91
DHCP server formulates DHCP ACK containing clientrsquos IP address IP address of first-hop router for client name amp IP address of DNS server
DHCP
UDP
IP
Eth
Phy
DHCP
DHCP
DHCP
DHCP
DHCP
UDP
IP
Eth
Phy
DHCP
DHCP
DHCP
DHCP
DHCP
encapsulation at DHCP server frame forwarded (switch learning) through LAN demultiplexing at client
Client now has IP address knows name amp addr of DNS
server IP address of its first-hop router
DHCP client receives DHCP ACK reply
A day in the lifehellip connecting to the Internet
router
(runs DHCP)
Link Layer 5-92
A day in the lifehellip ARP (before DNS before HTTP)
before sending HTTP request need IP address of wwwgooglecom DNS
DNS
UDP
IP
Eth
Phy
DNS
DNS
DNS
DNS query created encapsulated in UDP encapsulated in IP encapsulated in Eth To send frame to router need MAC address of router interface ARP
ARP query broadcast received by
router which replies with ARP reply giving MAC address of router interface
client now knows MAC address of first hop router so can now send frame containing DNS query
ARP query
Eth
Phy
ARP
ARP
ARP reply
router
(runs DHCP)
Link Layer 5-93
DNS
UDP
IP
Eth
Phy
DNS
DNS
DNS
DNS
DNS
IP datagram containing DNS query forwarded via LAN switch from client to 1st hop router
IP datagram forwarded from campus network into comcast network routed (tables created by RIP OSPF IS-IS andor BGP routing protocols) to DNS server
demuxrsquoed to DNS server
DNS server replies to client with IP address of wwwgooglecom
Comcast network
68800013
DNS server
DNS
UDP
IP
Eth
Phy
DNS
DNS
DNS
DNS
A day in the lifehellip using DNS
router
(runs DHCP)
Link Layer 5-94
A day in the lifehellipTCP connection carrying HTTP
HTTP
TCP
IP
Eth
Phy
HTTP
to send HTTP request client first opens TCP socket to web server
TCP SYN segment (step 1 in 3-way handshake) inter-domain routed to web server
TCP connection established 64233169105
web server
SYN
SYN
SYN
SYN
TCP
IP
Eth
Phy
SYN
SYN
SYN
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
web server responds with TCP SYNACK (step 2 in 3-way handshake)
router
(runs DHCP)
Link Layer 5-95
A day in the lifehellip HTTP requestreply
HTTP
TCP
IP
Eth
Phy
HTTP
HTTP request sent into TCP socket
IP datagram containing HTTP request routed to wwwgooglecom
IP datagram containing HTTP reply routed back to client
64233169105
web server
HTTP
TCP
IP
Eth
Phy web server responds with
HTTP reply (containing web page)
HTTP
HTTP
HTTP HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
web page finally () displayed
Link Layer 5-96
Chapter 5 Summary
principles behind data link layer services error detection correction
sharing a broadcast channel multiple access
link layer addressing
instantiation and implementation of various link layer technologies Ethernet
switched LANS VLANs
virtualized networks as a link layer MPLS
synthesis a day in the life of a web request
Link Layer 5-97
Chapter 5 letrsquos take a breath
journey down protocol stack complete (except PHY)
solid understanding of networking principles practice
hellip could stop here hellip but lots of interesting topics wireless
multimedia
security
network management
router
(runs DHCP)
Link Layer 5-90
A day in the lifehellip connecting to the Internet
connecting laptop needs to get its own IP address addr of first-hop router addr of DNS server use DHCP
DHCP
UDP
IP
Eth
Phy
DHCP
DHCP
DHCP
DHCP
DHCP
DHCP
UDP
IP
Eth
Phy
DHCP
DHCP
DHCP
DHCP DHCP
DHCP request encapsulated in UDP encapsulated in IP encapsulated in 8023 Ethernet
Ethernet frame broadcast
(dest FFFFFFFFFFFF) on LAN received at router running DHCP server
Ethernet demuxed to IP demuxed UDP demuxed to DHCP
router
(runs DHCP)
Link Layer 5-91
DHCP server formulates DHCP ACK containing clientrsquos IP address IP address of first-hop router for client name amp IP address of DNS server
DHCP
UDP
IP
Eth
Phy
DHCP
DHCP
DHCP
DHCP
DHCP
UDP
IP
Eth
Phy
DHCP
DHCP
DHCP
DHCP
DHCP
encapsulation at DHCP server frame forwarded (switch learning) through LAN demultiplexing at client
Client now has IP address knows name amp addr of DNS
server IP address of its first-hop router
DHCP client receives DHCP ACK reply
A day in the lifehellip connecting to the Internet
router
(runs DHCP)
Link Layer 5-92
A day in the lifehellip ARP (before DNS before HTTP)
before sending HTTP request need IP address of wwwgooglecom DNS
DNS
UDP
IP
Eth
Phy
DNS
DNS
DNS
DNS query created encapsulated in UDP encapsulated in IP encapsulated in Eth To send frame to router need MAC address of router interface ARP
ARP query broadcast received by
router which replies with ARP reply giving MAC address of router interface
client now knows MAC address of first hop router so can now send frame containing DNS query
ARP query
Eth
Phy
ARP
ARP
ARP reply
router
(runs DHCP)
Link Layer 5-93
DNS
UDP
IP
Eth
Phy
DNS
DNS
DNS
DNS
DNS
IP datagram containing DNS query forwarded via LAN switch from client to 1st hop router
IP datagram forwarded from campus network into comcast network routed (tables created by RIP OSPF IS-IS andor BGP routing protocols) to DNS server
demuxrsquoed to DNS server
DNS server replies to client with IP address of wwwgooglecom
Comcast network
68800013
DNS server
DNS
UDP
IP
Eth
Phy
DNS
DNS
DNS
DNS
A day in the lifehellip using DNS
router
(runs DHCP)
Link Layer 5-94
A day in the lifehellipTCP connection carrying HTTP
HTTP
TCP
IP
Eth
Phy
HTTP
to send HTTP request client first opens TCP socket to web server
TCP SYN segment (step 1 in 3-way handshake) inter-domain routed to web server
TCP connection established 64233169105
web server
SYN
SYN
SYN
SYN
TCP
IP
Eth
Phy
SYN
SYN
SYN
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
web server responds with TCP SYNACK (step 2 in 3-way handshake)
router
(runs DHCP)
Link Layer 5-95
A day in the lifehellip HTTP requestreply
HTTP
TCP
IP
Eth
Phy
HTTP
HTTP request sent into TCP socket
IP datagram containing HTTP request routed to wwwgooglecom
IP datagram containing HTTP reply routed back to client
64233169105
web server
HTTP
TCP
IP
Eth
Phy web server responds with
HTTP reply (containing web page)
HTTP
HTTP
HTTP HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
web page finally () displayed
Link Layer 5-96
Chapter 5 Summary
principles behind data link layer services error detection correction
sharing a broadcast channel multiple access
link layer addressing
instantiation and implementation of various link layer technologies Ethernet
switched LANS VLANs
virtualized networks as a link layer MPLS
synthesis a day in the life of a web request
Link Layer 5-97
Chapter 5 letrsquos take a breath
journey down protocol stack complete (except PHY)
solid understanding of networking principles practice
hellip could stop here hellip but lots of interesting topics wireless
multimedia
security
network management
router
(runs DHCP)
Link Layer 5-91
DHCP server formulates DHCP ACK containing clientrsquos IP address IP address of first-hop router for client name amp IP address of DNS server
DHCP
UDP
IP
Eth
Phy
DHCP
DHCP
DHCP
DHCP
DHCP
UDP
IP
Eth
Phy
DHCP
DHCP
DHCP
DHCP
DHCP
encapsulation at DHCP server frame forwarded (switch learning) through LAN demultiplexing at client
Client now has IP address knows name amp addr of DNS
server IP address of its first-hop router
DHCP client receives DHCP ACK reply
A day in the lifehellip connecting to the Internet
router
(runs DHCP)
Link Layer 5-92
A day in the lifehellip ARP (before DNS before HTTP)
before sending HTTP request need IP address of wwwgooglecom DNS
DNS
UDP
IP
Eth
Phy
DNS
DNS
DNS
DNS query created encapsulated in UDP encapsulated in IP encapsulated in Eth To send frame to router need MAC address of router interface ARP
ARP query broadcast received by
router which replies with ARP reply giving MAC address of router interface
client now knows MAC address of first hop router so can now send frame containing DNS query
ARP query
Eth
Phy
ARP
ARP
ARP reply
router
(runs DHCP)
Link Layer 5-93
DNS
UDP
IP
Eth
Phy
DNS
DNS
DNS
DNS
DNS
IP datagram containing DNS query forwarded via LAN switch from client to 1st hop router
IP datagram forwarded from campus network into comcast network routed (tables created by RIP OSPF IS-IS andor BGP routing protocols) to DNS server
demuxrsquoed to DNS server
DNS server replies to client with IP address of wwwgooglecom
Comcast network
68800013
DNS server
DNS
UDP
IP
Eth
Phy
DNS
DNS
DNS
DNS
A day in the lifehellip using DNS
router
(runs DHCP)
Link Layer 5-94
A day in the lifehellipTCP connection carrying HTTP
HTTP
TCP
IP
Eth
Phy
HTTP
to send HTTP request client first opens TCP socket to web server
TCP SYN segment (step 1 in 3-way handshake) inter-domain routed to web server
TCP connection established 64233169105
web server
SYN
SYN
SYN
SYN
TCP
IP
Eth
Phy
SYN
SYN
SYN
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
web server responds with TCP SYNACK (step 2 in 3-way handshake)
router
(runs DHCP)
Link Layer 5-95
A day in the lifehellip HTTP requestreply
HTTP
TCP
IP
Eth
Phy
HTTP
HTTP request sent into TCP socket
IP datagram containing HTTP request routed to wwwgooglecom
IP datagram containing HTTP reply routed back to client
64233169105
web server
HTTP
TCP
IP
Eth
Phy web server responds with
HTTP reply (containing web page)
HTTP
HTTP
HTTP HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
web page finally () displayed
Link Layer 5-96
Chapter 5 Summary
principles behind data link layer services error detection correction
sharing a broadcast channel multiple access
link layer addressing
instantiation and implementation of various link layer technologies Ethernet
switched LANS VLANs
virtualized networks as a link layer MPLS
synthesis a day in the life of a web request
Link Layer 5-97
Chapter 5 letrsquos take a breath
journey down protocol stack complete (except PHY)
solid understanding of networking principles practice
hellip could stop here hellip but lots of interesting topics wireless
multimedia
security
network management
router
(runs DHCP)
Link Layer 5-92
A day in the lifehellip ARP (before DNS before HTTP)
before sending HTTP request need IP address of wwwgooglecom DNS
DNS
UDP
IP
Eth
Phy
DNS
DNS
DNS
DNS query created encapsulated in UDP encapsulated in IP encapsulated in Eth To send frame to router need MAC address of router interface ARP
ARP query broadcast received by
router which replies with ARP reply giving MAC address of router interface
client now knows MAC address of first hop router so can now send frame containing DNS query
ARP query
Eth
Phy
ARP
ARP
ARP reply
router
(runs DHCP)
Link Layer 5-93
DNS
UDP
IP
Eth
Phy
DNS
DNS
DNS
DNS
DNS
IP datagram containing DNS query forwarded via LAN switch from client to 1st hop router
IP datagram forwarded from campus network into comcast network routed (tables created by RIP OSPF IS-IS andor BGP routing protocols) to DNS server
demuxrsquoed to DNS server
DNS server replies to client with IP address of wwwgooglecom
Comcast network
68800013
DNS server
DNS
UDP
IP
Eth
Phy
DNS
DNS
DNS
DNS
A day in the lifehellip using DNS
router
(runs DHCP)
Link Layer 5-94
A day in the lifehellipTCP connection carrying HTTP
HTTP
TCP
IP
Eth
Phy
HTTP
to send HTTP request client first opens TCP socket to web server
TCP SYN segment (step 1 in 3-way handshake) inter-domain routed to web server
TCP connection established 64233169105
web server
SYN
SYN
SYN
SYN
TCP
IP
Eth
Phy
SYN
SYN
SYN
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
web server responds with TCP SYNACK (step 2 in 3-way handshake)
router
(runs DHCP)
Link Layer 5-95
A day in the lifehellip HTTP requestreply
HTTP
TCP
IP
Eth
Phy
HTTP
HTTP request sent into TCP socket
IP datagram containing HTTP request routed to wwwgooglecom
IP datagram containing HTTP reply routed back to client
64233169105
web server
HTTP
TCP
IP
Eth
Phy web server responds with
HTTP reply (containing web page)
HTTP
HTTP
HTTP HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
web page finally () displayed
Link Layer 5-96
Chapter 5 Summary
principles behind data link layer services error detection correction
sharing a broadcast channel multiple access
link layer addressing
instantiation and implementation of various link layer technologies Ethernet
switched LANS VLANs
virtualized networks as a link layer MPLS
synthesis a day in the life of a web request
Link Layer 5-97
Chapter 5 letrsquos take a breath
journey down protocol stack complete (except PHY)
solid understanding of networking principles practice
hellip could stop here hellip but lots of interesting topics wireless
multimedia
security
network management
router
(runs DHCP)
Link Layer 5-93
DNS
UDP
IP
Eth
Phy
DNS
DNS
DNS
DNS
DNS
IP datagram containing DNS query forwarded via LAN switch from client to 1st hop router
IP datagram forwarded from campus network into comcast network routed (tables created by RIP OSPF IS-IS andor BGP routing protocols) to DNS server
demuxrsquoed to DNS server
DNS server replies to client with IP address of wwwgooglecom
Comcast network
68800013
DNS server
DNS
UDP
IP
Eth
Phy
DNS
DNS
DNS
DNS
A day in the lifehellip using DNS
router
(runs DHCP)
Link Layer 5-94
A day in the lifehellipTCP connection carrying HTTP
HTTP
TCP
IP
Eth
Phy
HTTP
to send HTTP request client first opens TCP socket to web server
TCP SYN segment (step 1 in 3-way handshake) inter-domain routed to web server
TCP connection established 64233169105
web server
SYN
SYN
SYN
SYN
TCP
IP
Eth
Phy
SYN
SYN
SYN
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
web server responds with TCP SYNACK (step 2 in 3-way handshake)
router
(runs DHCP)
Link Layer 5-95
A day in the lifehellip HTTP requestreply
HTTP
TCP
IP
Eth
Phy
HTTP
HTTP request sent into TCP socket
IP datagram containing HTTP request routed to wwwgooglecom
IP datagram containing HTTP reply routed back to client
64233169105
web server
HTTP
TCP
IP
Eth
Phy web server responds with
HTTP reply (containing web page)
HTTP
HTTP
HTTP HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
web page finally () displayed
Link Layer 5-96
Chapter 5 Summary
principles behind data link layer services error detection correction
sharing a broadcast channel multiple access
link layer addressing
instantiation and implementation of various link layer technologies Ethernet
switched LANS VLANs
virtualized networks as a link layer MPLS
synthesis a day in the life of a web request
Link Layer 5-97
Chapter 5 letrsquos take a breath
journey down protocol stack complete (except PHY)
solid understanding of networking principles practice
hellip could stop here hellip but lots of interesting topics wireless
multimedia
security
network management
router
(runs DHCP)
Link Layer 5-94
A day in the lifehellipTCP connection carrying HTTP
HTTP
TCP
IP
Eth
Phy
HTTP
to send HTTP request client first opens TCP socket to web server
TCP SYN segment (step 1 in 3-way handshake) inter-domain routed to web server
TCP connection established 64233169105
web server
SYN
SYN
SYN
SYN
TCP
IP
Eth
Phy
SYN
SYN
SYN
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
SYNACK
web server responds with TCP SYNACK (step 2 in 3-way handshake)
router
(runs DHCP)
Link Layer 5-95
A day in the lifehellip HTTP requestreply
HTTP
TCP
IP
Eth
Phy
HTTP
HTTP request sent into TCP socket
IP datagram containing HTTP request routed to wwwgooglecom
IP datagram containing HTTP reply routed back to client
64233169105
web server
HTTP
TCP
IP
Eth
Phy web server responds with
HTTP reply (containing web page)
HTTP
HTTP
HTTP HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
web page finally () displayed
Link Layer 5-96
Chapter 5 Summary
principles behind data link layer services error detection correction
sharing a broadcast channel multiple access
link layer addressing
instantiation and implementation of various link layer technologies Ethernet
switched LANS VLANs
virtualized networks as a link layer MPLS
synthesis a day in the life of a web request
Link Layer 5-97
Chapter 5 letrsquos take a breath
journey down protocol stack complete (except PHY)
solid understanding of networking principles practice
hellip could stop here hellip but lots of interesting topics wireless
multimedia
security
network management
router
(runs DHCP)
Link Layer 5-95
A day in the lifehellip HTTP requestreply
HTTP
TCP
IP
Eth
Phy
HTTP
HTTP request sent into TCP socket
IP datagram containing HTTP request routed to wwwgooglecom
IP datagram containing HTTP reply routed back to client
64233169105
web server
HTTP
TCP
IP
Eth
Phy web server responds with
HTTP reply (containing web page)
HTTP
HTTP
HTTP HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP
web page finally () displayed
Link Layer 5-96
Chapter 5 Summary
principles behind data link layer services error detection correction
sharing a broadcast channel multiple access
link layer addressing
instantiation and implementation of various link layer technologies Ethernet
switched LANS VLANs
virtualized networks as a link layer MPLS
synthesis a day in the life of a web request
Link Layer 5-97
Chapter 5 letrsquos take a breath
journey down protocol stack complete (except PHY)
solid understanding of networking principles practice
hellip could stop here hellip but lots of interesting topics wireless
multimedia
security
network management
Link Layer 5-96
Chapter 5 Summary
principles behind data link layer services error detection correction
sharing a broadcast channel multiple access
link layer addressing
instantiation and implementation of various link layer technologies Ethernet
switched LANS VLANs
virtualized networks as a link layer MPLS
synthesis a day in the life of a web request
Link Layer 5-97
Chapter 5 letrsquos take a breath
journey down protocol stack complete (except PHY)
solid understanding of networking principles practice
hellip could stop here hellip but lots of interesting topics wireless
multimedia
security
network management
Link Layer 5-97
Chapter 5 letrsquos take a breath
journey down protocol stack complete (except PHY)
solid understanding of networking principles practice
hellip could stop here hellip but lots of interesting topics wireless
multimedia
security
network management